Kinetics

Question 1

Define rate of reaction

Answer 1

The change in concentration of a product (or reactant) per unit time.

Question 2

How to find the rate of reaction at a given time

Answer 2

Plot graph of concentration of product (or reactant) against time during the reaction. Draw a line of best fit. The rate is the magnitude of the gradient so draw a tangent to the curve at the time you want and find its gradient.

Question 3

What does the rate expression for a reaction show?

Answer 3

How the rate of a chemical reaction depends on the concentrations of the various chemical species involved.

Question 4

What is the rate constant, k?

Answer 4

The constant of proportionality in the rate expression. It is different for every reaction and varies with temperature. If the concentrations of all the species in the rate equation are 1M then the rate of reaction is equal to the value of k.

Question 5

What is the order of reaction?

Answer 5

The sum of the powers to which the concentrations of all the species involved in the reaction are raised. For the order with respect to a certain species, it is the power that just that specie’s concentration is raised to.

Question 6

Can catalysts appear in the rate equation?

Answer 6

Yes

Question 7

How to find units of k

Answer 7

Rearrange the rate equation but subbing in mol/dm^3 for each species and mol/dm^3 for the rate. Still raise units to powers.

Question 8

How to find order of reaction with respect to a particular species, A, using rate-concentration graphs

Answer 8

Plot graph of [A] against time. Draw tangents to line at different points to find rates at different concentrations of A. Plot a second graph of rate against [A]. If graph is horizontal straight line, order is 0. If graph is diagonal straight line through origin, order is 1. If graph is not a straight line, order is greater than 1.

Question 9

How to find order of reaction with respect to a particular species, A, using initial rate method

Answer 9

Series of experiment carried out at constant temperature with different combinations of initial concentrations of reactants and catalysts, etc. Between any pair of experiments only the concentration of one reactant changes. Time 0 when reactants mixed. Concentration-time graph are plotted and tangent drawn at t=0 to find initial rate. The concentrations of all substances are known exactly at this time. Compare concentrations of reactants and rates to find orders. Or do graph of 1/time (as measure of rate) against [A] graph and interpret gradient.

Question 10

How to find rate constant

Answer 10

Sub in any set of values for rate and all species in rate equation. Rearrange the equation to find k.

Question 11

Why does the rate constant increase with temperature?

Answer 11

Temperature is the measure of the average kinetic energy of particles. Particles will only react if their collisions have enough energy to start bond breaking, Ea. As temperature increases, a greater proportion of molecules have enough energy to react so there are more frequent successful collisions.

Question 12

What is the Arrhenius equation?

Answer 12

k=Ae^(-Ea/RT)

k is rate constant
A is Arrhenius constant
R is gas constant
T in temperature in K

Question 13

Graph of lnk against 1/T

Answer 13

Straight line. Gradient of -Ea/R. y-intercept of lnA. Can use it to find Ea by measuring rate at different temperatures.

Question 14

What does the e^(-Ea/RT) tell you?

Answer 14

The fraction of molecules with energy greater than the activation energy.

Question 15

What is a reaction mechanism?

Answer 15

The series of simple steps that lead from reactants to product in a chemical reaction

Question 16

What is the rate determining step?

Answer 16

The slowest step in the reaction mechanism. It governs the rate of the overall reaction. Any step that occurs after the rate determining step will not affect the rate, so the species involved don’t appear in the rate expression. Species up to and including reactants for RDS are in rate equation.

Question 17

Methods to measure rate of reaction at different times throughout the whole reaction.

Answer 17

If there is a colour change, can use colorimeter to measure the concentration of product or reactant. Take readings at suitable equal time intervals. If the reaction is too fast, a computer or data logger can be used to record the readings. If a gas is produced, you can collect it in a gas syringe and record the volume collected at suitable equal time intervals.