5.1 - Rates, Equilibrium and pH

Question 1

Describe the generalised rate equation.

Answer 1

For the following reaction aA + bB –> cC + dD the rate equation is: rate = k[A]^m x [B]^n
The square brackets indicate the concentrations of the reactants in mol dm-3.

Question 2

Define rate of reaction.

Answer 2

The change in concentration of a substance per unit time. It is usually in mol dm-3 s-1.

Question 3

Define order of reaction.

Answer 3

The power to which the concentration of a reaction is raised in the rate equation.

Question 4

Define overall order of reaction.

Answer 4

The sum of the orders for each reactant.

Question 5

Define rate constant.

Answer 5

The constant of proportionality in the rate equation. The larger the value of k, the faster the reaction goes.
The units of k depend on the overall order of reaction. It must be worked out from the rate equation.
The value of k is independent of concentration and time. It is constant at a fixed temperature.

Question 6

How does the concentration of reactants affect the rate constant?

Answer 6

Concentration will not affect it.

Question 7

How does a catalyst affect the rate constant?

Answer 7

It will increase it.

Question 8

How does temperature affect the rate constant?

Answer 8

it increases if temperature increases.

Question 9

How do you calculate rate constant and units?

Answer 9

k = rate / [A]^m x [B]^n

Units are calculated by dividing the rate units over all the concentration units.

Question 10

How do you deduce the rate of a reaction from a concentration-time graph?

Answer 10

The gradient of a concentration-time graph gives the rate. Draw a tangent for the concentration you’re trying to find the rate for. This allows you to calculate the rate for different concentrations including the initial rate. y / x

Question 11

How can you deduce the order with respect to a reactant from a concentration time graph?

Answer 11

Zero order: a straight line with a constant slope.
First order: downward sloping curve with a constant half life.
Second order: steep downward sloping curve which levels out quicker.

Question 12

What is half-life?

Answer 12

The time taken for the concentration of a reactant to drop to half its original value.

Question 13

How do you calculate k from the half life of a first order reaction?

Answer 13

k = ln x 2 divided by t1/2.

Question 14

How can you deduce the order with respect to a reactant from a rate concentration graph?

Answer 14

Zero order: straight line horizontal to the x axis, rate is independent of concentration.
First order: straight line with constant slope, rate is proportional to concentration.
Second order: upward sloping curve.

Question 15

How do you calculate the rate constant from a first order rate concentration graph?

Answer 15

If the overall reaction is first order then k will be the gradient of the rate concentration graph for the first order reactant.

Question 16

Outline the procedure used to investigate rates via the initial rates method.

Answer 16

Initial rates experiments produce rate-concentration graphs. The initial rates method gathers data for the time it takes to reach a particular point in the reaction. This should be very early in the reaction. You would vary the concentration of one substance and measure the amount of time it takes to reach the end point for each concentration. You would then keep that substance constant and vary the concentration of another substance. This allows you to work out the order of a reaction with respect to each of the reactants.

Question 17

Describe the different techniques that can be used to measure the time it takes to reach a specific point in a reaction for an initial rates experiment.

Answer 17

You are measuring the time for a particular amount of reactant to be used up (or product formed).
The time for a specific volume of gas to be produced could be measured.
The time for an amount of precipitate to be formed could be measured: disappearing cross (less accurate), colorimetry.
The time for a colour change to appear could be measured: colorimetry.
The time for a specific pH to be reached if the reaction produces a change in acidity.

Question 18

What are the key features of clock reactions?

Answer 18

This is an initial rates experimental method where a clear colour change is seen when a certain amount of product has been made. The colour change occurs at exactly the same point in the reaction each time and so can be used to measure the rate. The concentration of a reactant can be changed and the reaction can be timed for the colour change at each concentration. The process can then be repeated for the other reactants.

Question 19

What are the key features of iodine clock experiments?

Answer 19

Most common version of the clock experiment. If iodine is a product of the reaction you can add starch and the solution will turn a very deep blue if iodine is present. Problem - only a tiny amount of iodine is needed for the colour change so if iodine is a product then all solutions will turn blue immediately. Solution - add a substance that reacts with the iodine produced. If the same amount of substance is added each time then the colour change will happen when the same amount of iodine has been produced every time (fixed end point). Thiosulphate is used. When it has all been used up the iodine will react with the starch and the deep blue colour will appear. This allows you to measure the rate at which a specific amount of iodine is produced.