C11 Chemical kinetics

Question 1

what are rate laws?

Answer 1

mathematical expressions which relate the rate of the reaction to the concentration of reactants

Question 2

for the following reaction, write a formula for the rate of loss of reactant A

aA + bB –> cC + dD

Answer 2

rate of loss of reactant A =
k x [A]^a x [B]^b

Question 3

describe zero-order reactions and their rate equations

Answer 3

• simplest type of rate equations
• rate of disappearance of a reagent is constant and doesn’t depend on its concentration

Question 4

how do you work out whether a reaction has zero order kinetics from experimental data?

Answer 4

• plot conc (y) against time (x)
• if this forms a straight line, your reaction is zero order
• gradient = -k

Question 5

describe what is meant by a first order reaction

Answer 5

the rate of disappearance of a reagent is proportional to the amount of material remaining

Question 6

for the first order reaction shown below, what is the equation that shows rate of loss of reactant A?

A –>P

Answer 6

k[A]

Question 7

plotting [A] against time for a first order reaction gives an exponential curve. how do we plot a linear graph for first order reactions?

Answer 7

• plot the natural log of [A] against time
• gives a linear graph for first order reactions

Question 8

what is half-life?

Answer 8

the time taken for half the concentration of reactant to decompose

Question 9

what is the overall reaction order?

Answer 9

• the sum of the orders with respect to each reactant
• eg. 2 reactants both first order means the overall order would be second

Question 10

explain the relationship between temperature and rate

Answer 10

• reactions tend to get faster as temperature increases
• as temperature increases, more molecules have sufficient energy to overcome the activation energy barrier

Question 11

what 2 things must happen for 2 molecules to react?

Answer 11

• collision
• collide with sufficient energy to overcome Ea

Question 12

what is collision rate linearly dependent on? what is exponentially dependent on temperature?

Answer 12

• linearly dependent on temperature
• fraction of molecules having sufficient energy to overcome Ea is exponentially dependent on temperature

Question 13

after logging the Arrhenius equation to get a linear graph, what formula can we get for the gradient?

Answer 13

-Ea / R

Question 14

what does ΔG equal when a reaction is at equilibrium?

Answer 14

0

Question 15

what is the Arrhenius equation? what does it show?

Answer 15

k=Ae^(-Ea/RT)

shows the effect that temperature has on rate

Question 16

integrated rate law for zero order reaction

Answer 16

[A] = -kt + [A]₀

Question 17

integrated rate law for first order reaction

Answer 17

[A]t = [A]₀ e ^ -kt

Question 18

what shape is the integrated form of the differential equation for zero order reactions?

Answer 18

linear

Question 19

what shape is the integrated form of the differential equation for first order reactions? how can we change its shape?

Answer 19

exponential, must use logs to make it linear