18 - Rates of Reaction

Question 1

How do you calculate rate?

Answer 1

Rate = change in concentration (mol dm-3) / change in time (s)

Question 2

What is reaction rate?

Answer 2

Is the change in the amount of reactants or products per unit time.

Question 3

What is the units for rate?

Answer 3

mol dm^-3 s^-1

Question 4

What are four ways of following the rate of reaction?

Answer 4

• Measure the volume of gas evolved.
• Measure the loss in mass as a gas is evolved.
• Use Colorimetry to measure the colour change of a reaction.
• Measure the pH change of a reaction.

Question 5

What is the rate constant (k)?

Answer 5

The constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation.

Question 6

What is the rate equation definition?

Answer 6

The mathematical relationship between the concentrations of the reactants and the reaction rate.

Question 7

What is the actual mathematical rate equation

Answer 7

Question 8

What is order?

Answer 8

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

Question 9

What is the overall order?

Answer 9

The sum of the individual order of reactants in the rate equation: m + n

Question 10

How can you only find orders of reaction?

Answer 10

From experiments. Can’t work them out from chemical equations.

Question 11

What does a zero order reaction look like and what does it mean?

Answer 11

If you double the reactants concentration then the rate will stay the same.

Question 12

What does a first order reaction look like and what does it mean?

Answer 12

If you double the reactants concentration then the rate will also double.

Question 13

What does a second order reaction look like and what does it mean?

Answer 13

If you double the concentration then the rate will quadruple. (2 squared = 4)

Question 14

How do you calculate the units of the rate constant (k)?

Answer 14

1. Rearranging the equation to make k the subject.
2. Substitute units into the expression for k.
3. Cancel common units.

Question 15

For a generic reaction: A + B —> C

What are the two experimental data used to work out the order?

Answer 15

1. Continuously monitoring the change in concentration of A against time, to construct a rate-concentration graph.
2. Uses an initial rates method to find out how the initial rate changes as you vary the concentration of A.

B must stay constant so that the change in rate can only be due to a concentration in A so you can work out the order of A.