Rates of Reaction and Energy Changes

Question 1

Core Practical: Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions:

Answer 1

In these experiments, you are investigating the effect on rate of changing the size of the marble chips (smaller chips=larger surface area= faster rate) and also the effect of changing the concentration of hydrochloric acid (greater concentration= greater number of particles in a given volume= faster rate).

Question 2

Practical methods for determining the rate of a given reaction

Answer 2

Rates of reactions can be measured using the amount of product used, or amount of product formed over time.
Quantity of reactant or product can be measured by the mass in grams or by a volume in cm3.
To measure reactant used: if the product is a gas, which will be given off, you can carry out the reaction on a set of weighing scales and measure how much mass is lost.
To measure product formed: if the product is a gas, you can measure the volume of gas produced in a gas syringe.

Question 3

How reactions occur when particles collide

Answer 3

Chemical reactions only occur when the reacting particles collide with enough energy, the minimum amount of energy required is called the activation energy.
In order to increase the rate of a reaction, you need to increase the frequency/energy of collisions, so that more of them reach the activation energy.
This can be done by: increasing temperature, pressure, concentration, surface area or by using a catalyst.

Question 4

The effects on rates of reaction of changes in temperature

Answer 4

Increasing the temperature increases the rate of reaction. As increasing temperature increases the kinetic energy of particles, so they collide more frequently and energetically.

Question 5

The effects on rates of reaction of changes in concentration

Answer 5

Increasing concentration of reacting solutions increases the rate of reaction, as it increases the number of particles in a given volume and so increases the frequency of collisions.

Question 6

The effects on rates of reaction of changes in pressure

Answer 6

Increasing pressure in reacting gases increases the rate of reaction, as it increases the number of particles in a given volume so increases the frequency of collisions.

Question 7

The effects on rates of reaction of changes in surface area of solid reactants

Answer 7

Increasing the surface area of solid reactants increases the rate of reaction, as it increases the frequency of collisions so increases the rate of reaction.

Question 8

What are catalysts?

Answer 8

Catalysts are substances that speed up chemical reactions without being changed or used up during the reaction. They are the same and have the same mass at the end of the reaction.

Question 9

Explain how the addition of catalyst increase the rate of a reaction in terms of activation energy

Answer 9

Catalysts decrease the activation energy; this increases the proportion of particles with energy to react, leading to more particles with energy to react, leading to more frequent successful collisions and so an increased rate of reaction.

Catalysts lower the activation energy by providing a different pathway for a chemical reaction that has a lower activation energy

Question 10

Recall that changes in heat energy accompany the following
changes: salts dissolving in water

Answer 10

Salts dissolving in water is either exothermic or endothermic.

Question 11

Recall that changes in heat energy accompany the following
changes: neutralisation reactions

Answer 11

Neutralisation reaction is exothermic.

Question 12

Recall that changes in heat energy accompany the following
changes: displacement reactions

Answer 12

Displacement is an exothermic or endothermic reaction.

Question 13

Recall that changes in heat energy accompany the following
changes: precipitation reactions

Answer 13

Precipitation is an exothermic reaction.

Question 14

What happens when these reactions take place in a solution?

Answer 14

When these reactions take place in solution,
temperature changes can be measured to reflect the heat changes.

Question 15

Describe an exothermic change

Answer 15

An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
Examples of exothermic reactions include; combustion, many oxidation reactions and neutralisation.
Everyday examples of exothermic reactions include; self-heating cans (e.g for coffee) and hand warmers.

Question 16

Describe an endothermic change or reaction as one in which heat energy is taken in

Answer 16

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
Examples of endothermic reactions are thermal decomposition and the reaction of citric acid and sodium hydrogen carbonate.

Question 17

Describe the making and breaking of bonds

Answer 17

During a chemical reaction:
Energy must be SUPPLIED to BREAK bonds in the reactants.
Energy taken in to break > energy released when formed = ENDOTHERMIC (because overall energy has been taken in).
Energy is RELEASED when bonds in the products are FORMED.
Energy taken in to break < energy released when formed = EXOTHERMIC (because overall energy has been released).

Question 18

Calculate the energy change in a reaction given the energies of bonds (in kJ mol-1)

Answer 18

Sum of energy taken in to break bonds – sum of energy released to form bonds = overall energy change
If the energy out > energy in, the energy change will be negative showing an exothermic reaction and if the energy out < energy in, the energy change will be positive showing an endothermic reaction.

Question 19

Explain the term Activation energy

Answer 19

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
Activation energy = minimum amount of energy that particles must have to react.

Question 20

Draw and label reaction profiles for endothermic and exothermic reactions, identifying activation energy

Answer 20

For the exothermic diagram, the products have less energy than the reactants, because the energy has been released to the surroundings.
For the endothermic diagram, the reactants have less energy than the products, because the energy has been taken in from the surroundings.