5.1 Rates Of Reactions Flashcards
Rate of reaction
Reaction rates are measured by observing the changes in the quantities of reactants and products over time
Rate = quantity reacted or produced/time
Rate = change in concentration/change in time
mol dm-3 s-1
Order of reaction
Changing the concentration often changes the rate of reaction. The rate of reaction is proportional to the concentration of a particular reaction raised to the power. For example, for reactant [A] and power n the rate is given by: rate dir prop [A]^n
For each reaction, the power is the order of reaction for that reactant. In a reaction, different reactants can have different orders and each may affect the rate in different ways.
Zero order
When the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant:
Zero order: rate directly proportional to [A]^0
In a zero order reaction:
– any number raised to the power zero is 1
– concentration does not influence the rate
First order
A reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of 1:
First order: rate directly proportional to [A]^1
In a first order reaction:
– if the concentration of A is doubled, the reaction rate increases by factor of 2^1 = 2
– If the concentration of A is tripled, the reaction rate increases by a factor of 3^1 = 3
Second order
A reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of two:
2nd order: rate directly proportional to [A]^2
In a second order reaction:
– if the concentration of A is doubled, the reaction rate increases by a factor of 2^2 = 4
-if the concentration of A is tripled, the reaction rate increases by a factor of 3^2 = 9
The rate equation
The rate equation gives the mathematical relationship between the concentrations of the reactants and the reaction rate. For two reactants, A and B, the reaction equation is:
Rate= k [A]^m [B]^n
Rate constant
The rate constant k is the proportionality constant. It is the number that mathematical converts between the rate of reaction and concentration and orders
Overall order
The overall order of reaction gives the overall effect of the concentrations of all reactants on the rate of reaction
Overall order= sum of orders with respect to each reactant
Rate= k [A]^m [B]^n overall order= m + n
Continuous monitoring of rate
Concentration-time graphs can be plotted from continuous measurements taken during the course of a reaction
Orders from shapes of concentration-time graphs
Gradient is rate of the reaction
Order with respect to a reactant can also be deduced from shape of graph
Order with respect to reactant can only be obtained if all other reactant concs remain constant
Zero order conc-time graph
Produces straight line with negative gradient
Reaction rate doesn’t change at all during course of the reaction
Value of gradient is equal to rate constant k
First order conc-time graph
Produces downwards curve with a decreasing gradient over time
As the gradient decreases with time, the reaction gradually slows down
The time for the conc of the reactant to halve is constant
This time is called the half-life and the rate constant of a first order reaction can be determined using this value
Second order conc-time graph
Also a downward curve, steeper at the start but tailing off more slowly
Half life
Time taken for half of a reactant to be used up
First order reaction half life
Have constant half life with the conc halving every half life
Pattern is called exponential decay
A first order relationship can be confirmed by measuring successive half lives and if they’re the same the reaction is first order with respect to the reactant
Calculating rate constant from half life
k = ln2/t1/2
Zero order rate-concentration graph
Produces a horizontal straight line with zero gradient
Intercept on y axis gives rate constant k
Reaction rate doesn’t change with inc conc
First order rate-concentration graph
Produces straight line graph through origin
Rate is dir prop to conc
k can be determined by measuring the gradient
Second order rate-concentration graph
Produces an upward curve with increasing gradient
By plotting second graph with rate against conc squared, the result is a straight line through the origin, gradient is k
Clock reaction
Convenient way of obtaining initial rate of a reaction by taking a single measurement
The time from the start of an experiment is measured for a visual change to be observed, often a colour or precipitate
Provided there is no significant change in rate during this time, it can be assumed that the average rate of reaction over this time will be the same as the initial rate
Initial rate is then prop to 1/t
Clock reaction is repeated several times with different concs, and values of 1/t are calculated for each experimental run
Iodine clocks
Clock that relies on the formation of iodine
As aqueous iodine is coloured orange-brown, the time from the start of the reaction and the appearance of the iodine colour can be measured
Starch is usually added since it forms a complex with iodine which is an intense dark blue-black colour
Rate determining step
The steps in a multi step reaction will take place at different rates
The slowest step in the sequence is the rate determining step
Predicting reaction mechanisms
- the rate equation only includes reacting species involved in the rate-determining step
- the orders in the rate equation match the number of species involved in the rate-determining step
So the rate determining step provides important evidence in supporting or rejecting a proposed reaction mechanism
Reaction mechanism
The series of steps that make up an overall reaction
Hydrolysis of haloalkanes
Hydrolysed by hot aqueous alkali
RBr + OH- —> ROH + Br-
Hydrolysis reactions of haloalkanes can be investigated experimentally to determine the overall order of reaction, the rate equation, and a possible mechanism for the reaction
Effect of temp on rate constants
As temp inc, rate inc and the value of the rate constant k will also increase
Increasing temp shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy
As the temp inc, particles move faster and collide more frequently
Arrhenius equation
k= A e^(-Ea/RT)
Exponential relationship between the rate constant k and temperature T
Exponential factor
e^(-Ea/RT)
Represents proportion of molecules that exceed Ea and have sufficient energy for a reaction to take place
Pre-exponential term
A
Takes into account the frequency of collisions with the correct orientation
Logarithmic form of Arrhenius equation
ln k = - Ea/RT + ln A
Plot graph of ln k against (1/T)/k-1
Gradient of -Ea/R
Intercept of ln A on the y axis
Homogeneous equilibria
Contains equilibrium species that all have the same state or phase
E.g.
N2(g) + 3H2(g) <==> 2NH3(g)
Heterogenous equilibria
Contains equilibrium species that have different states or phases
E.g.
C(s) + H2O(g) <==> CO(g) + H2(g)
Conc of solids and liquids are essentially constant so any species that are s and l are omitted from the Kc expression
Mole fraction
Under the same conditions of temp and pressure, the same volume of diff gases contains the same number of moles of gas molecules
The mole fraction of a gas is the same as its proportion by volume to the total volume of gases in a gas mixture
Mole fraction x = number of moles of A/total no of moles in gas mixture
Partial pressure
The contribution that the gas makes towards the total pressure
The sum of the partial pressures of each gas equals the total pressure
Partial pressure p = mole fraction of A x total pressure
Equilibrium constant Kp
E.g.
H2(g) + I2(g) <==> 2HI(g) Kp=(HI)^2/p(H2) x p(I2)
p is the equilibrium partial pressure
Kp only includes gases because only gases have partial pressures
Do equilibrium constants change?
Magnitude of an equilibrium constant k indicated the extent of a chemical equilibrium
K=1 indicates an equilibrium halfway between reactants and products
K=100 indicates an equilibrium well in favour of the products
K=1x10-2 indicates an equilibrium well in favour of the reactants
K only changes if temp is changed
Effect of temp on equilibrium constant k
If forward reaction is exothermic- k decreases with inc temp and inc temp decreases equilibrium yield of products
If forward reaction is endothermic- k inc with inc temp and inc temp inc equilibrium yield of products
how do changes in conc and pressure affect equilibrium constants?
the value of an equilibrium constant K is unaffected by changes in conc or pressure.
equilibrium shift results from the fact that the equilibrium constant doesn’t change
fewer moles of gaseous products effect on equilibrium
ratio is smaller than K
effect of inc pressure- products increase, reactants decrease
equilibrium shift to right
more moles of gaseous products effect on equilibrium
ratio is larger than K
effect of inc pressure- products decrease, reactants inc
equilibrium shift to left
same number of moles of gaseous reactants and products effect on equilibrium
ratio = K
effect of inc pressure- no change
no equilibrium shift
how does a catalyst affect equilibrium constants?
equilibrium constants are unaffected by the presence of a catalyst
- catalysts affect the rate of a chemical reaction but not the position of equilibrium
- catalysts speed up both the forward and reverse reactions in the equilibrium by the same factor
equilibrium is reached quicker but the position is not changed
