Module 2

Question 1

How do you write the rate law for the following reaction? a A + b B → c C + d D

Answer 1

Can use any reactant to write the rate law.

• 1 d [A]
  ———
  a dt

Question 2

What is the unit for rate law?

Answer 2

Concentration/time - usually mol L-1 s-1

Question 3

What does the rate law describe?

Answer 3

How the rate depends on the concentration. (rate constant multiplied by the concentration of acid and/or base reactants) - rate = k [A]x [B]y

Question 4

What will the rate law be for the general reaction of

a A + b B → c C + d D

Answer 4

rate = k [A]x [B]y

x and y are exponents.

x is the order with respect to A.
y is the order with respect to B.
x + y is the overall order of the reaction.

Question 5

How can the values of the orders in the rate law be obtained?

Answer 5

They can only be obtained experimentally, they can not be obtained from the stoichiometry of the reaction.

Question 6

What does the order of the reactant(s) tell us?

Answer 6

The speed at which is reacts and forms into the product

Question 7

How could be tell if the rate order of a reactant is 0?

Answer 7

If we varied the concentration of the reactant, the rate would stay the same (as anything to the power of 0 = 1, so rate law = k).

Question 8

How could we tell if the rate order of a reactant is 1?

Answer 8

If we doubled the concentration of the reactant, the rate would also double, and if we halved the concentration of the reactant, the rate would also be halved.

Question 9

How could we tell if the rate order of a reactant is 2?

Answer 9

The rate is equal to the square of the concentration, so if the rate were to equal 4x10-3 at a concentration of 0.2, if the concentration were doubled to 0.4, the rate would be the the concentration to the power of 2, hence 16x10-3.

Question 10

When trying to out the reaction order for a reactant, why measure the initial reaction rate?

Answer 10

Because if the reaction order of the reactant of being investigated, is non-zero, the reaction rate will decrease over time. (as if the order = 0 it is a linear rate).

Question 11

What are the 3 factors that affect the rate at which molecules react?

Answer 11

• collision rate
• collision effectiveness
• collision orientation

Question 12

What factors affect collision rate?

Answer 12

• higher concentrations (more particles available to react).

- Greater speed (kinetic energy - from heat) = more collisions

Question 13

What is collision effectiveness?

Answer 13

Collisions must have enough energy to break bonds or form bonds. This energy threshold is called the activation energy (Ea).

Question 14

How can we improve the collision effectiveness?

Answer 14

Increase the temperature, giving molecules more force, as an increase in temperature gives molecules more energy meaning the energy of the molecules can exceed activation energy.

Question 15

What is collision orientation?

Answer 15

Molecules must collide in the correct orientation, for a successful collision.

Question 16

What is molecularity?

Answer 16

The total number of molecules that must collide for a reaction to occur.

1 molecule - unimolecular
2 molecules - bimolecular
3 molecules - termoleular (unlikely, hence we have elementary reactions).

Question 17

What is an elementary reaction?

Answer 17

The steps in an overall reaction, which has an intermediate (a molecule in both equations that doesn’t appear in the products of the final reaction.

Question 18

What is the process of breaking of the bonds a protein called, and under what circumstances does it occur?

Answer 18

When the temperature exceeds the core human body temperature there is sufficient molecular motion to overcome interactions that maintain protein structure. This is called denaturation.

Question 19

Why can we not predict the rate law from the overall chemical equation?

Answer 19

Because almost all reactions take place in a series of steps called elementary reactions.

Question 20

Why do reaction intermediates not appear in the rate law?

Answer 20

Because they are not a product or reactant.

Question 21

What is a series of elementary steps describing an overall reaction called?

Answer 21

A reaction mechanism.

Question 22

Are you allowed to write the rate laws from a chemical reaction for elementary reactions?

Answer 22

Yes

Question 23

Can rate laws ever be proven?

Answer 23

No, they can only be supported by experimental rate laws, it can never prove it.

Question 24

What is the rate determining step?

Answer 24

The elementary step of a reaction that is considered the slowest. The predicted rate law can be written down from the stoichiometry of the rate determining step.

Question 25

Which step has the highest activation energy in a reaction mechanism?

Answer 25

The rate determining step, as it is the longest step.

Question 26

If the 1st elementary reaction has the highest Ea, what does this mean?

Answer 26

That this step is the rate determining step.

Question 27

What does a catalyst do?

Answer 27

Increases the rate of reaction, without being used up (provides an alternative pathway with a lower Ea).

Question 28

What is an induced fit?

Answer 28

When a substrate binds onto a enzyme and both structures can change.

Question 29

What is the enzyme-substrate complex?

Answer 29

When the substrate is attached to the active site of the enzyme.

Question 30

What is the difference between a lock and key fit and an induced fit with an enzyme?

Answer 30

Lock and key - substrate and active site of the enzyme have the right shape for each other and fit together.

Induced fit - shape of the active site can change to fit the substrate appropriately.

Question 31

How is the activation energy (Ea) for a reaction lowered and made faster by a enzyme-substrate complex?

Answer 31

The enzyme helps hold the substrate (reactant) in the correct orientation for the reaction to occur.

Question 32

Increasing the number of substrates will continue to increase the rate of reaction until…

Answer 32

All the enzymes are being used, hence the reaction can not proceed any faster (Vmax)

Question 33

If the substrate is limiting..

Answer 33

Increasing substrates will increase rate of reaction, until all enzymes are being used.

Question 34

If enzyme is limiting..

Answer 34

Increasing substrates will not increase the rate of reaction.

• there is a maximum velocity (Vmax)