C10- Reaction Rates And Equilibrium

Question 1

What is meant by rate of reaction?

Answer 1

The rate of a chemical reaction measures how fast a reactant is being used up or a product forms.
- can be defined as the change in concentration of a reactant/product within a given time.

Question 2

Formula for rate of reaction?

Answer 2

Rate (moldm-3s-1) = change in conc (moldm-3) / time(s)

Question 3

Describe the change in rate as the reaction takes place.

Answer 3

1. The rate of reaction is the fastest at the start of the reaction, as each reactant is at its highest conc.
2. Rate slows down as reaction proceeds as reactants are being used up and conc decreases.
3. Once reactants are completely used up, concentrations stop changing and rate is zero.

Question 4

Factors affecting rate of reaction?

Answer 4

1. Temperature.
2. Use of catalyst.
3. Conc (pressure if gases)
4. Surface area of solid reactants.

Question 5

What is the collision theory?

Answer 5

States that two reacting particles must collide in order for a reaction to occur.
- usually only a small proportion of collisions result in a chemical reaction.

Question 6

When is a collision effective?

Answer 6

When:

• the particles collide with the correct orientation.
• particles have sufficient energy to overcome the activation barrier of the reaction.

Question 7

Effect of increasing concentration on rate of reaction?

Answer 7

Increasing conc = increases rate.

• increased number of particles in the same volume.
• particles are closer and collide more frequently.
• more successful collisions within a given time.

Question 8

Effect of increasing pressure of a gas on rate of reaction?

Answer 8

Increased pressure = increased rate.

• the same numbers of gas molecules occupy a smaller space (conc increases).
• more frequent successful collisions in given time.

Question 9

How can you follow the progress of a chemical reaction?

Answer 9

1. Monitoring the removal (decrease in conc) of a reactant.

2. Following the formation (increase in conc) of a product.

Question 10

How can you determine rate of a reaction that produces gas?

Answer 10

1. Monitor the vol of gas produced using a gas syringe or inverted measuring cylinder in water.
   - plot the volume of gas against time and calculate gradient at given point to find rate.
   (Initial rate= tangent at 0)
2. Monitor the loss of reactants using a balance. Reaction is complete when there is no more change in mass. Plot graph of mass lost against time.

Question 11

what is a catalyst?

Answer 11

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

• not used up in reaction.
• may react with a reactant to form an intermediate. Or may provide a surface on which the reaction can take place.
• regenerated at the end of the reaction.

Question 12

How does a catalyst work?

Answer 12

Increases the rate of a reaction by providing an alternative pathway of lower activation energy.

Question 13

Enthalpy profile diagram?

Answer 13

• reactants and products should be shown at the correct levels with respect to eachother.
• 🔺H (enthalpy change) should be shown by an arrow pointing in correct direction.
• activation energy shown with upward arrow.
  Learn endo and exo diagrams from book!

Question 14

Homogeneous catalyst?

Answer 14

Has the same physical state as the reactants.
- it reacts with the reactants to form an intermediate.
- intermediate breaks down to give the product and regenerates the catalyst.
(Eg. Cl radicals are the catalysts in ozone depletion)

Question 15

Heterogeneous catalyst?

Answer 15

Has a different physical state than the reactants.
- are usually solids in contact with gaseous/aq reactants.
- reactant molecules are adsorbed (weakly bonded) onto the surface of the catalyst where the reaction takes place.
- after reaction, product molecules leave the surface by desorption.
(Eg. Fe is haber process. Ni in hydrogenation of alkenes.)

Question 16

Sustainability and economic importance of catalysts?

Answer 16

• by lowering activation energy they reduce the temperature and energy requirements needed for the process.
• if a chemical reaction requires less energy, less electricity/fossil fuels are used. Increasing rate can cut costs and increase profitability.
• using less fossil fuels cuts co2 emissions, and so global warming.

Question 17

What is the Boltzmann distribution?

Answer 17

In a gas, liquid or solution some molecules move slowly with low energy and some move fast with high energy.
Most move close to average speed and have around average energy.
This spread of molecular energies in gases is known as the boltzmann distribution.

Question 18

Features of the boltzmann distribution curve?

Answer 18

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

Question 19

Effect of temp on boltzmann distribution.

Answer 19

At higher temperature:

• more molecules with energy greater than or equal to activation energy.
• therefore greater proportion of successful collisions so rate increases.
• collisions are also mire frequent as molecules are moving faster but increased energy is more important.
• area under graph is still the same as no of molecules is unchanged.

Question 20

Effect of catalysts on boltzmann distribution.

Answer 20

Catalyst provides an alternative reaction route with lower activation energy.

• greater proportion of molecules now have energy equal to or greater than activation energy.
• therefore on collisions more molecules will react to form products.
• rate increases.

Question 21

What is a reversible reaction?

Answer 21

Reactions that take place in both forward and reverse directions.
- usually important in industrial processes. (Eg. Haber process in manufacturing ammonia)

Question 22

Features of an equilibrium system?

Answer 22

• the rate of the forward reaction = rate of the reverse reaction.
• therefore concentrations of reactants and products do not change.
  Equilibrium systems are dynamic:
• at equilibrium, both the forward and reverse reactions are taking place at the same time.
  The system must be closed in order to remain in equilibrium.

Question 23

Define a closed system.

Answer 23

A closed system is isolated from its surroundings, so the temperature, pressure, and concentrations of reactants and products are unaffected by outside influences.

Question 24

What is le Chatelier’s principle?

Answer 24

States that when a system in equilibrium is subjected to an external change, the position of equilibrium shifts to cancel out the change.

Question 25

Effect of concentration changes on equilibrium.

Answer 25

Changing concentration of a reactant or product will change the rate of the forward/reverse reaction and so position of equilibrium will also change:
- if there are more products formed, position of equilibrium has shifted right.
- if there are more reactants formed, position of equilibrium has shifted left.
You can see this using a reaction where the products and reactants are different colours.

Question 26

Effect of temperature changes on equilibrium.

Answer 26

The direction in which equilibrium shifts depends on the sign of 🔺H.

• an increase in temperature shifts the equilibrium position in the endothermic direction (delta H is positive).
• a decrease in temp shifts equilibrium in the exothermic direction (delta H is negative).

Question 27

Effect of pressure changes on equilibrium.

Answer 27

Changing pressure in a system of gases will only cause position of equilibrium to shift if there are more moles on one side.

• increase in pressure = shifts towards fewer gaseous molecules.
• decrease in pressure = shifts towards more gaseous molecules.

Question 28

Effect of catalyst on equilibrium.

Answer 28

Catalyst does not change position of equilibrium.

• it speeds up the rate of the forward and reverse reactions equally.
• it does i crease the rate at which an equilibrium is established.