Chapter 10 - Reaction Rates and Equilibrium

Question 1

What is the rate of reaction?

Answer 1

The change in concentration of a reactant or product in a given time

Question 2

What is the equation and units for rate of reaction?

Answer 2

rate = change in concentration/time
units = mol dm-3 s-1

Question 3

How does the rate of reaction change during a reaction?

Answer 3

Rate is fastest at the start of the reaction, as each reactant is at its highest concentration
It slows down as the reaction proceeds as the reactants become used up and their concentrations decrease
Once one of the reactants has been completely used up, the concentrations stop changing and rate = 0

Question 4

What factors can alter the rate of a chemical reaction?

Answer 4

concentration/pressure
temperature
use of a catalyst
surface area of solid reactants

Question 5

What 2 conditions lead to an effective collision?

Answer 5

Particles colliding with correct orientation

Particles having sufficient energy to overcome the activation energy barrier of the reaction

Question 6

How does increasing concentration affect the rate of reaction?

Answer 6

Rate generally increases
This is because there is an increased number of particles in the same volume. There will therefore be more effective collisions and an increased rate

Question 7

How does increasing the pressure of a gas affect the rate of reaction?

Answer 7

Rate increases
The concentration of gas molecules increases as the same number of gas molecules occupy a smaller volume. The gas molecules are closer together and collide more frequently - more effective collisions

Question 8

How can the progress of a chemical reaction be measured?

Answer 8

• monitoring the removal (decrease in concentration) of a reactant
• following the formation (increase in concentration) of a product

Question 9

How can you determine the rate of reaction of a gas producing reaction?

Answer 9

• monitoring the volume of gas produced at regular time intervals using gas collection
• monitoring the loss of mass of reactants using a balance

These are both proportional to the change in concentration of a reactant or product

Question 10

How can you collect gas from a reaction?

Answer 10

Upturned measuring cylinder/burette in water bath

Gas syringe

Question 11

What is a catalyst?

Answer 11

A substance that changes the rate of a chemical reaction without undergoing any permanent change itself

Question 12

How does a catalyst increase the rate of reaction?

Answer 12

It provides an alternative reaction pathway of lower activation energy

Question 13

What is a homogeneous catalyst?

Answer 13

A catalyst that has the same physical state as the reactants

Question 14

How does a homogeneous catalyst work?

Answer 14

The catalyst reacts with the reactants to form an intermediate. The intermediate then breaks down to give the product and regenerates the catalyst

Question 15

What is a heterogeneous catalyst?

Answer 15

A catalyst that has a different physical state from the reactants

Question 16

How does a heterogeneous catalyst work?

Answer 16

Reactant molecules are adsorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place. After reaction, the product molecules leave the surface of the catalyst via desorption.

Question 17

What are the advantages to industry of using a catalyst?

Answer 17

Reduces the temperature needed for industrial processes and the energy requirements.
Requiring less energy means less electricity or fossil fuels - reduce global warming. Making product faster and using less energy can cut costs and increase profitability.

Question 18

What is the Boltzmann distribution?

Answer 18

The spread of molecular energies in gases

Question 19

What does Ea stand for?

Answer 19

Activation energy

Question 20

What are the features of the Boltzmann distribution?

Answer 20

• no molecules have zero energy, the curve starts right at the origin
• the area under the curve is equal to the total number of molecules
• there is no maximum energy for a molecule, the curve does not meet the x-axis at high energy

Question 21

How does increasing temperature affect a Boltzmann distribution curve?

Answer 21

As temperature increases, the average energy of the molecules also increases.
Therefore more molecules have an energy greater or equal to the activation energy - area under graph stays the same so peak lowers and is shifted to right.

Question 22

How does increasing temperature increase the rate of reaction?

Answer 22

More molecules have activation energy, meaning a greater porportion of collisions will lead to a reaction, increasing the rate of reaction. Collisions will also be more frequent as the molecules are moving faster.

Question 23

How does the presence of a catalyst affect a Boltzmann distribution?

Answer 23

In the presence of a catalyst, a greater proportion of molecules exceeds the new, lowered activation energy

Question 24

What are the 2 conditions of an equilibrium system?

Answer 24

Rate of forwards reaction is equal to rate of reverse reaction
The concentrations of reactants and products do not change

Question 25

What is a closed system?

Answer 25

One that is isolated from its surroundings, so the temperature, pressure and concentrations of reactants and products are unaffected by outside influences

Question 26

What does the position of equilibrium indicate?

Answer 26

The extent of a reaction

Question 27

What does Le Chatelier’s principle state?

Answer 27

That when a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of that change

Question 28

What is the effect of concentration changes of a reactant or product on an equilibrium system?

Answer 28

Causes the rate of forward or reverse reactions to change, shifting the position of equilibrium
If more products formed - equilibrium has shifted to right (counteract to left)
If more reactants formed - equilibrium has shifted to left (counteract to right)

Question 29

What does equilibrium position shifting due to temperature changes depend on?

Answer 29

The sign of ∆H

Question 30

What does an increase in temperature do to the position of equilibrium?

Answer 30

Shifts the equilibrium position in the endothermic direction (∆H is positive)

Question 31

What does a decrease in temperature do to the equilibrium position?

Answer 31

It shifts the equilibrium position in the exothermic direction (∆H is negative)

Question 32

How do pressure changes effect an equilibrium?

Answer 32

Changing the pressure of a system containing gases in equilibrium may result in the position of equilibrium changing, but only if there are more gaseous molecules on one side of the equation than the other

Question 33

What is the effect of a catalyst on equilibrium?

Answer 33

Does not change the position of equilibrium, but will increase rate at which equilibrium is established

Question 34

What is the purpose of an equilibrium constant?

Answer 34

Provides the actual position of equilibrium

Question 35

What is the equilibrium law for the equilibrium constant Kc in terms of concentrations?

Answer 35

Kc = [C]^c [D]^d
—————-
[A]^a [B]^b
Where [ ] indicate the concentration, a b c d are the balancing numbers in the overall equation, and [A] [B] etc are the equilibrium concentrations of the reactants and products of this equilibrium

Question 36

What does the magnitude of Kc indicate?

Answer 36

The relative proportions of reactants and products in the equilibrium system

Question 37

What does a Kc value of 1 indicate?

Answer 37

A position of equilibrium that is halfway between reactants and products

Question 38

What does a Kc value > 1 indicate?

Answer 38

A position of equilibrium that is towards the products

Question 39

What does a Kc value <1 indicate?

Answer 39

A position of equilibrium towards the reactants