Topic 10 - Rates Of Reaction

Question 1

Activation energy

Answer 1

The energy needed to start a reaction.

Question 2

Exothermic reaction

Answer 2

Transfers heat energy to the surrounding so the temperature of the surroundings increases.

Question 3

Endothermic reaction

Answer 3

Absorbs heat energy from the surroundings so the temperature of the surroundings decreases.

Question 4

Breaking bonds

Answer 4

Endothermic

Question 5

Making bonds

Answer 5

Exothermic

Question 6

Acronym to remember breaking bonds and making bonds?

Answer 6

Bendomex

Question 7

Energy profile diagram:
Exothermic reaction

Answer 7

Overall, more heat energy is released in the making of bonds in the reactants, than is needed to break the bonds in the reactants.

Question 8

Energy profile diagram:
Endothermic reaction

Answer 8

More heat energy is needed in the breaking of bonds in the reactants than is released in the making of the bonds on the reactants.

Question 9

Bond energies

Answer 9

Total energy change = Total energy in - Total energy out

Question 10

What is needed for a reaction to occur?

Answer 10

• For a reaction to occur particles need to collide.
• The particles can be a molecule, ion or atoms.
• The particles need to collide with enough energy for particles to reaction - the activation energy is the minimum energy needed.

Question 11

Effective collision/Successful collision

Answer 11

Collisions between atoms or molecules that successfully transfer enough energy for the atoms or molecules to react together.

Question 12

Four way in which a reaction can be increased?
The collision model helps explain this.

Answer 12

1) Increasing the temperature.
2) Increasing the surface area.
3) Increasing the concentration in a reacting solution.
4) Increasing the pressure in a reacting gas.

Question 13

Concentration

Answer 13

• Increases the number of reactant particles in the same volume.
• Therefore collision occur more often
• Therefore rate increases.

Question 14

Temperature

Answer 14

• The higher the temperature, the faster the particles move.
• Therefore, particles will collide more frequently.
• And with more energy meaning more successful collisions
• Therefore rate increases.

Question 15

Surface area

Answer 15

• The greater the surface area, the more particles there are on the surface where collisions happen.
• Therefore, there will be more collisions per second.
• Therefore rate increases.

Question 16

Mean rate of reaction

Answer 16

A measure of how quickly a reacting is used up, or a product is formed.

Quantity of reaction used / Time
OR
Quantity of product formed / Time

Question 17

How to calculate rate of reaction at a certain time?

Answer 17

Draw a tangent on the curve at the specified time and use this equation to calculate the gradient.

Question 18

Catalysts

Answer 18

A substance that speeds up the rate of a reaction without altering the products of the reaction, being itself unchanged chemically and in mass at the end of the reaction.

Question 19

How do catalysts reduce costs in industrial processes?

Answer 19

• By making products more quickly.
• By allowing reactions to occur at lower temperatures.
• Catalysts do not usually need to be replaced because they are not used up.

Question 20

How does a catalyst increase the rate of reaction?

Answer 20

• A catalyst provides an alternative reaction route, which requires less activation energy.
• The catalyst does not alter the overall energy change.
• As less activation energy is needed to start the reaction, more reactant molecules have enough energy and so more collisions are successful.

Question 21

What are enzymes?

Answer 21

Large complex protein molecules that act as catalysts in biological reactions.

Question 22

How are enzymes used in the production of alcoholic drinks?

Answer 22

• Alcoholic drinks are produced using an enzyme in yeast.
• This enzyme catalyses the reaction in which glucose is converted into ethanol (alcohol) and carbon dioxide.

Question 23

Core practical:
Rates of reaction with syringe

Answer 23

1) Set up gas collection apparatus. Clamp the syringe in place.
2) 50cm3 2M Hydrochloric Acid.
3) 5g of marble chips.
4) QUICKLY place bung into conical flask AND start the timer.
5) Measure and record volume on gas syringe every 10 s, for 100s.
6) Decant out any excess liquid, discard any solid and rinse.
7) Repeat the experiment with different sized marble chips or different concentration of acid.

Question 24

Core practical:
Rates of reaction with syringe
Word equation and balanced equation

Answer 24

Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

Question 25

Core practical:
Temperature and rate of reaction

Answer 25

1) Add 20cm3 of sodium thiosulfate into boiling tube.
2) Add 2cm3 of hydrochloric acid into boiling tube.
3) Place both boiling tubes into a rack in a water bath for 5 minutes.
4) Pour the sodium thiosulphate and acid into the conical flask.
5) Swirl flask Place on cross Start timer.
6) Stop timer when cross has disappeared – record time.
7) Repeat experiment using water baths at different temperatures.
□ 20OC □ 30OC □ 40OC □ 50OC

Question 26

Core practical:
Temperature and rate of reaction
Word equation and balanced equation

Answer 26

Sodium thiosulfate + hydrochloric acid —> Sodium chloride + water + sulfur dioxide + sulfur

Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)

Question 27

The effect of temperature on the rate of reaction?

Answer 27

For a reaction to happen the reactant particles need to collide with enough energy to react. (Activation energy)

Increasing temperature increases the rate of reaction because when the temperature rises:
• The particles move faster and the particles collide more often.
• More of the collisions that happen are successful.
• There are more collisions every second.

Question 28

Reversible reaction

Answer 28

Where the products can react and reform the reactants.

Question 29

Dynamic equilibrium

Answer 29

A dynamic equilibrium occurs in a closed system, when forward and backward reactions happen at the same time and therefore there is no overall change in the concentrations of reactants or products.

There are no observable changes.

Question 30

What must occur for dynamic equilibrium to occur?

Answer 30

• Reversible reaction.
• Closed system - none of the reactants or products can escape.

Question 31

Haber process

Answer 31

During the Haber process, nitrogen and hydrogen react to form ammonia: N2 + 3H2 <—> 2NH3

The nitrogen is obtained from the air.
The hydrogen can be extracted from hydrocarbons from sources such as natural gas and crude oil.

The Haber process is carried out at 450°C with a pressure of 200 atmospheres and an iron catalyst.

Question 32

What 3 things can change the position of equilibrium?

Answer 32

• Temperature
• Pressure
• Concentrations

Question 33

Temperature

Answer 33

Increase in temperature means that equilibrium position shifts left because reverse reaction is endothermic.

Decrease in temperature means that equilibrium position shifts right because reverse reaction is exothermic.

Question 34

Pressure

Answer 34

Increase in pressure favours side with fewer molecules.

Decrease in pressure favours side with more molecules.

Question 35

Concentration

Answer 35

Increase in concentration favours side with lower concentration.

Decrease in concentration favours side with higher concentration.

Question 36

Why is 450° chosen for the Haber process?

Answer 36

• High temperatures give a faster rate of reaction.
• Low temperatures favour the production of ammonia.
A compromise is reached.

Question 37

Why is 200 atmospheres chosen?

Answer 37

• High pressures increase rate of reaction.
• However it can be dangerous and expensive.
A compromise is reached.

Question 38

Why is an iron catalyst used?

Answer 38

Catalysts do not change the equilibrium concentrations of reacting substances in reversible reactions. However, they do reduce the time taken to reach equilibrium.
• Iron is a cheap catalyst used in the Haber process.
• It helps to achieve an acceptable yield in an acceptable time.

Question 39

Le Chatelier’s Principal

Answer 39

When something changes in a reaction, the equilibrium will always shift to oppose the change.