Lesmahagow High School Acids and Bases
Lesmahagow High School
S4 Chemistry
Acids and Bases
The effect of soluble oxides on the pH of water and their environmental impact - Acid Rain.
National 4
The uses of acids on food and drink and their impact on health.
National 4
Dissociation of water into H+ and OH- ions and the excess of these ions producing acids or alkalis respectively.
National 5
Neutralisation of acids with alkalis produces salts which can be named from the reactants used.
National 4
Titration can be used to analyse acids or alkalis of unknown concentration by reacting them with known concentrations of a neutraliser.
National 5
Learning Outcomes – Acids and Bases
Circle a face to show how much understanding you have of each statement: J if you fully understand enough to do what the outcome says, K if you have some understanding of the statement, and L if you do not yet understand enough to do what the statement says. Once you have completed this, you will be able to tell which parts of the topic that you need to revise, by either looking at your notes again or by asking for an explanation from your teacher or classmates.
National 4 Learning Outcomes
By the end of this topic I will be able to:
1. Describe practical activities to compare the
properties of acids and bases.
2. Demonstrate ways of measuring and adjusting pH.
3. Describe the significance of pH in everyday life
e.g in soils, insect stings, toothpaste, indigestion
remedies, foods etc.
4. Participate in practical activities to evaluate
the effect of dilution on pH values.
5. State that acids are produced from soluble
non-metal oxides when they are dissolved in water.
6. State that alkalis are produced from soluble metal
oxides. Insoluble metals oxides and carbonates are
bases.
7. Give examples of the uses of acids and alkalis in everyday foodstuffs.
8. State that reactions like neutralisation, where heat is
given out, are known as EXOTHERMIC reactions.
National 5 Learning Outcomes
9. State that an acid solution contains more H+ions
than pure water
10. State that an alkaline solution contains more OH-ions
than pure water
11. Explain the effect of dilution on the pH of an acid or
alkali in terms of the decreasing concentration of hydrogen and hydroxide ions.
12. State that in water and neutral solutions, the
concentration of hydrogen ions is equal to the
concentration of hydroxide ions.
13. Write the equation for water dissociating into H+
and OH- ions.
14. Use the dissociation of water to explain how acids
and alkalis form.
15. Use titration calculations to determine the
unknown concentration of an acid or alkali.
16. State that insoluble salts formed in a neutralisation
are a type of precipitate.
17. Identify the spectator ions in a neutralisation or
precipitation reaction.
18. Identify the products formed when an acid is neutralised
by different bases.
19. Name the salts formed from a given acid and alkali.
Acids and Bases Notes
National 4 Notes
Acids and Alkalis (Revision)
You will have come across acids in the lab, e.g. hydrochloric acid, sulphuric acid and nitric acid. These bottles are labelled with the warning symbol for 'irritant'. Any spills must be washed off with water … if you don’t do this, your skin will soon feel itchy. We often say that an acid ‘burns’ the skin but this is not the same as what happens when a fuel burns!
Alkalis are also found in the lab, e.g. sodium hydroxide solution. Like acids, the bottles are labelled with the warning symbol for 'irritant'. Alkalis feel soapy if spilled on your skin.
Acids are all around us in many ‘things’ that we take for granted, even in foods and our bodies … so not all acids are dangerous! The same applies to alkalis. From your S3 work, you should be able to list some laboratory acids/alkalis and some household acids/alkalis.
The pH Scale and Solutions
Universal indicator and pH paper can be used to identify solutions as being acid or alkali. They can also be used to ‘measure’ just how much acid or aqueous alkali is in a solution.
The scale that is used is called the pH scale. This is a continuous scale that runs from below 0 (acid) to above 14 (alkali) with water and neutral solutions having a pH of 7. Acidity and alkalinity increase as the pH moves away from 7, i.e. the lower the pH of a solution (below 7), the greater the acidity and the higher the pH of a solution (above 7), the greater the alkalinity.
Universal indicator can only measure the pH to the nearest whole number. A pH meter can be used to give a more accurate measurement of acidity and alkalinity.
The indicator turns a different colour at each pH number and so the pH of acids and alkalis can be found by colour-matching with a chart.
Red Orange / Yellow Green Green/blue Blue/Purple
Remember that the pH of a substance can only be measured in aqueous solution - any solids must be dissolved first. A solution is formed when a substance dissolves in water.
Concentrated and Dilute
A concentrated solution has a lot of the dissolved substance in a certain volume of water …. think of concentrated orange juice. Adding water dilutes the solution.
If we compare the same volume of a concentrated and dilute solution, the dilute solution will have less dissolved substance than the concentrated solution.
In the same way, adding water to an acid or an alkali dilutes the solution.
This affects both the acidity/alkalinity and the pH of the solutions.
Note that diluting a solution by a factor of 10 (e.g. 1ml of solution diluted with water up to 10ml) will only change its pH value by 1. In other words a pH 2 solution is TEN times more acidic than a pH 3 solution.
Calculating Concentration of Solutions
The concentration of a chemical is measured in mol/l and tells us exactly how many moles of a solute is dissolved in 1 litre of solution, but how is this calculated?
Firstly, chemists need to use a measurement called the mole to help calculate this. The mole is a chemist’s way of measuring the amount of a substance. The formula mass in grams of any substance contains the same number of particles and is known as the mole.
If we know the exact concentration of a solution it is called a ‘standard solution’.
A solution labelled 1 mol l-1 contains one mole of substance dissolved in one litre of solution.
A solution labelled 2 mol l-1 contains two moles of substance dissolved in one litre of solution.
Example 1: How many moles are there in 100 cm3 of sodium hydroxide solution, concentration 0.4 mol l-1?
0.4 mol l-1 is 0.4 mol in a litre
1000 cm3 0.4 mol
100 cm3 0.04 mol
Example 2: What is the concentration of a solution of hydrochloric acid containing
0.1 mol in 50 cm3?
50 cm3 0.1 mol
1000 cm3 0.1 x 1000
50
concentration is 2 mol l-1
Example 3: What volume of a sodium carbonate solution, concentration 2 mol l-1, contains 0.5 mol?
2 mol l-1 is 2 mol in a litre
2 mol 1000 cm3
0.5 mol 1000 x 0.5
2
There is also a short way of calculating concentration, by rearranging the equation given below:
n = CV
so C = n/V
Remember the answer you get will be in litres.
Acids from Non-Metal Oxides
An element that reacts with oxygen forms an oxide. Carbon, nitrogen and sulphur are all non-metals. Carbon forms carbon dioxide (CO2), nitrogen forms nitrogen dioxide (NO2) and sulphur forms sulphur dioxide (SO2).
What is the effect of carbon dioxide, nitrogen dioxide and sulphur dioxide on the pH of water?
It is not just the oxides of carbon, nitrogen and sulphur that lower the pH of water … all non-metal oxides that dissolve in water form acidic solutions.
Sulphur dioxide is produced in the atmosphere by the burning of sulphur, either as the element or as part of a compound. Nitrogen dioxide is released into the atmosphere in car exhaust fumes. Both gases dissolve in water to form acid rain. The cost of finding solutions to the acid rain problem has to be weighed against the cost of long-term damage to the world around us. Check your S3 notes for more info on acid rain.
What is an acid?
Hydrochloric acid, sulphuric acid and nitric acid are common acids.
The chemical formulae for the three acids show symbols for non-metal elements and this suggests that the acids are made up of molecules. However, dilute acids conduct electricity. This means that ions (charged particles) exist in the solutions.
During electrolysis, hydrogen gas is produced at the negative electrode. This means that the hydrogen in an acid must exist as positive hydrogen ions, H+ (aq). At the negative electrode, the hydrogen ions, 2H+ (aq), gain electrons to form uncharged hydrogen molecules.
2H+ (aq) + 2e- H2 (g)
It is the presence of aqueous hydrogen ions, H+ (aq), that makes a solution an acid.
The term ‘acid’ is also used for compounds that dissolve in water to produce H+ (aq) ions,
e.g. solid formic acid and hydrogen chloride gas.
HCl (g) + H2O HCl (aq) [H+ (aq) + Cl- (aq)]
hydrogen chloride hydrochloric acid
HCOOH (s) + H2O HCOOH (aq) [H+ (aq) + HCOO- (aq)]
hydrogen formate formic acid
The substance added is made up of molecules. When the substance dissolves in water, the covalent bonds beak to form ions that become attached to water molecules, i.e. H+ (aq) ions.
Alkalis from Metal Oxides
When a metal reacts with oxygen, the metal oxide is formed, e.g. sodium produces sodium oxide, calcium produces calcium oxide.
Some metal oxides are soluble in water, e.g. sodium oxide and barium oxide.
What is the effect of sodium oxide and barium oxide on the pH of water?
It is not just the oxides of sodium and barium that increase the pH of water …
all metal oxides that dissolve in water form alkaline solutions.
Some metal oxides are insoluble in water, e.g. copper(II) oxide and zinc oxide.
What is the effect of copper(II) oxide and zinc oxide on the pH of water?
It is not just copper(II) oxide and zinc oxide that have no effect on the pH of water …
all metal oxides that are insoluble in water behave in this way.
The solubility of selected metal oxides is shown on page … of the Data Booklet.
Only oxides with the letter ‘i’ are sufficiently insoluble to have no effect on the pH of water.
Place each of the following metal oxides in the correct column in the table below.
potassium oxide, nickel oxide, lithium oxide, tin(II) oxide, calcium oxide, iron(III) oxide
Dissolves to form an alkali / No effect on the pH of waterWhat is an alkali?
Each of the three solutions shown is an alkali. All three conduct electricity showing the presence of ions.
The names and the chemical formulae indicate that the solutions contain hydroxide ions, OH- (aq).
It is the presence of aqueous hydroxide ions, OH- (aq), that makes a solution an alkali.
The term ‘alkali’ is also used for compounds that dissolve in water to produce OH- (aq) ions,
e.g. sodium hydroxide solid. The solid does not behave as an alkali unless water is present.
Name the ion that gives a solution the properties of an alkali.
Acids and Alkalis in Food
In S3, you tested some common food and drink items to determine their pH. We discovered that acid pH is good for preserving food, which is why some foods are pickled in vinegar, and citric acid can be added as a natural preservative. At the same time, some alkalis, like sodium hydrogen carbonate (bicarbonate of soda) can be added to create bubbles of carbon dioxide and make a foodstuff 'rise' or become less dense.
Bases
Acids have a pH less than 7; alkalis have a pH greater than 7; water and neutral solutions have a pH equal to 7.
Alkalis can be thought of as ‘opposites’ to acids because they react with acids, moving the pH towards 7 and forming water. Any substance that reacts with acids in this way is called a base. As well as alkalis, metal oxides and metal carbonates are examples of bases.
Alkalis are a subset of the set of bases. Alkalis are the solutions formed when bases dissolve in water. Alkalis have a pH greater than 7 due to the concentration of OH- (aq) ions.
Why can sodium hydroxide solution be described as ‘BOTH a base and an alkali’and yet nickel oxide can be described as ‘a base but NOT an alkali’?
Water
The chemical name for water is hydrogen oxide, H2O. With two non-metal elements, water can be expected to be made up of molecules with the atoms joined together by covalent bonds. Covalent compounds are non-conductors of electricity.
A sensitive meter can be used to show that water does conduct electricity although very poorly. This indicates that water does not just consist of molecules … a small number of ions must also be present in water.