6 Reaction Kinetics

Question 1

Define rate of reaction. State its units and whether it is a positive or negative quantity.

Answer 1

Change in concentration of a particular reactant or product per unit time. It is a positive quantity.

Question 2

Define instantaneous rate, initial rate and average rate. State when the initial rate can be approximated by the average rate.

Answer 2

Instantaneous rate is the rate at a particular time
Initial rate is the rate at t = 0
Average rate of a reaction during a specified time interval is the change in concentration of a reactant or a product over that time interval.

The initial rate can be approximated by the average rate when the time interval is small enough and the time interval starts from t = 0.

Question 3

State the relationship between the different rates of a reaction.

Answer 3

Reactants - include negative sign in front
Reactants and products - divide the rate of reaction by their stoichiometric coefficient

Question 4

Explain how the rate of reaction can be determined experimentally.

Answer 4

(Pg 5 of notes onwards)
By monitoring physical quantities using the continuous method or the ‘clock’ reaction method.

Continuous method monitors the concentration of a reactant or product species continuously over time.

1) Sampling (Quenching) and Titration (Keywords: aliquot portion from reaction mixture, quench the sample by adding a large volume of ice-cold water (dilutes and cools) or an excess of quenching agent (reacts immediately with the reactant or catalyst), composition of the reaction mixture does not continue to change, titrate quenched sample, plot graph of volume of titrant used against time)

2) Measuring the colour intensity at regular time intervals (Keywords, calibration curve, colourimeter, colour intensity, known concentration, rate = change in colour intensity over time)

3) Measuring the electrical conductivity at regular time intervals (Keywords: number and types of ions affect electrical conductivity, changes in ions present will result in changes in electrical conductivity, two inert electrodes in the reaction mixture and plotting graph of electrical conductivity against time)

4) Measuring the volume of gas produced at regular time intervals (Keywords: gas collected in a graduated gas syringe, volume measured at regular intervals, graph of volume evolved against time, rate of reaction is the change in the volume of the gas produced over a specified time interval)

5) Measuring the mass of the reaction mixture at regular time intervals (Keywords: gas is allowed to escape, determine mass loss over time, plotting a graph of mass loss against time and rate of reaction is proportional to the gradient of the tangent to the curve at that instant)

6) Measuring the pressure at regular time intervals (Keywords: calculate partial pressure and the rate of reaction can be determined from the change in partial pressure, at constant volume and temperature, of the reaction or product over time)

Clock reaction method involves measuring the time taken for a stated change to occur and is used for reactions that are accompanied by prominent visual changes such as the forming of a precipitate or obvious change in colour. By identifying the clock, the rate of reaction will be proportional to Vclock/time. If the clock is a product then the rate is proportional to 1/time.

1) Reaction between thiosulfate ions and hydrogen ions
2) Reaction between hydrogen peroxide and iodide ions in acidic medium

Question 5

Explain why the total volume of the reaction mixture must be kept constant.

Answer 5

The initial concentration of the reactant in a reaction mixture equals the volume of the reactant*concentration of the reactant/total volume of the reaction mixture. Since the concentration of the reactant remains constant, if the total volume of the reaction mixture is kept constant by adding an appropriate volume of water to each reaction mixture, the initial concentration will be directly proportional to the volume of the reactant used.

Question 6

Explain why the same beaker must be used for each experiment. Explain the relationship between the initial rate of reaction and the time taken for the cross to be obscured. (Clock reaction and formation of ppt)

Answer 6

The same beaker is used to ensure that the depth of the reaction mixture remains the same and the same amount of product needs to be precipitated before the cross is obscured. The initial rate of reaction is proportional to the time taken for the cross to be obscured. (Pg 11 of notes)

Question 7

State the 4 factors which influence the rate of a reaction. Explain how they influence the rate of a reaction.

Answer 7

Recall: Reactant particles must mix in order to collide and react. Thus, the frequency of collisions between reactant particles also depends on the physical states of the reactants.

1) Physical state of the reactants: For solids, the greater surface area per unit volume, the more contact it makes with the other reactant and the faster the rate of reaction.

2) Concentration of the reactants: As the concentration of a reactant increases, the reactant particles come closer together, frequency of collisions increases, the probability of atoms having the correct collision geometry and sufficient energy for a reaction to occur increases and the frequency of effective collisions increase, leading to an increase in rate of reaction.

For gases, concentration is measured by its partial pressure. An increase in the pressure of the system is equivalent to an increase in the concentration of reactants… (explain about coming closer, freq and eff. freq)

3) Temperature: Maxwell-Boltzmann Distribution curve shows the distribution of kinetic energies among particles in a reaction mixture. At a higher temperature, the area under the curve remains the same as the total number of particles in the system remain the same. However, the maximum of the curve is displaced to the right and takes on a smaller value and there is a greater spread of kinetic energies (curve broadens). When the temperature increases from X to XX, the average kinetic energy of the reactant particles increases. Hence, significantly more reactant particles have energy greater than or equal to the activation energy of the reaction (indicate greater shaded area). There is an increase in effective collision frequency and hence an increase in reaction rate. In addition, higher temperature results in a larger rate constant k hence there is an increase in rate of reaction.

4) Catalyst: Using the Maxwell Botlzmann Distribution curve, draw a vertical line on the curve and show the differentiation in the shaded area of the number of particles with energy greater than or equals to Ea for the catalyzed and uncatalysed reaction. A catalyst increase the rate of reaction by providing an alternative reaction pathway, one of lower activation energy than the uncatalysed reaction without itself undergoing any permanent chemical change. Hence, significantly more reactant molecules have energy greater than or equal to Ea, increase in eff. collision frequency and hence rate of reaction increases. In addition, it also results in a larger rate constant and hence increase in reaction rate.

Question 8

Define a rate equation.

Answer 8

Rate equation is a mathematical expression that relates the rate of reaction to the concentration of each reactant raised to the appropriate power. It shows the exact dependance of a reaction rate on the concentration of all the reactants. (Can only be obtained by experiment, may not include all reactants in equation)

Question 9

Define rate constant k. Explain how temperatures and the presence of a catalyst affects k.

Answer 9

Rate constant of a reaction is the constant of proportionality in the rate equation of a reaction. Its units depend on the over order of the reaction. K remains constant for a given reaction at a particular temperature and increases with increasing temperature or in the presence of a catalyst, leading to a faster rate of reaction.

Question 10

Define order of a reaction with respect to a reactant.

Answer 10

Order of reaction with respect to a reactant is the power to which the concentration of that
the reactant is raised in the experimentally
determined rate equation.

Question 11

Define the overall order of a reaction.

Answer 11

It is the sum of the powers of the concentration terms in the rate equation.

Question 12

Define the half-life of a reaction. State what happens when half-life is constant at constant temperature. Draw the graph of rate against concentration for a first-order reaction. State the formula related to half-life.

Answer 12

The half-life of a reaction is the time taken for the concentration of a reactant to decrease to half its initial value. When half-life is constant at constant temperature, it is a first-order reaction where half-life = ln 2/k and is independent of a initial concentration of the reactant.

The graph is a straight line, with a positive gradient that equals k, that passes through the origin. This shows that rate is proportional to concentration.

C = (1/2)^n C0 where n is the number of half-life, C0 is the initial concentration and C is the final concentration

Question 13

Draw the rate/concentration, [reaction]/time and [product]/time graphs for a zero-order reaction.

Answer 13

Pg 20 of notes

Question 14

State the three situations where a reaction is a Pseudo-order reaction.

Answer 14

(a) Presence of a large excess of reactant
(b) Reactant is also the solvent (reactant that is in large excess, its concentration remains essentially constant)
(c) Presence of a catalyst (Increases the rate of a reaction by participating in the reaction but is not consumed by the reaction hence its concentration can be regarded as essentially constant and k’ = k[catalyst] = constant

If the reaction becomes 1st order, it is a pseudo-first order reaction and k’ is the pseudo-first-order rate constant.

Question 15

Explain how to find the order of a reaction from initial rates data.

Answer 15

Pg 22 of notes
Inspection
Calculation

Question 16

Explain how to find the order of a reaction from concentration time graph

Answer 16

Pg 22 of notes
Include construction lines

Question 17

Define reaction mechanism of a reaction / elementary step / molecularity of an elementary step / intermediate.

Answer 17

Collection of elementary steps in the proper sequence showing how reactant particles are converted into products.

Elementary step is a distinct step in a reaction mechanism which describes a single molecular event that involves bond breaking and/or forming.(Cannot be broken down into simple steps)

Molecularity of an elementary step in a reaction mechanism is the number of reactant particles (e.g. atoms, molecules or ions) taking part in that step. (one reactant particle: unimolecular, bimolecular, termolecular - rare esp with correct orientation and required activation energy)

Intermediate is a species that is formed in one step of a reaction mechanism and consumed in the subsequent step.

Question 18

Define a single-step reaction.

Answer 18

It is one that takes place in a single step and is termed an elementary reaction. (Hence, it identical to the stoichiometric equation for the reaction and the rate equation can be deduced directly)

Question 19

Explain what a multi-step reaction is and how the rate equation can be deduced from a given reaction mechanism.

Answer 19

A multi-step reaction takes place by two or more steps that take place at different rates and the rate of the overall reaction depends on the rate of the rate-determining step which is the slowest step in the reaction mechanism (highest Ea) and other steps are referred to as fast steps.

When the first step is the slow step, the reactants that participate in the slow step are those that appear in the rate equation. The order of the reaction with respect to the reactant X is the number of X particles that participate in the slow step. (E.g. one particle X, order of reaction with respect to X = 1)

When the first step is not the first step, the rate equation for the overall reaction can be deduced by considering the slow step and all the fast steps before the slow step. Since intermediates should not appear in the rate equation, the reactions that appear should only be those that participate in the slow step and any fast step before the slow step.

Question 20

State the Collision Theory.

Answer 20

The collision theory states that reactant particles must collide in order to react and must collide in a favourable orientation with a certain minimum amount of energy.

Question 21

Transition State Theory

Answer 21

.

Question 22

Differentiate kinetic feasibility and thermodynamic feasibility.

Answer 22

Delta G indicates thermodynamic feasibility of a reaction; whether it can occur and gives no information about the kinetic feasibility of a reaction; whether it occurs at an observable rate - depends on the activation energy.

Kinetically stable reactants take part in thermodynamiccally feasible reactions with high activation energy.

Question 23

Define a catalyst. Define an inhibitor.

Answer 23

Catalyst is a substance which increase the rate of a reaction without itself undergoing any permanent chemical change.

An inhibitor is a substance which decreases the rate of a chemical reaction.

Question 24

State the two types of catalysis and their examples.

Answer 24

Homogeneous catalysis - reactants and catalyst are in the same phase (E.g. Reaction between peroxodisulfate ions and iodide ions, Catalytic oxidation of atmospheric sulfur dioxide by atmospheric oxides of nitrogen)

Heterogeneous catalysis - reactants and catalyst are in different phases. Usually, the reactants are liquids or gases and the catalyst is usually in the solid phase. For this to occur, diffusion first takes place: Reactsnt molecules diffuse towards the catalyst surface. Then, adsorption takes place: Reactant molecules become adsorbed onto the active sites of the catalyst surface. This involves the formation of weak bonds between the reactant molecules and catalyst surface. It increases the concentration of reactants at the catalyst surface, allowing the reactant molecules to come into close contact with proper orientation for reaction and also weakens the covalent bonds in the molecules, lowering the Ea for the reaction. This increases the rate of reaction. Then, Desorption takes place: It is the reverse of adsorption and the product molecule eventually breaks free from the catalyst surface. Finally, diffusion takes place: Product molecules diffuse away from the surface and the vacant sites are now available for adsorption of other reactant molecules. (E.g.Haber Process, the addition of H2 to alkenes catalysed by nickel-metal - conversion of vegetable oil into margarine)

Pg 33 to 35

Question 25

Explain the catalytic oxidation of atmospheric sulfur dioxide by atmospheric oxides of nitrogen and its implications.

Answer 25

Atmospheric sulfur dioxide can be oxidised by oxygen but the reaction is very slow in the absence of a catalyst. In the presence of nitrogen dioxide, the rate of oxidation of sulfur dioxide is increased. The catalysed reaction occurs in two steps.

In the first step, the nitrogen dioxide from car exhaust fumes oxidised the atmospheric sulfur dioxide from the burning of fossil fuels to form sulfur trioxide. and nitrogen oxide. In the second step, the catalyst nitrogen dioxide is regenerated when nitrogen oxide reacts with oxygen.

SO3 formed is a secondary pollutant that can react with acid rain to form sulfuric acid and results in acid rain, amplifying the problem of atmospheric SO2 as a pollutant. Hence, there is a need to reduce the amount of sulfur dioxide and nitrogen dioxide in our atmosphere.

Question 26

State the 3 conditions required for the Haber Process to take place.

Answer 26

250 atm
450 d.c
Fe as a catalyst

Question 27

Explain how carbon monoxide, oxides of nitrogen and unburnt hydrocarbons from exhaust gases can be removed in cars fitted with catalytic converters.

Answer 27

The catalytic converter consists of a ceramic honeycomb structure coated with platinum and palladium and rhodium which act as catalysts. It maximises the surface area on which the heterogeneously catalysed reactions take place. These catalysts can be de-activated by lead as lead is preferentially adsorbed onto the catalyst surface thereby occupying the active sites. Hence, cars fitted with catalytic converters must run on unleaded petrol.

Unburnt hydrocarbons are oxidised to CO2 and H2O with Platinum and palladium catalysts.

CO is oxidised to CO2 with Pt and Pd catalysts.

Oxides of nitrogen such as NO and CO are reduced to N2 by the excess CO present, with Rh acting as the catalyst, forming N2 and CO2.

Question 28

Define autocatalysis and explain how the rate of reaction changes during the catalytic reaction. Give examples of such reactions and explain how to test for an autocatalyst.

Answer 28

Autocatalysis is a type of catalytic action whereby the product of a reaction acts as a catalyst for the reaction (autocatalytic reaction). Initially, the reaction will be slow since it is not catalysed. However, as the autocatalyst is produced, the rate of reaction is increased. Towards the end of the reaction, the concentration of the reactants had fallen to a low level and so the rate of reaction decreases, even though there is an adequate supply of catalyst. (E.g. oxidation of ethanedioate ions by manganate ions, the reaction is accelerated by the Mn2+ ions produced during the reaction, the decomposition of arsine by heat is catalysed by the metallic arsenic formed).

To test for an autocatalyst, add it at the start of the reaction and reaction should become very rapid.

Question 29

Define enzymes. State the properties of enzymes.

Answer 29

Enzymes are proteins that catalyse chemical reactions in living systems (often called biological catalysts or biocatalysts) by providing an alternative pathway with lover activation energy. (They do so by forming a complex with the substrate (reactant) of a reaction. Without them, most biochemical reactions would be too slow to sustain life. Examples of enzymes include amylase, trypsin and lipase.)

1) Nature and size
Enzymes are globular porteins with active sites contained in their three-dimensional structure.

2) Efficiency
They are required in very small amounts as they are very effective catalysts and the enzyme molecules are regenerated during their catalytic activity. (A million times in one minute)

3) Specificity
Enzymes are very specific to a particular reaction or type of reaction due to their three-dimensional conformation of the active site

4) Temperature
The enzymes operate most efficiently at body temperature. At higher temperatures, it can result in a change in the three-dimensional conformation of the active site which causes the enzyme to become denatured and inactive.

5) Sensitivity to pH
Enzymes only work well over a narrow pH range (that of blood) and have different optimum pH levels.

Question 30

State the factors affecting the rate of an enzyme-catalysed reaction. Draw the graph of substrate concentration against the rate of reaction. (Note that a similar graph is observed for the reaction involving heterogeneous catalyst although concentration will now be partial pressure for gases)

Answer 30

1) Temperature
2) pH
(refer to page 39 of notes)

3) Concentration of enzyme: In an enzyme-catalysed reaction, the concentration of enzyme is very small compared to the concentration of the substrate hence the rate of reaction is directly proportional to the concentration of the enzyme.

4) Concentration of the substrate: At a constant concentration of the enzyme, there is a finite number of active sites in the enzyme. At low substrate concentration, not all of the active sites are occupied hence the rate of the reaction increases with increasing substrate concentration until a maximum is reached. Here, the rate is directly proportional to the concentration of the substrate and the reaction is first order wrt substrate. At high substrate concentration, all the active sites are occupied and the enzyme becomes saturated with the substrate. At this point, any increase in the concentration of the substrate will not have any effect on the reaction rate and the reaction is zero-order wrt the substrate.