Energy

Question 1

what is an exothermic reaction

Answer 1

a reaction in which energy is transferred from the reacting substances to their surroundings

Question 2

what is an endothermic reaction

Answer 2

a reaction in which energy is transferred to the reacting substances from their surroundings

Question 3

Explain why some reactions are exothermic and some are endothermic in terms of the processes of bond making and bond breaking

Answer 3

Energy has to be supplied to break chemical bonds. This means that breaking bonds is an endothermic process as energy is taken in from the surroundings. However, when new bonds are formed, energy is transferred to the surroundings, this shows that making bonds is an exothermic process.

Question 4

describe the chemical reaction which takes place in a disposable hand warmer

Answer 4

• the iron reacts with oxygen and water in an oxidation reaction
• sodium chloride is used as a catalyst too speed up the rate of reaction
• the reaction produces hydrated iron (iii) oxide
• it is an endothermic reaction, meaning that heat is released which heats up the hand warmer

Question 5

Describe how a reusable hand warmer works

Answer 5

• a supersaturated solution is prepared by dissolving as much sodium ethanoate as possible in hot water and letting it cool
  -when you press the small metal dis in the hand warmer small particles of matel are scraped off
  -these start off the crystallisation
• the crystals spread throughout the solution, transferring energy to the surroundings in an exothermic reaction

Question 6

One advantage and one disadvantage of disposable hand warmers

Answer 6

Longer lasting heat but single use

Question 7

Give one advantage and one disadvantage of reusable hand warmers

Answer 7

Can be reused multiple times but provides heat for a shorter duration