Rates & Equilibrium

Question 1

RATE OF REACTION

Answer 1

Change in concentration of a reactant or product with time.

Question 2

ORDER, m

Answer 2

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

Question 3

RATE CONSTANT, k

Answer 3

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 4

HALF-LIFE

Answer 4

The time taken for the concentration of the reactant to reduce by half.

Question 5

RATE DETERMINING STEP

Answer 5

The slowest step in the reaction mechanism of a multi-step reaction.
• If a reactant appears in the rate equation, it must also appear in the rate determining step
• The order with respect to the reactant tells you how many particles of the reactant are involved in the rate determining step.

Question 6

Zero Order Graphs

Answer 6

Rate-Concentration Graph:
• horizontal line parallel to x-axis
• rate is constant

Concentration-Time Graph:
• straight diagonal line going down, with a negative gradient
• half-life decreases with time

Question 7

First Order Graphs

Answer 7

Rate-Concentration Graph:
• straight line through 0,0, going up, with a positive gradient

Concentration-Time Graph:
• curve sloping down
• constant half-life

Question 8

Second Order Graphs

Answer 8

Rate-Concentration Graph:
• curve sloping up, starting at 0,0

Concentration-Time Graph:
• curve sloping down
• half-life increases with time

Question 9

What can be said about the half-life of a first-order reaction?

Answer 9

It is independent of the concentration

Question 10

How can a concentration-time graph be used to determine the rate of reaction at a given time, (t=0)?

Answer 10

• Drawing a tangent to the curve at time, t.

* Finding the gradient of the tangent to the curve

Question 11

Effect of temperature change on rate constant (k) and rate of reaction

Answer 11

If the temperature of a reaction is increased then the rate constant k increases and hence the rate of a reaction also increases

Question 12

Effect of concentration on half life in 1st order reactions

Answer 12

No effect

Question 13

Effect of temperature increase on equilibrium

Answer 13

Equilibrium shifts in the endothermic direction

Question 14

Effect of temperature increase on Kc

Answer 14

In endothermic reaction- Kc increases

In exothermic reaction- Kc decreases

Question 15

Effect of pressure on equilibrium

Answer 15

Shifts in the direction of the side with fewer gaseous moles.

Question 16

Effect of pressure on Kc

Answer 16

No effect

Question 17

Effect of changing concentration on Kc

Answer 17

No effect

Question 18

Effect of changing concentration on equilibrium

Answer 18

Shifts in direction where conc has been minimised until equilibrium is achieved again

Question 19

Effect of catalyst on Kc and equilibrium

Answer 19

No effect Rate of reaction increases in both directions

Question 20

LE CHATELIER’S PRINCIPLE

Answer 20

When a system in dynamic equilibrium is subjected to a change, the equilibrium position will shift to minimise the change.

Question 21

Equilibrium

Answer 21

• rate of forward reaction=rate of reverse reaction

* concentration of reactants and products remain constant

Question 22

Kc >10¹⁰

Answer 22

Very much to RHS

Question 23

Kc >10² (100)

Answer 23

Just to RHS

Question 24

Kc Approximately 1

Answer 24

In the middle

Question 25

Kc below 10² (100)

Answer 25

Just to LHS

Question 26

Kc above 10¹⁰

Answer 26

Very much to LHS

Question 27

How could you use the results from various experiments to show the order with respect to the reactants of a reaction.

Answer 27

• vary one reactant whilst keeping the other constant
• repeat the other way round
• Calculate rate, rate=conc/time
• Relate order to rate/time graph

Question 28

Explain why it is difficult to predict how the yield of a reactant will change.

Answer 28

• Increasing/decreasing temp would have this effect…
• However increasing/decreasing pressure would…
• Relative effect of temp and pressure are unknown - don’t know which has biggest effect