Topic 3- Chemical Reactions

Question 1

Why do chemical reactions occur?

Answer 1

Elements can achieve a more stable energy state by gaining a full outer shell of electrons

Question 2

What is chemical bonding?

Answer 2

OId bonds are broken and new bonds are formed

Question 3

What does chemical bonding involve?

Answer 3

Transfer of energy into and out of reaction mixtures (system and surroundings), energy comes from chemical bonds themselves and is often in form of heat, light or sound

Question 4

What happens in exothermic reactions?

Answer 4

Energy is given out to surroundings so temperature of surroundings increase
> Energy transferred from chemical energy store of chemical system so energy of system falls
> Overall transfer is from system to surroundings

Question 5

Examples of exothermic reactions

Answer 5

• Combustion
• Oxidation
• Neutralisation

Question 6

What are endothermic reactions?

Answer 6

Energy is taken in from surroundings so temperature of surroundings decreases
> Energy transferred to chemical energy store of chemical system so energy of system increases
> Overall transfer is from surroundings to system

Question 7

Examples of endothermic reactions

Answer 7

• Less common than exothermic reactions
• Electrolysis, thermal decomposition and first stages of photosynthesis

Question 8

How can you measure energy transfer?

Answer 8

• Changes in heat content can be determined and measured with thermometer or temperature probe

Question 9

Conservation of energy during chemical reaction

Answer 9

• Overall amount of energy does not change as energy is conserved in reactions
• Cannot be created or destroyed but can only be transferred

Question 10

What is activation energy?

Answer 10

When atoms react with each other, they must come into contact with each other in a collision. A minimum amount of energy is required for collision to be successful (particles to react) and this is called activation energy

Question 11

What is the importance of activation energy?

Answer 11

Different reactions have different activation energies and higher activation energies require more energy to start to those with lower activation energies.

Question 12

What is a reaction profile?

Answer 12

• Also called energy level diagrams
• Graphical representations of the relative energies of the reactants and products in chemical reactions

Question 13

How are reaction profiles laid out?

Answer 13

• The energy of the reactants and products are displayed on the y-axis and the reaction pathway (a bit like time) is shown on the x-axis
• The difference in height between the energy of reactants and products represents the overall energy change of a reaction.

Question 14

What is the activation energy in a reaction profile?

Answer 14

Initial increase in energy

Question 15

What is shown in a reaction profile where energy is given out in exothermic reaction?

Answer 15

• The energy of the products will be lower than the energy of the reactants, so the change in energy is negative
• Downwards arrow in reaction profile

Question 16

What is shown in a reaction profile where energy is taken in a endothermic reaction?

Answer 16

• The energy of the products will be higher than the energy of the reactants, so the change in energy is positive
• Upwards arrow in reaction profile

Question 17

What process is bond breaking?

Answer 17

• Endothermic process as energy is needed to break bonds which is absorbed from reaction surroundings

Question 18

What process is bond forming?

Answer 18

• Exothermic process as it releases energy back into surroundings

Question 19

How do you calculate energy change in a reaction using bond energies?

Answer 19

1) Identify and sum the bond energies of all bonds in reactants (Energy In)
2) Identify and sum the bond energies of all bonds in products (Energy Out)
3) Calculate overall energy change:
Energy Change = Energy In - Energy Out
- Positive value indicates energy absorbed (endothermic)
- Negative value indicates energy released (exothermic)