National 5 Chemistry
Unit1
Chemical changes and structures
Topic / Page1 – Rates of Reaction / 1
Minitest / 7
2–ElementsAtomicStructure / 10
Minitest / 14
3- BondingRelatedtoProperties of Materials / 15
Minitest / 19
4 – Chemical Formulae / 21
5–BalancingEquations / 23
Minitest / 24
6–TheMole / 25
Minitest / 27
4 – Acids and Bases / 28
Minitest / 33
Glossary / 34
Material sourced from BBC Bitesize Revision Website
1 - Rates of reaction
There are four factors that affect the rate of a chemical reaction:
- temperature
- concentration
- particlesize
- useofacatalyst
Forareactiontooccur,theparticlesthatarereactingmustcollidewitheachother. Onlysomeofallthecollisionsthattakeplacecauseachemicalchangetohappen. Thesearecalled'successful'collisions.Thegreaterthenumberof'successful'collisions, thefastertherateofareaction.Thisiscalledthe'collisiontheory'.
a)Temperature
Ifthetemperatureisincreased,theparticleshavemoreenergyandsomovequicker. Increasingthetemperatureincreases therateof reaction becausethe particlescollidemoreoftenandwithmoreenergy.Thehigherthetemperature,thefasterthe rateofareactionwillbe.
Lookatthegraphofthereactionbetweenhydrochloricacidandcalciumcarbonate. Noticehowanincreaseintemperatureleadstoanincreaseinthespeedofreleaseof carbondioxide,butnotthetotalvolumeofcarbondioxidereleased.
b)Concentration
If the concentration of reactants is increased, there are more reactant particles movingtogether.Therewillbemorecollisionsandsothereactionrateisincreased. Thehighertheconcentrationofreactants,thefastertherateofareactionwillbe.
c)Particlesize
Bydecreasingtheparticlesizeofareactant,weareincreasingitssurfacearea.The greaterthesurfacearea,thehigherthechanceofcollisions,thusthefastertherateof reaction.Thesmallertheparticlesizethefasterthereaction.
Thinkofacubewherethelengthofeverysideis2cm. Theareaofonefaceofthecubewillbe2x2=4cm2. Thecubehassixfaces,sothetotalsurfaceareais
4 cm2 x 6 = 24 cm2.
Wecouldcutthatcubehorizontallyandverticallyalongeachfacesothatwehave eightsmallercubes.
Each of the small cubes has a face area of 1 cm x 1 cm = 1 cm2.
Thesixfacesgiveatotalsurfaceareaforeachsmallercubeof6cm2. Thereareeightcubessothetotalsurfaceareais6cm2x8=48cm2.
d)Useofacatalyst
Acatalystspeedsuptherateofareactionbutitisnotusedupinthereaction.Ifa catalystispresent,thereactingparticlescancollidemoresuccessfullywithlessenergy andsothereactioncantakeplaceatalowertemperature.
Enzymes
Enzymes are biological catalysts.
They occur naturally in the body and help with digestion. They are used in the productionofalcohol(zymase)anddigestionoffood(amylase).Enzymesarespecific. Thismeansthattheycanonlycatalyseonereaction.
Many enzymes work best at body temperature. While they do work at lower temperatures,theyarelesseffective.Athightemperatures,theenzymemolecule changesshape(denatures)andnolongerspeedsupthereaction.
d)Monitoringtherateofareaction
Therateofachemicalreactionisameasureofhowfastthereactantsarebeingused upandhowfasttheproductsarebeingmade.
The rate can be determined by measuring:
- changesintheconcentrationofthereactantsorproducts
- changesinthemassofthereactantsorproducts
- changesinthevolumeofthereactantsorproducts
For example, when dilute hydrochloric acid reacts with zinc metal, zinc chloride and hydrogen gas are produced.
There are two simple ways to measure the rate of this reaction in the lab.
1.Ifthereactionissetuponabalanceasshown,themassoftheapparatuscan be monitored throughout the reaction. As hydrogen bubbles escape, the apparatus will losemass.
2.Analternativewaytomonitorthereactionisbycollectingandmeasuringthe volumeofthegasasitisproduced.Thegascanbecollectedinagassyringeor overwater(aslongasitisinsolubleinwater).
Twodifferentwaystomeasurethevolumeofagasthatisproduced
e)Calculating the rate of areaction
Usingtheresultsofexperimentslikethese,theaveragerateofthereactioncanbe calculated.Nomatterwhichquantityismeasuredduringthecourseofareaction,the averagerateofreactioncanbecalculatedusingtheequationbelow.
Thechangeinmeasurablequantitycanbereadfromatableofresultsorfroma graphproducedfromresults.
Astherateischangingthroughoutthereaction,wearecalculatingtheaveragerate overagiventimeperiod.
Forexample,thegraphbelowcouldbeusedtocalculatetheaveragerateoverany period oftime
Usingthisgraph,wecancalculatetheaverageratebetween30secondsand120 seconds.
The unit that rate is measured in depends on the measurable quantity. Since a change in mass is measuredingramsandachangeintimeinsecondsinthisexample,theunitofratewouldbegrams persecond(gs-1).Similarly,ifachangeinconcentrationismeasured(inmoll-1),thenratewillhavethe unit moles per litre per second (mol l-1 s-1) or a change in volume measured in cubic centimetres, centimetrescubedpersecond(cm3s-1).
f)Rategraphs
Inchemistry,graphscanbeusedtofollowthecourseofareaction.Agraphcantell usmanythingsaboutareaction.
Thegraphbelowshowstwosimilarreactions.
Themagentalinehasasteepergradientandrepresentsconditionsfavouringafaster reactionthanthegreenline.Whenthereactionisfinished(theend-point)thegraph goesflatasnomoreproductsarebeingproduced.
RatesofReactionMinitest
1Whichofthefollowingfactorswillnotspeedupachemicalreaction?
- Increasingthetemperatureofthereaction
- Increasingtheparticlesizeofreactants
- Increasingtheconcentrationofreactants
2Apupilplaces10gofpotatointo1mol/lhydrogenperoxidesolution.
Thepotatohasabiologicalcatalystinsideitthatbreaksthehydrogenperoxide downtowaterandoxygengas.
What term is used to describe a biological catalyst such as catalase?
- Fermentation
- Enzyme
- Protein
3Apupilplaces10gofpotatointo1mol/lhydrogenperoxidesolution.
Thepotatohasabiologicalcatalystinsideitthatbreaksthehydrogenperoxide downtowaterandoxygengas.
The pupil counts 15 bubbles of oxygen given off every minute.
Whatcouldshedotothe10gofpotatothatwouldincreasethenumberof bubblesperminute?
- Cookitbeforeuse
- Cutitintosmallerpieces
- Use5ginstead
4Apupilplaces10gofpotatointo1mol/lhyrogenperoxidesolution.
Thepotatohasabiologicalcatalystinsideitthatbreaksthehydrogenperoxide downtowaterandoxygengas.
Whatwouldhappentothenumberofbubblesperminuteifsheusedaless concentratedhydrogenperoxidesolutioninsteadoftheoriginal1mol/l?
- Staythesame
- Increase
- Decrease
5Apupilplaces10gofpotatointo1mol/lhyrogenperoxidesolution.
Thepotatohasabiologicalcatalystinsideitthatbreaksthehydrogenperoxide downtowaterandoxygengas.
Whatwouldhappentothenumberofbubblesifthetemperaturewasincreased to30°C?
- Increase
- Decrease
- StaytheSame
6Whichofthefollowingreactionswouldbethefastest?
- 1mol/lhydrochloricacidreactingwithzincpowder
- 1mol/lhydrochloricacidreactingwithalumpofzinc
- 0.5mol/lhydrochloricacidreactingwithzincpowder
7Inwhichofthefollowingreactionswouldhydrogenbeproducedthefastest?
- Calciumpowderand2mol/lhydrochloricacid
- Calciumlumpsand2mol/lhydrochloricacid
- Calciumpowderand4mol/lhydrochloricacid
8What does a catalystdo?
- Speedsupareactionandisnotusedupinthereaction
- Slowsdownareactionandisnotusedupinthereaction
- Speedsupareactionandisusedupinthereaction
9Apupilmonitorstherateofareactionbymeasuringthevolumeofgasthatis beingproducedevery15seconds.
What will her units be when calculating the rate of reaction?
- gs-1
- cm3s-1
- moll-1s-1
10
Whichlineonthisrategraphshowstheslowestreaction?
- A
- B
- C
2–ElementsandAtomicStructure
a)ThePeriodicTable
Theperiodictablebringsordertoinformationaboutthechemicalelements.Ithelps chemiststounderstandwhyelementsreactastheydo.
Therearemorethan100elementswhicharearrangedintheperiodictableandeach is represented by a symbol. A section of the periodic table is shown below which illustrateshowtheelementsarearranged.
TheSQADataBookletgivesmoreinformation.
The chemical elements are arranged in order of increasing atomic number.
Thehorizontalrowsarecalledperiodsandtheverticalcolumnsarecalledgroups. Elementswithsimilarchemicalpropertiesareinthesamegroupofelements.
The properties of the main groups of elements
- Theelementsingroup1areknownasalkalimetals.Theyreactrapidly(veryfast) withwater,producinganalkalinesolutionandhydrogengas.Themetalsbecome morereactiveasyougodownthegroup.
- Theelementsingroup7areknownashalogens.Fluorineandchlorinearegases. Bromineisoneofonlytwoliquid elements.Iodine issolid.Theyexist asdiatomicmolecules-theyhavetwoatomsineachmolecule.Asyougodownthegroup thehalogensbecomelessreactive.
- Theelementsingroup0areknownasnoblegases.Theyareveryunreactiveand exist as individual atoms(monatomic).
- Thetransitionmetalsareelementswhicharefoundbetweengroups2and3.Well known examples are iron, copper and gold. They are generally quite dense (heavy)andmanyformbrightlycolouredcompounds.
b)Structureoftheatom
Everythingintheworldismadeupofatoms.Thestructureoftheatomiswhatgivesan elementitschemicalandphysicalproperties.
Atomsaremadeupofthreesmallerparticlescalledelectrons,protonsandneutrons.
Electrons
Negativelychargedparticlesthatspinaroundthepositivecentreoftheatomin circlescalledenergylevels.Theirmassissosmallitisnearlyzero.
Protons
Positivelychargedparticlesthatarecontainedinthenucleusoftheatom(thecentre) theyhaveamassof1amu(atomicmassunit).
Neutrons
Particleswithnochargearealsocontainedinthenucleusoftheatom.Theytoohave amassof1amu.
The nucleus has an overall positive charge as it contains the protons.
Particle / Mass / Charge / LocationElectron / Approx 0 / -1 / Energy level
Proton / 1 amu / +1 / Nucleus
Neutron / 1 amu / 0 / Nucleus
Every atom has no overall charge (neutral). This is because they contain equal numbersofpositiveprotonsandnegativeelectrons.Theseoppositechargescancel eachotheroutmakingtheatomneutral.
Atomic number
Each element has its own atomic number.
Elementsarearrangedintheperiodictableinorderofincreasingatomicnumber.For examplehydrogenhastheatomicnumberofone,heliumtwo,lithiumthreeetc.
Mass number
Themassnumberisgivenatthetopleftoftheelement'ssymbol,forexample,sodium hasamassnumberof23.
Weknowthattheatomicnumberofsodiumis11.Thistellsusthatsodiumhas11 protonsandbecauseitisneutralithas11electrons.
ThemassnumberofanelementtellsusthenumberofprotonsANDneutronsinan atom(i.e.thetwoparticlesthathaveameasureablemass).
Sodiumhasamassnumberof23amu.Sincesodiumhas11protons,thenumberof neutronsisgivenbymassnumber–numberofprotons(23-11)=12neutrons.
Element / Mass number / Protons / NeutronsMagnesium / 24 / 12 / 12
Potassium / 39 / 19 / 20
Carbon / 12 / 6 / 6
Isotopes
Isotopesareatomswiththesameatomicnumberbutdifferentmassnumber.
Thesetwocarbonatomsareisotopes.Eachhasthesamenumberofprotonsbuta different number ofneutrons.
Relativeatomicmass
Elementsaremadeupofisotopessothemassgiveninthedatabookletiscalledthe relative atomic mass(RAM).
Therelativeatomicmassofeachelementshowninthedatabookiscalculatedfrom the masses of all the isotopes of an element taking into account the percentage proportion ofeach.
Example: Therearetwoisotopesofchlorine:
SincetheRAMofchlorineis35.5,chlorine-35mustbemoreabundantbecauseits massnumberisclosertotheRAM.
ElectronArrangement
The electron arrangement of all atoms can be found in the data booklet. All the electronsarearrangedintoenergylevels.Theseenergylevelscanonlyholdacertain numberofelectrons.
Thefirstenergylevel(theonenearestthenucleus)canholdamaximumof2electrons withtheothersbeingabletoholduptoamaximumof8electrons(onlytrueforthe first 20elements).
Example: sodium has the electron arrangement 2,8,1.
Anatomofsodiumhas11electrons.Thefirsttwofilltheinnermostenergylevel.The second energy level is also full, holding eight electrons and one electron remainsin theouterenergylevel.
Elements in the same group of the periodic table have the same number of outer electrons.
Itisthenumberofouterelectronsthatgiveanelementitschemicalproperties.Thisis whyelementsinthesamegroupoftheperiodictablehavesimilarproperties.
Atomic Structure Minitest
1Whatisthecentreofanatom called?
- Theprotons
- Thenucleus
- Theelectrons
2Whatisthechargeonan electron?
- Onepositive
- Nocharge
- Onenegative
3If atoms contain charged particles,whydotheynothavea charge?
- Theycontainthesamenumber ofprotonsaselectrons
- Thechargeislockedawayin thenucleus
- Theycontainequalnumbersof protons andneutrons
4Whatistheatomicnumberofan atom equalto?
- Thenumberofprotonsinthe nucleus
- Thenumberofneutronsinthe nucleus
- Thenumbersofprotonsand neutronsinthenucleus
5Whatareisotopes?
- Compoundswiththesame molecular formulae but different structuralformulae
- Atoms with the same mass numberbutdifferentatomic number
- Atomswiththesamenumberof protonsbutadifferentnumber ofneutrons
6Wherearetheelectronsinsidean atom?
- Theelectronsareinthenucleus of anatom
- Theyarearrangedinenergy levels
- The electrons are spread equallythroughouttheatom
7Whichofthethreesub-atomic particlesisthelightest?
- Theproton
- Theneutron
- Theelectron
8Whichelementshavesimilar chemicalproperties?
- Elementsinthesameperiodof the PeriodicTable
- Elementswiththesamenumber of electrons in their outer energylevel
- Elementswithsimilarmass numbers
9Howmanyneutronsdoesthisatom of carbonhave?
- 6
- 8
- 14
10Hydrogenhasthreeisotopesanda relative atomic mass of 1.0079. Which isotope is the most abundant?
- Hydrogenwithatomicmassof1
- Hydrogenwithatomicmassof2
- Hydrogenwithatomicmassof3
3–BondingRelatedtoPropertiesofMaterials
Atoms can be held together by chemical bonds. When atoms form bonds, they can achieve a stable electron arrangement. To achieve a stable electron arrangement atomscanlose,gainorshareelectrons.Therearedifferenttypesofbondsthathold atomstogether.
a)Covalentbonding
A covalent bond is a shared pair of electrons between atoms of two non-metal elements.
A covalent bond happens when the positive nuclei from two different atoms are held together by their common attraction for the shared pair of electrons held between them.
Covalent bonds are strong bonds.
Atomsthatsharepairsofelectronsformmolecules.Amoleculeisagroupofatoms heldtogetherbycovalentbonds.
A diatomic molecule is a molecule containing only two atoms. There are seven diatomicelementsthatyouhavetorememberandasimplemnemonictohelpwith this.Ifyouremember"IBringClayForOurNewHouse”thenyouwillhaveremembered thatthesevendiatomic elementsareIodine,Bromine,Chlorine,Fluorine, Oxygen,NitrogenandHydrogen.
Diagrams can be used to show how the outer electrons are shared to form the covalent bonds in amolecule.
Hydrogen (H2)
Both hydrogen atoms have only one electron, but by forming a singlecovalentbond,bothcanhaveafulloutershell.Theshape ofthemoleculeformediscalledlinear.
Asharedpairofelectronsbetweentwohydrogenatoms ThiscanalsobeshownasH-H.
Methane (CH4)
Carbon atoms have four outer electrons so need four moreforafulloutershell.Thecarbonformsfoursingle bonds to the hydrogen atoms, so all the atoms now haveafulloutershellofelectrons.Theshapeformedis calledtetrahedral.
A methane molecule has four shared pairs of electrons
Ammonia (NH3)
Nitrogenatomshavefiveouterelectronssoneedsthreemoreforafull outer shell. Nitrogen forms three single covalent bonds to hydrogen atoms.Theshapeformediscalled trigonal pyramidal.
Water (H2O)
Oxygen atoms have six outer electrons so need two more for a full outer
shell. The oxygen forms two single covalent bonds with the two
hydrogen atoms. The shape formed is called bent.
More than one bond can be formed between atoms leading to double and triple bonds. Examples of these are diatomic oxygen (double bond) or nitrogen (triple bond).
Oxygenmoleculeshaveadouble bond:twosharedpairsofelectrons
Thiscouldalsobewrittenas:
Substances that consist of covalent molecules are usually gases or liquids at room temperature.Covalentsubstancesthataresolidswithhighmeltingpointshavemuch largermolecules.
A covalent network structure consists of a giant 3-dimensional lattice of covalently bonded atoms.
Boron, carbon and silicon are all examples of covalent network elements. Diamond and graphite, two forms of carbon and compounds like silicon dioxide and silicon carbide are all covalent networks.
Diamond has a tetrahedral structure. Graphite has a layered, planar structure.
b)Ionicbonding
Ionic bonds are formed between a metal and non-metal, for example sodium chloride.
- Anatomofsodiumwillloseanelectronandformapositiveion.
- Anatomofchlorinewillgainanelectronandformanegativeion.
The ionic bond is the electrostatic force of attraction between a positively charged metal ion and a negatively charged non-metal ion.
Metalsformpositiveionsbecausetheyloseelectronstobecomestable. Example:
Magnesium (Mg)has theelectronarrangement2,8,2.
Tobecomestableitmustloseitstwoouterelectronstoobtainafullouterenergylevel.
Atoms are neutral because they have equal numbers of protons and electrons however,whentheylosetwoelectronstheyarenolongerneutral.Theychangeinto ionswithatwopositivecharge.
Non-metalsformnegativeionsbecausetheygainelectronstobecomestable. Example:
Chlorine (Cl) has an electron arrangement 2,7.
To become stable it must gain an electron to obtain a full outer energy level.
Whenthesetwochargedparticlescometogether they form an ionic bond because the positive magnesium ion is attracted to the negatively charged chlorideion.
Ionic compoundsformwhat isknownasa latticestructure.Thisisaregularrepeatingarrangement of metal and non-metal ions which creates compounds with very high melting points which conduct when molten or in solution but never whensolid.
Ionic compoundsdissolveinwatereasily,when they do this their lattice breaks up completely. Thereforetheycanconductastheirionsarefree tomove.
Electrolysis
Electrolysisisthebreaking upofanionicsolutionusing electricity.
The copper chloride solution is broken up because electricity is passed through thesolution. Adirectcurrent(DC)power supply must be used if the products of electrolysis are to beidentified.
Thepositivecopperionsareattractedtothenegativeelectrode.Whenthecopper ionsgettotheelectrodetheypickuptwoelectronstoformcoppermetal.
Thenegativelychargedchlorideionsareattractedtothepositiveelectrode.When the chloride ions get to the electrode they lose their extra electrons to become chlorinegas.
c)Summaryofbonding
Ioniclattice / Covalent network / Discrete covalent moleculesBoilingandmelting points / High / Very high / Low
Stateatroom temperature / Solid / Solid / Liquid or gas
Conduction of electricity / Onlywhenmoltenor insolution / Never (except graphite) / Never
Ioniclattice
Allioniccompoundshaveahighmeltingpointandboilingpoint.Theyconductwhen molten or in solution as the ions are free to move. They can be broken down by electrolysis.
Covalent network
Allcovalentnetworkstructureshaveveryhighmeltingpointsandboilingpoints.They areallhardanddonotconductelectricity.
Covalent molecular
Theyhavelowmeltingpointsandboilingpoints.Theydonotconductelectricity.Some covalentmolecularcompoundshavehighermeltingpointsthanexpected.
BondingRelatedtoPropertiesofMaterialsMinitest
1Whichofthefollowingpairsare both diatomicelements?
- Hydrogenandhelium
- Iodineandnitrogen
- Boronandfluorine
2Howcanthebondinginsilicon dioxide(SiO2)bedescribed?
- Ionic
- Discrete covalentmolecular
- Covalentnetwork
3Why do ionic compoundsnot conduct when they aresolid?
- Theiratomsarenotfreeto move
- Theirelectronsarenotfreeto move
- Theirionsarenotfreetomove
4Inthecompoundtin(IV)chloride,a small group of atoms are held together by shared pairs of electrons.Whattypeofbondingis presentintin(IV)chloride?
- Ionic
- Discrete covalentmolecular
- Covalentnetwork
5Which of these compounds are made up of diatomicmolecules?
- Nitrogenmonoxide(NO)
- Magnesium oxide(MgO)
- Carbon dioxide(CO2)
6Whichofthesedoesnothavea covalent networkstructure?
- Chlorine o Diamond oGraphite
7How can the shape ofmethane (CH4) molecules bedescribed?
- Bent
- Tetrahedral
- Trigonal pyramidal
8
Substance / Melting Point(°C) / Boiling Point (°C) / Electrical conductivityas asolid / Electrical conductivity asa liquid / Electrical conductivity in aqueous solutionA / 1538 / 2862 / Good / Good / Insoluble
B / 1414 / 3625 / Poor / Poor / Poor
C / 650 / 1091 / Good / Good / Insoluble
D / 801 / 1413 / Poor / Good / Good
E / -182 / -162 / Poor / Poor / Poor
F / 851 / 1633 / Poor / Good / Good
Thetableshowsthepropertiesofsomedifferentsubstances. Whichsubstances are covalent?
- B+E
- A+ C
- C+E
9Whichofthesediagramsbestrepresentstheattractionsthatformthecovalentbondholdingtheatomstogetherinthemolecule?
o o o
4– Chemicalformulae
a)Formulafromnamesofcompounds
Thechemicalformulaofacovalentmolecularsubstancegivesthenumberofatoms permolecule.Theformulaofacovalentnetworkorioniccompoundgivesthesimplest ratioofatoms/ionsinthesubstance.
Sometimesthenameofthecompoundgivesinformationabouttheformulaofthat compound.Namesofthesecompoundshaveprefixesthatgivethenumberofatoms ofcertainelementsineachmolecule.
Prefix / Number of atomsMono- / One
Di- / Two
Tri- / Three
Tetra- / Four
Penta- / Five
Hexa- / Six
Forexample,carbonmonoxidecontainsonecarbonatomjoinedtooneoxygen atom,soithastheformulaCO.
b)Valency
Valencyisthecombiningpowerofanelement.Elementsinthesamegroupofthe periodictablehavethesamevalency.Thevalencyofanelementisrelatedtohow manyelectronsareintheoutershell.
Group number / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8Valency / 1 / 2 / 3 / 4 / 3 / 2 / 1 / 0
Thenoblegaseshavethevalency0astheydonotusuallycombinewithother elements.
Towrite thechemicalformulaforacompounditisbesttouse the S.V.S.D.Fsystem.
- Stepone-writedownthesymbolsofboththeelementsinvolved.
- Steptwo-beneatheachsymbol,writeit’svalency.Memorisingtheabovetable isuseful.
- Stepthree-swapthevalenciesover.
- Stepfour-ifthevalenciescanbesimplified,dividethembothbythesmallerof thetwonumbers.Ifoneofthenumbersisalreadyone,thentheycannotbe dividedandsimplifiedanyfurther.
- Stepfive-writetheformula.
Example one
Whatistheformulaforpotassiumoxide?
Example two :
Whatistheformulafor carbon sulfide?
c)Formulaeofcompoundscontaininggroupions
Group ions contain two or more atoms and usually have a negative charge. The formulaeoftheseionscanbefoundinthedatabooklet.Wecantakethenumberof chargesontheionasthevalencyoftheion.
Example:
What is the formula for calcium nitrate?
d)Formula using Romannumerals
Some elements, particularly the transition metals, do not always have the same valencyintheirdifferentcompounds.Thevalencyoftheseelementsisusuallygivenin roman numerals insidebrackets.
Roman Numeral / NumberI / One
II / Two
III / Three
IV / Four
V / Five
VI / Six
Example
What is the formula for copper (I) oxide?
5– Balancedequations
Achemicalequationissaidtobebalancedwhentherearethesamenumberofthe sametypeofeveryatomonbothsidesoftheequation.
Copper and oxygen reaction - getting a balanced equation
Balancedsymbolequationsshowwhathappenstothedifferentatomsinreactions. Takea look at thiswordequationforthereaction: Copperandoxygenarethereactantsbecausetheyareontheleftofthearrow.
Copper oxide is the product because it is on the right of the arrow.
Ifwejustreplacethewordsshownabovebythecorrectchemicalformulae,wewill getanunbalancedequation,asshownhere:
Noticethatthereareunequalnumbersofeachtypeofatomontheleft-handside comparedwiththeright-handside.Tomakethingsequal,youneedtoadjustthe numberofunitsofsomeofthesubstancesuntilyougetequalnumbersofeachtype ofatomonbothsides.
Hereisthebalancedsymbolequation:
Youcanseethatnowtherearetwocopperatoms andtwooxygenatomsoneachside.Thismatches whathappensinthereaction.
2copperatomsreactwithanoxygenmoleculeto form2unitsofcopperoxide(CuO)
Balancing equations
To balance equations on your own, follow these simple rules:
- Checkthatalltheformulaeintheequationarecorrect.
- Dealwithonlyoneelementatatime.
- Balancingis addingBIG numbers.Youcannotchangeany ofthesmall numbersin a chemical formula. If balancing is required, put the number in front of the substance.
- Check each element again and repeat step 3 again ifneeded.
Example
Thisequationisunbalanced.Therearefourcarbonatomsonthelefthandsideand only one on the right. To balance the carbon, add a big '4' in front of the carbon dioxide.
Next,tobalancethehydrogen.Wehave8atomsofhydrogenonthelefthandsidein C4H8andonly2ontherighthandside.Tobalancethehydrogenatoms,addabig '4'infrontofH2O.
We’renotfinishedyet.Nowthatthecarbonandhydrogenhavebeenbalanced,we onlyhavetobalancetheoxygen.Wehave2atomsofoxygenontheleft,butintotal on the right (taking into account what we have balanced already) we have 12 oxygen atoms. This can be balanced by adding a big '6' in front of the diatomic oxygenmoleculeonthelefthandside.
Thebalancedequationwillbe:
Chemical formulae and balanced equationsMinitest
1 / Whatisthechemicalformulafor sodiumbromide? / 5 / Whatisthechemicalformulafor ammoniumsulfate?o / o
o / o
o / o
2 / What is the chemical formula for / 6 / Whatisthechemicalformulafor
calcium phosphide? / iron(II) chloride?
o / o
o / o
o / o
3 / What is the chemical formula for / 7 / Whatisthechemicalformulafor
titanium tetrachloride? / copper(II)hydroxide?
o / o
o / o
o / o
4 / Whatisthechemicalformulafor dinitrogentrioxide? / 8 / Whatisthechemicalformulafor calciumhydroxide?
o / o
o / o
oo
9
Whichofthefollowingvaluesarethecorrectvaluesfora,bandcrequiredto balance theequation
- a=2,b=1andc=2
- a=3,b=1andc=3
- a=2,b=2andc=4
10
Whichofthefollowingvaluesarethecorrectvaluesfora,bandcrequiredto balance theequation
oa=1,b=3,c=2andd=4 oa=1,b=3,c=2andd=2 oa=1,b=2,c=2andd=2
a)GramFormulaMass(GFM)
6 – The Mole
The gram formula mass (GFM) of a substance is known as the mass of one mole.
RelativeatomicmassesofselectedelementscanbefoundintheSQADataBooklet. Thesecanbeusedtocalculatethegramformulamassofasubstance.
Forexample
What is the mass of one mole of calcium chloride?
Tocalculatethis,workouttheformulaofcalciumchloridethenaddallthemass numberstogether.
One mole of calcium chloride has a mass of 111 g.
b)Molecalculationsbasedonmass
Tocalculatethemassofahighernumberofmoles,orevencalculatethenumberof molesinacertainmass,aformulatrianglecanbeused.
Usingtheformulatriangleisstraightforward.Simplycoverwhateverpartofthetriangle you are trying towork out.
Example
How many moles are present in 25 g of calcium carbonate?
Step One: Calculate GFM of calcium carbonate
Workouttheformulaweneedtouse,givenbycoveringupnumberofmoles. StepTwo:Putthenumbersintotheformula:
c)Concentration ofsolutions
Solutionsareformedwhensolutesdissolveinsolvents.Ifthenumberofmolesofsolute and the volume of solvent used is known, the concentration of the solution can be calculated.
Theconcentrationofasolutionismeasuredinmolesperlitre(moll-1)andcanbe calculated using this formulatriangle:
Using the formula triangle gives three useful formulae:
Example one
Calculatetheconcentrationofthesolutionformedwhen0.25molesofpotassium nitratearedissolvedin500cm3ofwater.
The formula we need to use, given by covering up concentration is:
Rememberthatthevolumealwayshastobechangedintolitres. Exampletwo
Calculatethenumberofmolesin200cm3of0.5moll-1sodiumhydroxide(NaOH) solution.
Example three
Calculatethevolumeofsodiumhydroxideusedin1moll-1solutioncontaining2molof solute.
ThemoleandconcentrationofsolutionsMinitest
1Whatisthegramformulamassof oxygen?
- 16g
- 32g
- 8g
2Whatisthegramformulamassof lithiumchloride?
o42.5g
- 7g
o35.5g
3Whatisthemassof1moleof carbondioxide?
- 12g
- 28g
- 44g
4Whatisthemassof1moleof sodiumsulphate?
- 119g o 71g o142g
5Howmanymolesaretherein24gof carbon?
- 2moles
- 1mole
- 0.5moles
6Howmanymolesaretherein50gof calcium carbonate(CaCO3)?
- 2moles
- 1mole
- 0.5moles
7Whatisthemassof0.5molesof magnesiumchloride?
o47.75g
o95.5g
- 59g
8What is the number of moles of potassiumhydroxideusedtomake 100cm3of0.1moll-1solution?
- 10moles
- 0.1mole
- 0.01moles
9What is the concentration of hydrochloricacidsolutionusedif2 moles are dissolved in 100cm3 of water?
- 20moll-1
- 2moll-1
- 0.02moll-1
10Whatvolumeofsodiumhydroxideis needed to produce a 0.1 mol l-1 solution containing 0.5 moles of solute?
- 5litres
- 5cm3
- 50litres
7 – Acids & bases
ThepHscalesmeasurestheacidityoralkalinityofasolution.Acidsandbaseshavea widevarietyofusesandcanreacttogetherinneutralisationreactions.
a)ThepHscale
How acidic or alkaline a substance is (the pH of the substance) can be measured usingthepHscale,acontinuousrangethatstretchesfrombelow0toabove14.Most commonpHvaluesoccurbetween0and14.
AcidshaveapHoflessthan7. AlkalishaveapHmorethan7.
Water and neutral solutions have a pH of exactly 7.
The pH is a measure of the concentration of hydrogen ions in a solution.
b)Formingacidsandalkalis
- MetalOxides
Alkalisaresolublebases.Analkalinesolutioncanbeformedwhenametaloxideis dissolved in water. An acidic solution can be formed when a non-metal oxide is dissolved inwater.
For example, magnesium oxide dissolves to form alkaline solutions.
Solublemetaloxidesproducealkaliswhendissolvedinwater. Exampleone
What will happen to the pH of water if barium oxide is added?
Usingthedatabookletwefindthatbariumoxideisametaloxide.Checkingits solubilityonpage8ofthedatabookletweseethatitwilldissolve.Itisthereforea solublemetaloxideanditwillincreasethepH.
Exampletwo
What will happen to the pH of water if aluminium oxide is added?
Usingthedatabookletwefinditisametaloxide.Checkingitssolubilityonpage8 wefinditisinsoluble.ItwillthereforehavenoeffectonthepHofwater.
- Non-metalOxides
Sulfurdioxideandnitrogendioxidehowever,willdissolvetoformacidicsolutions.
Soluble non-metal oxides produce acids when dissolved in water.
c)Hydrogenandhydroxideions
Acidicandalkalinesolutionscanconductelectricitybecausetheyhaveionsthatare freetocarrycharge.Lookattheformulaeoftheseacids.TheyallcontainH+ions.
Acid name / Formula / IonicFormulaHydrochloric acid / HCl / H+(aq) Cl-(aq)
Sulfuric acid / H2SO4 / 2H+(aq) SO42-(aq)
Nitric acid / HNO3 / H+(aq) NO3-(aq)
When an acid is diluted the concentration of H+ ions is decreased and the pH increasestowards7.Lookattheformulaeofthesealkalis.TheyallcontainOH-ions.
Alkali name / Formula / IonicFormulaSodium hydroxide / NaOH / Na+(aq) OH-(aq)
Calcium hydroxide / Ca(OH)2 / Ca2+(aq) 2OH-(aq)
Lithium hydroxide / LiOH / Li+(aq) OH-(aq)
The more concentrated an acid or alkali the better it conducts. When an alkali is dilutedtheconcentrationofOH-ionsisdecreasedandthepHdecreasestowards7.
d)Water and neutralsolutions
Water molecules can break down into hydrogen ions and hydroxide ions.
Thisisareversiblereaction.Asmallproportionofwatermoleculesbreakuptoform hydrogenionsandhydroxideions.Someofthesehydrogenandhydroxideionsthen reacttogetheragaintoformwatermolecules.
This is called an equilibrium and is presentin water and all aqueous solutions.
Inwaterandneutralsolutions,theconcentrationofhydrogenionsisequaltothe concentrationofhydroxideions.
Allacidicsolutionscontainmorehydrogenionsthanhydroxideions. Allalkalinesolutionscontainmorehydroxideionsthanhydrogenions.
e)Dilutingacidsandbases
AddingwatertoanacidorbasewillchangeitspH.Waterismostlywatermoleculesso addingwatertoanacidorbasereducestheconcentrationofionsinthesolution.
WhenanacidicsolutionisdilutedwithwatertheconcentrationofH+ionsdecreases andthepHofthesolutionincreasestowards7.
TomakethepHchangeby1,atenfolddilutionisrequired(egadding9cm3ofwater to1cm3acid).Theacidisbecominglessacidic.
Similarly,whenanalkaliisdilutedwithwatertheconcentrationofOH-ionsdecreases. ThiscausesthepHofthealkalitofalltowards7,makingthesolutionlessalkalineas morewaterisadded.
f)Neutralisation
NeutralisationisthereactionofanacidwithabasethatresultsinthepHmoving towards7.
Itisausefulprocessthatoccursineverydaylifesuchasinthetreatmentofacid indigestionandthetreatingofacidicsoilbyaddinglime.
Neutralisation also moves the pH of an alkali down towards seven.
Severaldifferentbasescanneutraliseacids,andwaterisalwaysproducedasaresult ofthesereactions.
- Equations forneutralisation
Metaloxidesandalkalisaretwotypesofbase.Basicsubstancesneutraliseacids, resultinginthepHoftheacidincreasingtowards7,andwaterbeingproduced.A solublebasedissolvesinwatertoformanalkalinesolution.
Acidscanalsobeneutralisedbymetalcarbonates.Intheneutralisationreaction betweenanacidandametalcarbonate,therearethreeproducts.Thehydrogen ions(H+)fromtheacidreactwiththecarbonateions(CO32-)toformwaterand carbondioxidegas.Asaltisalsoproduced.
Carbondioxidecanbetestedforusinglimewater(turnsfromcolourlesstochalky white).
- Namingsalts
Tonamethesalt,themetalionfromthealkali(orbase)replacesthehydrogenion fromtheacid-(alkalitofront,acidtoback).
For example:
Acid name / Saltname endingHydrochloric acid / ...chloride
Sulfuric acid / ...sulfate
Nitric acid / ...nitrate
DuringneutralisationtheH+ ionfromtheacidjoinswiththeOH- ionfromthealkali.This iswhywaterisformedinthesereactions.
This is true for all neutralisation reactions.
g)Ionicequationsandspectatorions
Ionicequationsaredifferenttosymbolequations.
Forexample,anequationshowinghydrochloricacidbeingneutralisedbysodium hydroxide:
Theionicequationfortheaboveprocessshowsthechargesinvolved.Statesymbols areaddedandionsinsolutionareseparatedwitha sign.
Spectatorionsareionsthatarepresentduringthereactionbutareunchangedbythe reaction, and so are present in the same state and charge on both sides of the equation.
This ionic equation can be shortened further by removing the spectator ions.
Re-writingtheequationwithoutthespectatorionsgives: This changeoccursin every neutralisationreaction.
h)Titrations
Atitrationexperimentcanbecarriedouttoaccuratelymeasurethevolumeof substancesthatreactinchemicalreactions.
Thistechniqueiscommonlyusedinneutralisationreactionsandcanalsobeusedto calculateanunknownconcentration(ofeithertheacidoralkali).
Whentitrating,itisessentialtomeasurethingsasaccuratelyaspossible. Theexperimentiscarriedoutbyfollowingthesesteps:
1.Ifthesampleisasolid,itisweighedusinganaccuratebalance,andthen dissolvedtomakeupaknownvolumeofsolution(usually100cm3).
2.Apipetteisusedtomeasureaccuratelyavolumeofthissolution-forexample, 10cm3.Asafetypipettefillerisusedtodrawsolutionintothepipette.Thisis emptiedintoaconicalflask.
3.Afewdropsofanindicatormaybeaddedtotheconicalflask.Thiswillshowa changeofcolourwhenthetitrationiscomplete.
4.Asecondchemicalisplacedinaburette.Thisothersolutionisofachemical thatwillreactwiththesampleintheconicalflask.Oftenthesolutioninthe buretteisanacidofaprecise,knownconcentration.
5.Thesolutionfromtheburetteisrunintotheconicalflask.Thesolutionisadded one drop at a time, with swirling to mix the solutions as the end-point is approached.Eventually,acolourchangeshowsthatthecorrectamounthas beenaddedtoreactcompletelywiththesampleintheconicalflask.
6.
Thevolumeofsolutionaddedfromtheburetteisnoted.Thetitrationresultscan thenbeusedtocalculatetheconcentrationofthesampleintheconicalflask.
Titrationapparatus:aburette,conicalflask,safetypipettefillerandpipette.
AcidsandBasesMinitest
1Omitting the spectator ions, which ofthefollowingistheionicequation for the reaction of an acid and an alkali?
o
o o
2What type of solution would be formedwhensulfurdioxidedissolves inwater?
- Alkalinesolution
- Acidicsolution
- Neutralsolution
3What would be the pH of the solutionformedwhensodiumoxide dissolves inwater?
- Above7
- Equalto7
- Lessthan7
4Potassiumchloridedissolvestoform aneutralsolution.Whatdoesthistell youabouttheionsinvolved?
- Therearemorehydrogenions than hydroxideions
- Therearemorehydroxideions thanhydrogenions
- There are the same concentrationsofhydrogenions as hydroxideions
5Which salt is made when calcium carbonatereactswithhydrochloric acid?
- Sodiumchloride
- Calciumchloride
- Carbonchloride
6Which of the following are the correctproductsfromthereaction between magnesium hydroxide and sulfuricacid?
- Magnesiumsulfate+water
- Magnesiumsulfate+water+ carbondioxide
- Magnesiumsulfate+hydrogen
7Ifequalvolumesofequimolar(same concentration) solutions of the chemicals below were measured, which would contain the most hydrogenions?
- Sodiumsulfate
- Lithiumcarbonate
- Nitricacid
8Whichofthefollowinghappensto the pH of an acid when it is neutralised?
- The pH does notchange
- ThepHincreasestowards7
- ThepHdecreasestowards7
9Which of the following has the correctpHvaluesforacids,alkalis and neutralsolutions?
- Acid, Alkali and Neutral
- Acid, Alkali and Neutral
- Acid, Alkali and Neutral