TO OUR TEACHERS AND CLASSMATES,
WE DEDICATE THESE NOTES
IN TOKEN OF
AFFECTION AND GRATITUDE
By all F.6B A chem students(00/01)
Method of Following a Reaction and Factors Affecting its Rate
The rate of a reaction: expressed as fall in concentration of the reactants or rise in conc.of the products in a given time.
(N.B. as we normally work with +ve reaction rates,any rate expression involving a reactant will have a -ve sign.)
Following a reaction(By means of physcial methods measuring initial rate)
1.Concentration:By withdrawal of samples and titration.(Before titration,the reactants should be quenched with ice or removing the catalyst or diluting with cold water for lowering the reaction tempt. and conc. In other words, remove the factors of the reaction)
2.Gas volume change:Using graduated glass for collecting gas and stop watch for measuring time.The final vol. recorded,V0, will be proportional to the init. conc. of the reactant.So (V0 - Vt),for Vt is the vol. of gas at time t ,will be proportional to the conc.of the reactant at t.
3.Mass change:Similar to gas volume change but this time we should measure the change in mass with a balance.(use stopwatch!)
4.Intervals of time(colour change):For a reaction that involve a sudden change of colour,by measuring the time taken to do so by a stop watch,the average rate of reaction can then be found.(the rate is proportional to 1/time taken)
5.Colorimetry:Measure the conc. of colored substances in solution. It
measures the extent of the absorption of a particular wavelength of light. Plot conc. against intensity to follow the rate.
6.Conductivity:The change in the conductivity in a reaction as the total
no.of ions or the conducting mobility of the ions are changing.(e.g.the conducting mobility of OH- and H+ ions are higher,change in their conc. will cause a change in the conductivity)
(N.B. measurement is carried out by following the a.c.for prevention of electrolysis in the conductance bridge)
7.Gas Presssure Change:Connect the reaction flask to a manometer to measure the rate as reaction that invlove gases,a change in total P may occour if the tot. no. of moles of product (P prop. to n) is changed.
Factors influencing the rates of reaction
1.Catalyst:By decreasing the Eact.
2.Surface Area(for solid of equal masses):increase the area of contact open to the reactants.
3.Tempt.
4.Light:leads to the breakdown of covalent bond/excite the outer electrons
e.g.homolytic fission
Br-Br à (presence of uv) 2Br
Graph showing the typical result for the reaction between Mg and HCl
Rate Equation, Order of Reaction.
A rate equation(showing how the rate depend on the conc. of reactant )
e.g. xA + yB à products
Rate=k[A]a[B]b
Points to note:
1. a is the order of reaction w.r.t. reactant A,while b is w.r.t. B
2. a + b is the overall order of reaction(it is an experimentally established quantity, showing the sum of powers of the conc. in the equation, so it may be an integer ,fraction, +ve or-ve. Also, as it must be determined by experiment ,it cannot be deuced from the balanced equ.
3. k is the rate constant, and it depends on the tempt, not conc.
First Order Reaction(a typical one)
e.g. Radioactive decays(614C à 714 N + -10 e) used for radio-carbon dating, 2H2O2(l) à2H2O(l) + O2(g)
The rate expression
A à products of reaction
=>rate = - d[A]t/dt = k[A]t
- ln[A]t = kt + C
Half Life , t1/2 = 0.693/k (for[A]t = ½[A]0)
Graphical representation.
Second Order Reaction
e.g. H2(g) +I2(g) à 2HI(g)
2A à products of reaction
=> rate= - d[A]t/dt = k[A]2
=> 1/[A] = 1/[A]0 + kt
Half Life t1/2 = 1/k[A]0
Graphical representation
Zero Order Reaction
Example: the decomposition of NH3 on a hot tungsten wire, or HI on a hot gold wire.
A à products
=>rate= - d[A]t/dt = k
=>[A]t = [A]0 - kt
Half Life : [A]0/2k
(N.B. the rate is independent of the initial conc.)
To find the order of a reaction, we can:
1. By graphical method: plot of experimental data against t(in the integrated rate expression),which gives a straight line.
2. Half life method: the half life is prop. to [A]0 for zero order, independent of [A]0 for first order and prop. to 1/[A]0 for second order with equal int. conc.
Collision Theory, Effects of tempt. and Cat. On Reaction Rate
Maxwell-Boltzmann distribution curve
*characteristics
---the distribution curve is not symmetrical and the curve always starts at the origin.
---the total area under the curve prop. no. of molecules
=>area is constant
---the greater the molecular mass, the narrower the spread of speed.
Reason: Lighter, faster =>curve shifts to the right.
Simple Collision Theory
---Refer to bimolecular reaction in the gases phase as well as in solution.
It states that, in order to react, molecules must
1.Collide.
2.Collide in a proper orientation.
3.Collide with enough energy to react.
Effects of conc.
---the conc. increases, the frequency of collision also increases and the probability of collision having sufficient energy for a reaction to occour also increase.
Effects of tempt.
The arrhenius equation
k = Ae-Eact/RT
---When T increases, the fraction of effective collision (e-Eact/RT ) and collision frequency also increases, so the no. of molecules having the Ea also increases, the rate then increases.
(N.B. In fact, the rate is also governed by steric factor)
*Finding the Eact
---ln k = lnA - Eact/RT
Plotting ln k against 1/T, then Eact= -R(slope).
Energy profile
Reaction Mechanism
---A series of one-step reactions(elementary step) building up for a reaction mechanism.It must agree with the experimentally determined rate law.
Molecularity
---the no. of species involve in a reaction.
e.g. AB à A + B (unimolecular)
e.g. A + B à AB (bimolecular)
---It is a theoretical postulate and not an experimentally measured quantity, i.e. it may not be the same with Rate Law.
---It must be a whole number because fraction of atoms or molecules cannot be involve in the reaction.(N.B. the order of reaction can be, so it is not a must for them to be the same)
---The overall molecularity of a complex reaction is, although not the same, generally taken as the molecularity of the rate-determining step(the slowest reaction in the reaction mechanism which controls the overall rate).
Catalyst
---It is a substance which alters the rate of a chemical reaction.
---It becomes temporarily involved in a reaction, providing an alternative reaction path of lower Ea than that for the uncatalysed reaction and it is chemically unchanged at the end of the reaction.
---It catalyses both forward and backward reactions to the same extent in a reversible reaction and thus have no effect on the equilibrium constant.
Energy profiles of catalysed and uncatalysed reaction
---If the catalyst and the reactants are in the same phase, the process is described as homogeneous catalyst. It increases the rate by increasing the amt. of cat. e.g. Esterification using acid as catalyst.
---Heterogeneous catalyst takes place at the surface of a catalyst which is in a different phase from the reactant and product. Therefore , the rate can be enhanced by increasing the surface area of cat. e.g.decomposition of H2O2 using MnO2 as catalyst(power form is better).
Autocatalyst---Mn2+ in acidic medium
Equ:2MnO4-(aq) + 16H+(aq) +5C2O42-(aq)à2Mn2+(aq) + 8H2O(l) +10CO2(g)
Application of Catalyst
1.d-block metal
---Catalytic converters(honeycomb platinum in car)
2CO(g) + 2NO(g) à 2CO2(g) + N2(g)
unburnt hydrocarbon à CO2(g) + H2O
(N.B. Pb poison)
---Haber process
N2(g) + 3H2(g) ⇌(Fe cat.)NH3(g)
2. semi-conducting oxides
---Contact Process
SO2(g) + 1/2O2(g) ⇌ (V2O5) SO3(g)
CHEMICAL EQUILIBRIA
An equilibrium exsists when two opposing forces or rates balance each other out.
*characteristics
---reversible(both forward and backward)
---dynamic(not stationary)
---overall properties(e.g. density, conc.)remain unchanged
---achieved in a closed system only.(no exchange of materials with the surroundings)
Equilibrium Law
aA + bB ⇌ xX + yY
e.g. conc. à[X]xeq[Y]yeq
[A]aeq[B]beq =Kc
where Kc is the molar conc. eq. At a particular tempt.
For reaction involving gases, we can measure and express the gas conc. as partial pressure.
e.g. aX(g) + bY(g) ⇌ rC(g)
(pC)req
(pX)aeq(py)beq = a constant at a given tempt. =Kp
The effect of conditions on the position of equilibrium
Le Chatelier’s Principle
---This states that in any equilibrium, when a change is made to some external factors(such as tempt, pressure), the change in the position of equilibrium is such ass to trend to change the external factors in the opposite direction.
Conc. change
---the position of the equilibrium is shifted to the direction that trends to reduce the change in conc.
Pressure change
---operates like the conc. change(i.e. depends on the volume, in terms of the change in the no. of mole in the reaction)
e.g. A + B⇌ C ,the increase in pressure will casuse a shift in the equ. position that trends to reduce the pressure,i.e. to the right.
e.g. A + B ⇌ C + D ,there will be no change in the equ. position
(N.B. the value K only depends on tempt. Pressure does not takes a role in affecting the K as the conc. does the same)
Ways to change the pressure
1. add or remove a gaseous reactant or product.
2. add an inert gas(since the partial pressure unchanged and the pressure exerted by that gas is just on the container)
3. change the volume of the container.
Tempt. change
The contribution of it is similar to that of the above stated,but it also changes the value of K and the rate of both forward and backward reaction, however may not in the same extent.(i.e. increase intempt. will cause a shift in the equ. position that absorbs heat and vice versa).The value K is related to tempt. in this way:
lnK = -H/RT + C ,where C is a constant.
value of H can be found if K at diff tempt. are known.
The equ. shown below is endothermic as written from to right
(N.B. N2O4 is colorless while NO2 is brown)
Haber process
Heterogeneous reactions
e.g. CaCO3(s) <--> CaO(s) + CO2(g),then (Pco2)eq = constant if the conc. of the soilds are assumed to be constant.Therefore Kp =(Pco2)eq
ACID BASE EQUILIBRIA
Bronstes-Lowry definition: Acid: give up protons to a base
Base: accept protons from an acid
e.g. A ⇌ H + B
acid base
(proton-donor) (proton-acceptor)
A and B are said to be conjugate pair.B is the conjugate base of acid A
, and A the conjugate acid of base B.
The strengths of conjugate pairs
A is a strong acid if the equ. position
A ⇌ H+ + B
lies to the right; B is a strong base if the equ. lies to the left. Thus the conjugate base of a strong acid is a weak base and vice versa. Similarly, the conjugate acid of a strong base is a weak acid, and vice versa.
The strength are given by:
Ka = [H+][A-(aq)] Kb = [HA(aq)][OH-(aq)]
[HA(aq)] [A-(aq)]
The stronger the acid/base, the lower value of pKa / pKb.
then , Ka Kb =[H][OH] =Kw=K[H2O]=10-14(ionic product of water in a particular tempt.i.e. at 298K,for which the conc. of water is considered to be constant.)
or pKa +pKb = 14
When an acid reacts with a base,the change is genrealised as
acid(1) + base(2) <--> acid(2) + base(1)
acid(1) is conjugate with base(1) and acid(2) is conjugate with base(2).
The equ. position will depend on the relative strength of thte acids and bases.If acid(1) is stronger than acid(2) , then base(2) will be stronger than base(2).
e.g.HCl(g) + H2O(l) à H3O+ +Cl-(aq)
pH value and its measurement
pH=log10 1
[H3O+(aq)]
or =-log10[H+(aq)]
pH meter
It consists of a glass and a reference electrode, for which the glass electrode is immersed in a buffer solution containing a platinum wire. The glass electrode, sealed in a thin-walled bulb made of special glass, develops a potential depending on the H+(aq) ion conc. and it is measured against the reference electrode. This value is calibrated and the pH of the solution being measured is known.
The strength of oxoacids in aq solution
Generally, it can be represented as XOm(OH)n. e.g. nitric acid NO2(OH).
The higher the electronegativity of X the stronger the acid will be, because the conjugate base ‘s(which is –ve) negative charge spread through more evenly, thus stabilizing the conj. base ,shifting the equ. position to the right. Other effects such as conjugation with benzene ring does the same effect.(i.e. lower pKa value ,more acidic)
Lewis acids and bases
An acid is a substance that can accept a pair of electrons.(O.A.)
A base is a substance the can donate a pair of electrons.(R.A.)
Buffer and indicator for titration
Definition of a buffer
It is a solution which maintains a fairly constant pH when small amounts of acid or alkali are added to it.
Acid buffer solution solutions can be made by mixing a weak acid with a salt of the same acid; alkaline buffer solution, by mixing a weak base with a salt of the weak base.
How buffer works
For acidic buffer solution such as
HEt(aq) ⇌H+(aq) + Et-(aq)
NaEt(aq) àNa+(aq) + Et-(aq)
When H+(aq) ions are added,
Et-(aq) + H+ (aq) à HEt(aq)
Or when OH- ions are added,
HEt(aq) + OH-(aq) à H2O(l) + Et-(aq)
The pH of buffer solution is thus resisted.
ð pH = pKa + log[salt]
[acid]
Alkaline buffer works on the same principle as acidic one.
e.g. NH4+(aq) + OH(aq) à NH4OH(aq)
NH3(aq) + H+(aq) à NH4+(aq)
=> pH = pKw – pKb + log[base]
[ salt]
(N.B. the capacity of buffer can be increased by enhancing the conc. of acid(base)/salt present in it. On the other hand, it can also be used to make a solution of known pH by adjusting the acid(base)/salt ratio.)