SC19 Heat energy changes in Chemical reactions

Question 1

State the 2 types of ways chemical reactions can be described.

Answer 1

1. Exothermic
2. Endothermic

Question 2

What is an exothermic reaction?

Answer 2

= reactions that give out heat energy.

i.e. energy is transferred from stores of energy in chemical bonds to the surroundings.
During an exothermic reaction the temperature of the reaction mixture and the surroundings
increases.

Question 3

give 3 examples of exothermic reactions

Answer 3

1. any combustion reaction – burning (reaction with oxygen on heating)
2. a neutralisation reaction.
3. rusting of iron
4. Hot packs, involve an exothermic reaction

Question 4

What is an endothermic reaction?

Answer 4

= reactions that take in heat energy.

i.e. energy is transferred from the surroundings to stores of energy in chemical bonds.
During an endothermic reaction the temperature of the reaction mixture and the
surroundings decreases.

Question 5

give 3 examples of endothermic reactions

Answer 5

1. Photosynthesis
2. Melting ice cubes
3. Evaporating liquid water
4. Dissolving ammonium chloride in water
5. Cold packs used to treat injuries

Question 6

What does a energy level diagram show?

Answer 6

• Used to model the energy change during a chemical reaction
• In such diagrams, energy stored in bonds is represented as a horizontal line – the greater the energy stored, the higher the line

Question 7

How do energy profile diagram differ from energy level diagram?

Answer 7

• Differ from energy level diagrams because they show activation
  energy (= the minimum amount of energy needed by colliding
  particles for a reaction to happen)

Question 8

Is the breaking of bonds endothermic or exothermic? why?

Answer 8

Most chemicals will break up (decompose) if they are heated strongly enough. This means
that energy is needed to break chemical bonds – an endothermic process.

Question 8

Is the making of bonds endothermic or exothermic? why?

Answer 8

Because bond-breaking is endothermic, bond-making must therefore be exothermic. This
means that energy is released when chemical bonds are made.

Question 9

What does it mean if a chemical reaction is exothermic in terms of bonds?

Answer 9

Therefore, a reaction is exothermic overall, when more energy is
released when the bonds of the products are formed than the energy
taken in to break the bonds of the reactants

Question 10

What does it mean if a chemical reaction is endothermic in terms of bonds?

Answer 10

Therefore, a reaction is endothermic overall, when more energy is
absorbed when the bonds of the reactants are broken than the energy
released in to making the bonds of the products