4.3 How Fast? - Rates

Question 1

What is reaction rate?

Answer 1

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

Question 2

What is the purpose of a rate equation?

Answer 2

A rate equation tells you how the rate is affected by different concentrations of reactants.

Question 3

What is the order of reaction with a respect to a reactant?

Answer 3

The order tells you how the concentration effects the rate.

Question 4

How do you work out the overall order of a reaction?

Answer 4

The overall order of reaction is the sum of the order of the reactants.

Question 5

What is the rate constant, k?

Answer 5

It relates concentrations to the rate. A greater k means a faster reaction. It is constant for a certain reaction at a particular temperature.

Question 6

What is the half-life of a reaction?

Answer 6

The time it takes for the concentration of a reactant to half.

Question 7

What is the rate-determing step?

Answer 7

The slowest step in a reaction mechanism. Reactants that appear in the rate equation are involved in the rate-determining step.

Question 8

What is the activation energy of a reaction?

Answer 8

The minimum amount of kinetic energy that particles need to react.

Question 9

What is a homogenous catalyst?

Answer 9

A catalyst that is in the same state as a reactant.

Question 10

What is a heterogenous catalyst?

Answer 10

A catalyst that is in a different physical state from reactants.

Question 11

Name one example of a homogenous catalyst.

Answer 11

Enzymes.

Question 12

Name threes examples of heterogeneous catalysts.

Answer 12

Vanadium peroxide in the contact process of making sulfuric acid. Nickel in the hydrogenation of vegetable oils. Platinum in catalytic converters.

Question 13

Name fives properties that may changes as a reaction progresses.

Answer 13

Gas Volume. Loss of mass. Colour change. Clock Reaction. Electrical conductivity.

Question 14

How can you use gas volume to follow the progress of a reaction?

Answer 14

Collect it in a gas syringe and record volume at regular time intervals.

Question 15

How can you use loss of mass to follow the progress of a reaction?

Answer 15

Carry out the reaction on a balance and record mass at regular time intervals.

Question 16

How can you use colour change to follow the progress of a reaction?

Answer 16

Using a colorimeter.

Question 17

How can you follow the progress of a clock reaction?

Answer 17

Measure the time it takes for colour change to happen. A shorter time means a faster rate.

Question 18

Why can you use electrical conductivity to follow the progress of a reaction?

Answer 18

As the number of ions change, the electrical conductivity of the solution will also change.

Question 19

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

Answer 19

Draw a tangent and find the gradient.

Question 20

If you increase a reactant’s concentration by x and the rate is unchanged, the order with respect to that reactant is…

Answer 20

0

Question 21

If you increase a reactant’s concentration by x and the rate increases by x, the order with respect to that reactant is…

Answer 21

1

Question 22

If you increase a reactant’s concentration by x and the rate increases by x^2, the order with respect to that reactant is…

Answer 22

2

Question 23

If a concentration-time graph for a reactant has a straight line, the order with respect to that reactant is…

Answer 23

0

Question 24

If a concentration-time graph for a reactant has a line with constant half-life, the order with respect to that reactant is…

Answer 24

1

Question 25

If a concentration-time graph for a reactant has a half-life that isn’t constant, the order with respect to that reactant is…

Answer 25

2

Question 26

If a rate-concentration graph for a reactant has a horizontal line, the order with respect to that reactant is… because…

Answer 26

0 because changing the concentration does not affect the rate.

Question 27

If a rate-concentration graph for a reactant has a straight line through the origin, the order with respect to that reactant is… because…

Answer 27

1 because the rate is proportional to the concentration of the reactant.

Question 28

If a rate-concentration graph for a reactant has a curve, the order with respect to that reactant is… because…

Answer 28

2 because the rate is proportional to [reactant]^2

Question 29

How would you find the activation energy of a reaction?

Answer 29

Plot ln(1/t) vs 1/T. Find the gradient and multiply it by -R where R is the gas constant (8.31 JK^-1mol^-1)

Question 30

What does a catalyst do?

Answer 30

A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged after the reaction.

Question 31

Primary halogenoalkanes are hydrolysed by OH- ions by which mechanism?

Answer 31

Sn2 because two molecules are involves in the rate determining step.

Question 32

Tertiary halogenoalkanes are hydrolysed by OH- ions by which mechanism?

Answer 32

Sn1 because only one molecule is involved in the rate determining step.

Question 33

What is the rate equation for the reaction of bromoethane with hydroxide ions and why?

Answer 33

rate=k[CH3CH2Br][OH-] because the hydrolysis happens in one step using both the bromoethane and hydroxide ion.

Question 34

How do you calculate the half-life of a zero order reactant?

Answer 34

[reactant]/2k

Question 35

How do you calculate the half-life of a first order reactant?

Answer 35

ln2/k

Question 36

How do you calculate the half-life of a second order reactant?

Answer 36

1/k[reactant]