Rates and Equilibrium

Question 1

What is the reaction rate?

Answer 1

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

Question 2

What are ways to measure the rate of a reaction?

Answer 2

Gas volume (if gas was given off)
Colour change (using colorimeter)
Electrical conductivity (if number of ions changes)

Question 3

How can you work out reaction rate from a concentration-time graph?

Answer 3

By repeatedly taking measurements during a reaction you can plot a concentration time graph.
The rate at any point in the reaction is given by the gradient at that point.

Question 4

When will the order with respect to the reactant be 0?

Answer 4

If you double the reactant’s concentration and the rate stays the same.

Question 5

When will the order with respect to the reactant be 1?

Answer 5

If you double the reactant’s concentration and the rate also doubles.

Question 6

When will the order with respect to the reactant be 2?

Answer 6

If you double the reactant’s concentration and the rate quadruples.

Question 7

How can you use a concentration time graph and rate-concentration graph to find the order?

Answer 7

Find the gradient (rate) at various points on the concentration time graph- this gives you a set of points for the rate-concentration graph.
Plot the points and join them up and the ned shape tell you the order.

Question 8

What is the initial rate of a reaction?

Answer 8

Is the rate right at the start of the reaction.

You can find this from a concentration-time graph by calculating the gradient of the tangent at time = 0.

Question 9

What is half life?

Answer 9

The time for half the reactant to disappear.

The half life of a first order reaction is independent of the concentration.

Question 10

How does temperature change affect the rate constant?

Answer 10

Reactions happen when the reactant particles collide and have enough energy to break the existing bonds.
Increasing the temperature speeds up the reactant particles, so that they collide more often and more likely to have activation energy.

Question 11

What is the rate determining step?

Answer 11

Mechanisms can have one step or a series of steps.
In a series of steps, each step can have a different rate.
The overall rate is determined by the slowest step- the rate determining step.

Question 12

Important points to remember about the rate determining step?

Answer 12

1) The rate determining step doesn’t have to be the first step in a mechanism.
2) The reaction mechanism can’t usually be predicted from just the chemical equation.

Question 13

At equilibrium the amounts of reactants and products…

Answer 13

Stay the same.

Question 14

As the reactants get used up…
As more product is formed…
After a while…

Answer 14

…the forward reaction slows down.
…the reverse reaction speeds up.
…the forward and backwards reaction will be going at the same rate.

Question 15

What’s it called when the forward and backwards reaction are going at the same rate?

Answer 15

Dynamic equilibrium.

Question 16

If the position of the equilibrium moves to the left…

Answer 16

…you’ll get more reactants.

Question 17

If the position of the equilibrium moves to the right…

Answer 17

…you’ll get more products.

Question 18

If there’s a change in concentration, pressure or temperature…

Answer 18

…the equilibrium will move to help counteract the change.

Question 19

If you increase the temperature of a reaction…

Answer 19

…the equilibrium will shift in the endothermic direction to absorb the extra heat

Question 20

If you decrease the temperature of a reaction…

Answer 20

…the equilibrium will shift in the exothermic direction to replace the lost heat.

Question 21

If the change means more product is formed…

Answer 21

…kc will increase.

Question 22

If the concentration of one thing in the equilibrium mixture changes…

Answer 22

…then the concentrations of the others must change to keep the value of the Kc the same.

Question 23

Increasing the pressure shifts the equilbrium

Answer 23

…to the side with fewer gas molecules, this reduces the pressure.

Question 24

What effect do catalysts have on the position of the equilibrium?

Answer 24

No effect.

Question 25

Le Chatelier’s principle:

Answer 25

If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

Question 26

A + B -> C + D

Answer 26

r = k [A] [B]2

r- rate of reaction
k- rate constant
[ ]- concentration

Question 27

Individual order

Answer 27

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

Question 28

Experimental determination of the order:

Answer 28

Plot a conc/time graph and calculate the rate (gradient) at points on the curve.
Plot another graph of the rate (y axis) versus the concentration at that point (x axis).
If it gives a straight line, the rate is proportional to concentration (1st order)
If the plot is a curve then it must have another order. Try plotting rate v.(conc.)2
A straight line would mean 2nd order. This methods is based on trial and error.

Question 29

State two features of a system that is in dynamic equilibrium.

Answer 29

rate of forward reaction = rate of reverse reaction concentrations of reactants and products are constant but they are
constantly interchanging

Question 30

Suggest two reasons why it is likely that many reactions cannot take place in one step.

Answer 30

(stoichiometry in) rate equation does not match
(stoichiometry) in overall equation

Collision unlikely with more than 2 ions/species/particles