Chapter 26 Reaction Kinetics

Question 1

State the rate equation.

Answer 1

Rate = k[A]^x [B]^y
- reaction is xth order in A
- reaction is yth order in B
- reaction is (x+y)th order overall
- k = rate constant
- rate in mol dm-3 s-1
- [A] & [B] = concentrations in mol dm-3
- x & y = orders of reaction

Question 2

Relationship of rate of reaction with the concentration of reactants.

Answer 2

Directly proportional to the orders.

Question 3

Define order of reaction.

Answer 3

• How much the concentration of a reactant affects the rate

Question 4

Define overall order of reaction.

Answer 4

• The sum of all the individual orders in the rate equation.

Question 5

Rate constant, k is dependent on? Units?

Answer 5

• Does not depend on the [reactant], but is affected by changes in temperature
• Units = mol dm-3 s-1

Question 6

Define half-life, t1/2.

Answer 6

• Time required for the concentration of a reactant to decrease to half of its initial concentration.

Question 7

Half-life equation.

Answer 7

• t1/2 = ln 2 / k = 0.693 / k

Question 8

What are intermediates?

Answer 8

• Species that appear in a reaction mechanism but not in the overall balanced equation.
• Always formed in an early elementary step & consumed in a later elementary step.

Question 9

Define reaction mechanism.

Answer 9

• The sequence of elementary steps that leads to product formation.

Question 10

Define rate-determining step.

Answer 10

• The slowest step in the sequence of steps leading to product formation.

Question 11

What is half-life’s relationship to concentration in a first-order reaction?

Answer 11

• Half-life is independent of concentration in a first-order reaction
• In a first-order reaction, half-lives are constant.

Question 12

What is used to calculate the initial rate of reaction on a graph?

Answer 12

• Tangent at the start of the reaction

Question 13

What is a zero-order reaction + its graph (rate of reaction against concentration & rate of reaction against time) ?

Answer 13

• Rate does not depend on concentration
• Rate against concentration - A straight line that is parallel to the x-axis
• Rate against time - A straight line showing a constant decline in concentration

Question 14

What is a first-order reaction + its graph (rate of reaction against concentration & rate of reaction against time) ?

Answer 14

• Rate is proportional to the concentration
• Rate against concentration - Straight line of fixed gradient ( gradient = rate constant)
• Rate against time -Slightly-sloping curve which drops with a constant half-life

Question 15

What is a second-order reaction + its graph (rate of reaction against concentration & rate of reaction against time) ?

Answer 15

• Rate is proportional to the square of the concentration
• Rate against concentration - Upwardly sloping curve
• Rate against time - Curve declines steeply & then levels out

Question 16

The 2 rules for reaction mechanism.

Answer 16

• Rate equation must be consistent with the slow step.
• Sum of the elementary steps must give the overall balanced equation for the reaction.

Question 17

The two types of catalysts?

Answer 17

• Heterogeneous - different phase
• Homogeneous - same phase

Question 18

What is the mode of action of a heterogeneous catalyst?

Answer 18

• Adsorption

Question 19

Define adsorption.

Answer 19

• When something sticks to a surface

Question 20

What processes rely on adsorption?

Answer 20

• Iron in the Haber process
• Palladium, platinum and rhodium in the catalytic removal of oxides of nitrogen from the exhaust gases of car engines

Question 21

Mode of action of adsorption.

Answer 21

• Adsorption - Incoming species lands on an active site & forms weak bonds with the catalyst (metal atoms).
• Reaction - Adsorbed gases may be held on the surface in the right orientation for reaction to occur. Increases chances of favourable collisions taking place.
• Desorption - re-arrangement of electrons & products are released from the active sites

Question 22

Characteristics of a good catalyst for adsorption.

Answer 22

• Adsorbs the reactant molecules strongly enough for them to react
• But not so strongly that the product molecules stick permanently to the surface

Question 23

Mode of action of homogeneous catalysts.

Answer 23

• Catalyst & reactants are in the same phase
• Reaction proceeds through an intermediate species with lower energy
• Usually more than one reaction step
• Transition metals are often involved as their oxidation state changes

Question 24

What processes rely on homogeneous catalysts?

Answer 24

• Atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide
• Fe2+ or Fe3+ in the I–/S2O82– reaction

Question 25

Define autocatalysis.

Answer 25

• Reaction is catalysed by one of its products.

Question 26

Explain what is meant by overall order of reaction.

Answer 26

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