Chapter 6 - Reaction Kinetics

Question 1

Why ΔG cannot be used to tell the rate of reaction?

Answer 1

There is an energy barrier between the reactants and products that has to be overcome. The magnitude of the activation energy barrier does not depend on the magnitude of ΔG.

Question 2

Why are reactions with low Ea fast?

Answer 2

A large proportion of molecules have the necessary energy to overcome the energy barrier (kinetically favourable)

Question 3

Why are reactions with high Ea slow?

Answer 3

Only a small proportion of molecules have the necessary energy to overcome the energy barrier (kinetically unfavourable)

Question 4

What are 3 typical reactions with high Ea?

Answer 4

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 5

What are 3 typical reactions with low Ea?

Answer 5

1) reactions between 2 free radicals
2) reactions ions of opposite charge
3) acid-base reactions

Question 6

Define rate of reaction.

Answer 6

The rate of a reaction is define as the change in concentration of a reactant or a product per unit time.

Question 7

Define rate equation.

Answer 7

An experimentally determined equation that relates the rate of reaction to the concentrations of the reactants raised to appropriate powers.

Question 8

Define order of reaction.

Answer 8

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

Question 9

Define overall/total order of reaction.

Answer 9

It is the sum of individual orders.

Question 10

What is the rate constant (k)?

Answer 10

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

Question 11

What is half life?

Answer 11

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

Question 12

What are 2 ways to plot rate-concentration graphs?

Answer 12

1) measure the gradient at various points in the concentration-time graph and plot these rates against their corresponding reactant concentration
2) carry out the reaction a few time with different initial reactant concentrations and determine their rate by obtaining gradient at t=0 from a concentration-time graph

Question 13

What are 2 ways to study the rates and orders of reaction?

Answer 13

1) keep all reactants in large excess except the one being studied, so that their concentrations don’t appear to change significantly and may be regarded as constant - they do not seem to influence the rate of reaction. (pseudo-zero order wrt reactants in excess)
2) Measure the rate of several reaction mixtures, in which a different initial concentration of one reactant is used, while the initial concentrations of all other reactants are kept the same.

Question 14

What are 6 ways to measure the rate of reaction?

Answer 14

1) measure volume of gas produced, since volume is proportional to concentration.
2) measure the decrease in mass
3) follow a change in gas pressure at regular time intervals (using a manometer), since rate of reaction is directly proportional to the rate of change in pressure.
4) follow a change in electrical conductivity, since rate of reaction is directly proportional to the rate of change in conductivity. (applicable for reaction in aqueous solutions where there is a change in the concentration of ions)
5) follow a change in absorbance (using a colorimeter), measuring the amount of light absorbs as it passes through a coloured solution, since rate of reaction is directly proportional to the rate of change in absorbance. This is applicable for reactions with one of the substances being coloured/has a different color
6) extract a small fixed volume sample of the reaction mixture at various times, then quench the reaction sample. The quenched sample is then titrated against a suitable reagent to determine the amount of reactant that remains/amount of a product. Titre volume is proportional to concentration of reactant/product.
7) clock reaction: a reaction set up to produce a sudden visual change when a small, fixed amount of product is formed. Measure the time taken for the prescribed visual change to occur, which may be used as an approximation of the initial rate of reaction. It is only valid if it is small ,such that we can assume that the conc-time graph is a straight line.

Question 15

What is the rate-determining step (rds)?

Answer 15

It is the slowest step with the highest activation energy in a multi-step reaction mechanism,

Question 16

What does it mean to be an intermediate in a reaction? (3)

Answer 16

1) a chemical species produced in an early step of a mechanism and consumed in a later step.
2) Neither reactant nor ultimate product
3) Do not appear in overall balanced equation&overall rate equation

Question 17

How do we construct a rate equation from a given reaction mechanism? (3)

Answer 17

1) The coefficients of the reactants in the rate-determining step are used as the order with respect to each reactant. 2) Reactant species that react before on in the rds will appear in the rate equation, while reactant species that react after the rds will not appear in the rate equation.
3) since the intermediates cannot appear in the rate equation, substitute it with the reactants producing the intermediate.

Question 18

What are 5 factors affecting the rate of reaction?

Answer 18

1) concentration/pressure
2) surface area (solids)
3) temperature
4) catalyst
5) light

Question 19

How does concentration/pressure affect the rate of reaction?

Answer 19

At a higher concentration, there are more reactant particles per unit volume, thus reactant particles are closer together. Reactant particles collide more frequently, increasing the frequency of effective collisions, thus increasing the rate of reaction.

Question 20

How does surface areas of solids affect the rate of reaction?

Answer 20

Only particles on the solid surface are available for reaction. If the solid reactant becomes more finely divided, the surface area of the solids that can come into contact with other reactants is larger. Hence, frequency of effective collisions increase, increasing rate of reaction.

Question 21

How does temperature affect the rate of reaction?

Answer 21

When temperature increases, the reactant particles move faster since they have higher kinetic energy, colliding more frequently, thus increasing the proportion of particles with kinetic energy equal to or higher than activation energy. Frequency of effective collisions increase, thus increasing rate of reaction. (10C increase in temperature roughly doubles the rate of reaction)

Question 22

How does adding a catalyst affect the rate of reaction?

Answer 22

It lowers the activation energy through providing an alternative reaction pathway with lower Ea, increasing the fraction of reacting particles with enough energy to overcome the activation energy barrier, increasing the frequency of effective collisions, thus increasing the rate of reaction.

Question 23

How does concentration/pressure affect the rate of reaction?

Answer 23

When reactant molecules absorb light energy, the average kinetic energy of the particle increases, hence increasing the fraction of reacting particles with enough energy to overcome the activation energy barrier. This increases the frequency of effective collisions, thus increasing the rate of reaction.

Question 24

Why does changing temperature affect the rate constant?

Answer 24

With reference to the rate equation rate=k[A]^m[B]^n, if [A] and [B] are kept constant for 2 identical experiments in which only temperature is changed, rate will be directly proportional to k. Since increasing the temperature increases rate, and rate is proportional to k, increasing the temperature must have increased k.

Question 25

Define catalyst.

Answer 25

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 26

Why does adding a catalyst affect the rate constant?

Answer 26

With reference to the rate equation rate=k[A]^m[B]^n, if [A] and [B] are kept constant for 2 identical experiments in where one is catalysed and one is not, rate will be directly proportional to k. Since adding a catalyst increases rate of reaction, and rate is proportional to k, adding a catalyst must have increased k.

Question 27

Define homogenous catalyst.

Answer 27

A homogenous catalyst acts in the same phase as the reactants and is uniformly mixed with them.

Question 28

How does homogenous catalysis work?

Answer 28

It usually involves the formation of an intermediate between the catalyst and one of the reactants. The catalyst is then regenerated in a later step of the reaction. The homogenous catalyst must be involved in the rate-determining step in order for it to affect the rate of reaction. (thus it usually appears in the rate equation)

Question 29

Define heterogeneous catalyst.

Answer 29

Heterogeneous catalysts act in a different physical phase from the reactants.

Question 30

Why would a larger surface area be advantageous for a heterogeneous catalysis?

Answer 30

Catalytic action occurs on the surface of the solid catalyst, at what is called “active sites”. Thus, a larger surface area is more advantageous in increasing the rate of reaction since it exposes more active sites.

Question 31

How does heterogeneous catalysis occur? (3)

Answer 31

1) Adsorption: the reactant molecules diffuse towards the surface of the catalyst, and are adsorbed onto the active sites at the surface through forming weak attraction forces. The adsorption process brings reactant molecules closer together. The process also weakens the bonds in the reactant molecules, thus lowering the activation energy. The reactant molecules are also correctly orientated for the reaction to occur.
2) Chemical reaction occurs: bonds break and new bonds form.
3) Desorption: The product molecule desorbs and diffuses away from the catalyst surface. The active sites become free to adsorb “new” reactant molecules.

Question 32

How will the rate of reaction be affected in heterogeneous catalysis?

Answer 32

The rate of reaction is controlled by

1) how fast the gaseous reactants are adsorbed
2) how fast the products are desorbed
3) when the active sites are fully occupied (catalyst is saturated), there is no increase in reaction rate even if pressure increases.

Question 33

In a heterogeneous catalysis, when the catalyst is saturated, what is the order or reaction with respect to reactant molecules?

Answer 33

It will be zero order since the active sites are fully occupied.

Question 34

What is an autocatalyst?

Answer 34

It is when one of the PRODUCTS of the reaction is the catalyst for the reaction itself.

Question 35

What are enzymes?

Answer 35

Enzymes are biological catalysts that speed up reactions in living systems while remaining chemically unchanged.

Question 36

What are 4 characteristics of enzymes?

Answer 36

1) highly specific in action
2) most enzymes catalyse only one reaction/one group of similar reactions
3) enzyme-catalysed reactions have very low activation energy
4) three-dimensional shape

Question 37

Describe the mechanism of enzyme action.

Answer 37

1) enzyme and substrate bind together to form enzyme-substrate complex. Substrate binds to the active site of the enzyme through intermolecular attractions (hydrogen bonds/dispersion forces)
2) within the complex, the substrate molecule undergoes chemical change (bond reorganization/reaction with other molecules)
3) Once products are formed, they will be released from the enzyme and diffuse away. The reaction that forms enzyme + products from the enzyme-substrate complex ois the slow, rate determining step.
4) the enzyme’s active site is empty again and ready to take in another substrate.

Question 38

How does the concentration of substrates affect the rate of an enzyme-catalysed reaction and order of reaction? (2 scenarios)

Answer 38

1) when the enzyme concentration is greater than substrate concentration, so the rate of reaction increases proportionally with increasing substrate concentration. The reaction is first order with respect to the substrate.
2) when substrate concentration is greater than enzyme concentration, all the active sites are filled with substrate molecules, so any further increase in substrate concentration does not affect the rate. The reaction is now zero order with respect of the substrate.

Question 39

What are 2 factors that affect enzyme activity?

Answer 39

1) temperature

2) pH

Question 40

What happens when temperature goes beyond an enzyme’s optimum temperature?

Answer 40

Denaturation. The attraction forces holding the specific three-dimensional shape of the enzyme molecules are overcome. The denatured enzyme has a random structure and the active site has been irreversibly destroyed, thus being unable to catalyse reactions.

Question 41

How does poison affect catalyst activity?

Answer 41

Catalysts with active sites, whether biological or not, are vulnerable to poisons which may preferentially adsorb onto the active site. Once bound, these poisons have an inhibitive effect on the action of the catalyst, even when present in small amounts.