Rate Equations

Question 1

Define the rate of a reaction?

Answer 1

The change in molar concentration of a substance in a set time, usually a second

Question 2

What does rate equal?

Answer 2

Change in concentration divided by change in time

Question 3

In a graph of concentration against time for a first and second order reaction, why is the line curved?

Answer 3

The reactant is being used up and its concentration decreases more slowly in each successive time interval. This is because as the reaction proceeds, fewer particles per unit volume are available for collisions, so the time taken for a given number of particles to react increases

Question 4

What is the order of reaction?

Answer 4

The proportionality of rate to the concentration of reactants

Question 5

What is first order?

Answer 5

A reaction whose rate is proportional to the concentration of a reactant, reaction is said to be first order with respect to reactant A

Question 6

What is second order?

Answer 6

Where the rate of the reaction is proportional to the concentration of the reactant squared

Question 7

What is zero order?

Answer 7

Where the rate of reaction does not change when the concentration of the reactant changes

Question 8

What is the overall order of the equation?

Answer 8

All the orders with respect to each reactant added together

Question 9

What conditions do and do not affect the rate constant k?

Answer 9

• The temperature affects the k as well as the rate of the reaction
• The pressure or concentration of reactants does not affect k

Question 10

How can k be determined for a zero order graph of concentration against time?

Answer 10

K can be determined from the gradient

Question 11

How can you distinguish between first and second order reactions?

Answer 11

The rate must be determined at different concentrations of reactants and a graph must be plotted of rate against the concentration of one reactant

Question 12

When does k equal the gradient?

Answer 12

For a rate against concentration graph for a first order reaction

Question 13

What are the two ways to plot a graph of concentration against rate?

Answer 13

• The continuous monitoring method - conc. of reactant measured at set time intervals, tangents drawn, gradient of tangents is equal to the rate
• The initial rates method - multiple separate experiments with different starting conc. of a reactant, amount of product measured, tangent at t=0, gradient = initial rate

Question 14

What does the gradient equal on a concentration of reactant against time graph for a zero order reaction?

Answer 14

-k

Question 15

What do you do when trying to determine the order of a reaction with respect to a specific reactant when there is no suitable par of experiments?

Answer 15

Make a new line in the table for a hypothetical experiment where concentrations used are suitable

Question 16

What is the rate-determining step?

Answer 16

The slowest stage in reaction involving more than one intermediate stages
- Stages before and during the rate determining step have an effect on the overall rate of the reaction and appear in the rate equation, stages after do not

Question 17

Who was Svante Arrhenius?

Answer 17

A Swedish scientist who was awarded the Nobel prize for chemistry in 1903. He developed an equation that linked and can be used to calculate values relating to the rate constant, temperature and the activation energy of a reaction