Lecture 17 Using Reaction Rates

Question 1

Using reaction rates

Answer 1

aA+bB > cC

A and B are reactants C product

Lowercase a,b and c are stoichiometry of the reaction

Rate of reaction is defined with a rate equation/ law

Rate = constant of proportionality X a function of conc.Of A, B & sometimes C

Rate= k X function of conc.

Where k is the “rate constant” or “rate coefficient” e.g. rate = k (A)(B)

The 1/a etc. Make sure that we get the same numerical value for the rate

Rate has units of conc. / Time and the neg signs when we measure loss of A or B ensure that the rate is always a pos. Quantity (neg. Rate makes no sense)

Unit of rate constant and of those of the conc. Must multiply to give the same unit as the rate (conc. /Time)

Question 2

The Arrhenius equation - What does rate equation mean

Answer 2

Rate = k X concentrations

For reaction to occur:

1) must collide - collision freq. “A”

2) must have correct orientation
Need a probability of this P orient
Assume P orient = 1

3) must have enough energy to react
Need probability of this
P energy = e^(Ea/RT)

4) must collide in correct proportion
The concentrations bit

So gives the Arrhenius equation:
k= A x P orient x P energy = Ae^(-Ea/RT)

Question 3

Affect of temp. On rates

Answer 3

The pre-exponential factor or frequency factor A incorporates collision freq and orientation factors.

Arrhenius equation:
k= Ae^(-Ea/RT)
Or ln(k)=ln(A)-Ea/RT

Ea is the activation energy (Jmol-¹), R is the gas constant (J K-¹ mol-¹) and T is temp. In Kelvin.

An Arrhenius plot allows A and Ea to be determined

If you have only 2 data points (T1,K1) and (T2,K2) then calculate by

Ln (K1/K2) = Ea/R (1 T2- 1/T1)

Question 4

Conc factors and reaction orders

Answer 4

For reaction aA+bB>cC

Zero order:
no conc. terms in rate of equation, k has units of conc./time
Rate equation is rate = k
Integrated rate equation (A)=(A)t=0-kt

First order:
One conc. term in rate equation
Either (A) or (B)
K has units of 1/time
Rate equation is rate=k(A) or =k(B)
Integrated is (A) = (A)t=0 e^-kt

Second order
Two conc. terms in rate equation
Either (A) (B) or (A)² or (B)²
K has units of 1/conc. X time
Rate equation is rate=k(A)² or rate=k(B)²
Integrated is 1/(A) = kt+1/(A)t=0
Or rate=k(A)(B)

Pseudo first order
Version of a second order reaction

If rate equation is rate=(A)(B) then when (A)»(B) then A will hardly change during the reaction. In other words (A)~constant

so we can define k*=k(A) for a pseudo first order rate equation.

Rate equation is rate = k(B), k has units of 1/time and integrated rate equation is (B)=(B)t=0 e^-k*t

Question 5

Measuring rates: obtaining rate equation, reaction order and rate constant for reaction

Answer 5

Several ways depending on what you can measure in your experiment

Follow reactant or product conc. Continuously with time integration method

Conc. Variation. Vary (A) keeping (B) constant and vise versa using a few fiff conc e.g. doubling conc and measuring rate

Conc variation using a range of conc. and plot graphs of initial rate against conc.

Question 6

Calculate half lives for rate equations

Answer 6

By setting t= 0 and t=t 1/2

Integrated laws/half lives are not useful in biology as enzymes become inhibited as conc. of product builds up.

Hence we use the “initial rate method” at start of reaction while product quantity is small

Rate= k(A)^alpha

So log(rate) = log(k)+alpha log(A)

Straight line y=c+mx