kinetics

Question 1

what is half-life?

Answer 1

the half life of a reaction is the time taken for the concentration of a reactant to decrease to half its initial value

t(1/2) = ln2/k where k is rate constant

Question 2

what is the rate equation?

Answer 2

the rate equation is an experimentally determined equation that relates the rate of reaction to the concentration of the reactants raised to appropriate powers

Question 3

what is the order of reaction with respect to a reactant?

Answer 3

the order of reaction with respect to a reactant is the power of its concentration term in the rate equation, which must be experimentally determined

Question 4

what is the overall order of reaction?

Answer 4

the overall order of reaction is the sum of individual power on each concentration term in the rate equation, which must be experimentally determined

Question 5

what is the rate constant?

Answer 5

the rate constant is the proportionality constant in the experimentally-determined rate equation. it is constant at a given temperature

Question 6

for what types of graph can the half-life method be used to determine order of reaction?

Answer 6

concentration of reactant against time and concentration of product against time

Question 7

what is activation energy?

Answer 7

the activation energy of a reaction is the energy barrier the reactants must overcome in order to become the products

Question 8

what are examples of kinetically unfavourable reactions?

Answer 8

1. reactions between 2 neutral molecules/involve breaking strong covalent bonds
2. reactions between ions of similar charge
3. reactions where a bond breaks to form free radicals

Question 9

how does concentration/pressure affect rate?

Answer 9

higher concentration/pressure -> more reactant particles per unit volume -> reactant particles are closer together and collide more frequently -> frequency of effective collisions increases -> rate of reaction increases

Question 10

what is the rate determining steps?

Answer 10

the rate determining step is the slowest step in a mechanism

any step before the rds affects the overall rate but any step after the rds does not affect overall rate
if the rds is the first step, the order wrt a reactant is equal to its coefficient in the rds

Question 11

how does increase in temperature affect rate?

Answer 11

temp increase -> reactant particles have greater average kinetic energy -> move more quickly & collide more frequently + proportion of particles with kinetic energy greater than or equal activation energy increase -> frequency of effective collisions increase -> rate of reaction increases

rate constant increases since rate increases

Question 12

what is a catalyst?

Answer 12

a catalyst is a substance that speeds up a chemical reaction by providing an alternative reaction pathway of lower activation energy while remaining chemically unchanged at the end of the reaction

Question 13

how does catalyst affect rate of reaction?

Answer 13

catalyst provides alternative reaction pathway of lower activation energy -> proportion of particles that have kinetic energy greater than or equal to lower activation energy increases -> frequency of effective collisions increases -> rate of reaction increases

since rate increases, k increases

Question 14

what is the difference between homogeneous and heterogeneous catalysts?

Answer 14

homogeneous catalysts: act in the same phase as reactants
heterogeneous catalysts: act in a different phase from the reactants

Question 15

what is homogeneous catalysis?

Answer 15

homogeneous catalysis usually involves the formation of an intermediate between the catalyst and one of the reactants. the catalyst is regenerated in a later step of the reaction. the catalysed reaction may involve 2 or more steps, each of which has a lower activation energy than the uncatalysed reaction

Question 16

what is heterogeneous catalysis?

Answer 16

heterogeneous catalytic action occurs at the ‘active sites’ on the surface of the catalyst. a larger surface area exposes more active sites and increases the rate of reaction. heterogeneous catalysts are usually made into pellets

Question 17

describe the haber process.

Answer 17

adsorption
gaseous reactant molecules are adsorbed onto the active sites by formation of weak attraction forces
- this brings reactant molecules closer together,
- weakens the covalent bonds in the reactant molecules thus lowering activation energy,
- and orientates reactant molecules correctly for reaction to occur
reaction
occurs where bonds are broken and new bonds are formed
desorption
product molecules desorb from the catalyst surface so active sites are exposed for further reaction

Question 18

how do catalytic converters work?

platinum & rhodium work as heterogenous catalysts

Answer 18

exhaust gases are passed through the converter, which contains metals like platinum and rhodium on a honeycomb support. harmful gases are adsorbed onto the surface of the metals, go through a reaction and harmless gases then desorb from the surface

Question 19

what is autocatalysis?

Answer 19

autocatalysis is when the reaction is catalysed by one of its products

so rate is slow at the start due to low conc of autocatalyst, speeds up as the autocatalyst is produced, and slows down at the end due to decreasing conc of reactants