C3.2 - Energetics

Question 1

What is an exothermic reaction?

Answer 1

• A reaction in which there is an overall transfer of
  energy to the surroundings.
• The energy transferred to the surroundings when
  new bonds form is more than the energy taken in
  from the surroundings to break the bonds.

Question 2

What is an endothermic reaction?

Answer 2

• A reaction is which there is an overall transfer of
  energy from the surroundings, which cool down.
• The energy transferred to the surroundings when
  new bonds form is less than the energy taken in
  from the surroundings to break the bonds.

Question 3

What is the energy change of a reaction?

Answer 3

• The difference between the energy transferred
  from the surroundings to break bonds in the
  reactants, and the energy transferred from
  products when bonds form.
• Energy change = Energy transferred to break
  bonds - Energy transferred to make bonds.

Question 4

What are the steps to find out if a reaction is an endothermic reaction or an exothermic reaction?

Answer 4

• Measure the start temperature of a solution in an
  insulated container.
• Add the other reactant (reacting substance) and
  stir.
• Measure the end temperature of the mixture.
• Calculate the difference between the start
  temperature and the end temperature.
• If the temperature has increased then the reaction
  is exothermic and vice versa.

Question 5

What would you need to check if both reactants are solutions in an investigating temperature change practical?

Answer 5

If both temperatures are solutions, you may need to check that their temperatures are the same before mixing them.

Question 6

What is bond energy?

Answer 6

• The energy needed to be transferred to break 1
  mol of a particular bond (kJ/mol).
• Also the amount of energy transferred to the
  surroundings when 1 mol of these bonds form.

Question 7

Why might the calculated energy change for a reaction be different from the value found by experiment?

Answer 7

Bond energies are usually mean values for the same bonds in different substances.

Question 8

What are the surroundings in a chemical reaction?

Answer 8

Anything that is not a reacting particle.

Question 9

When is energy transferred in a chemical reaction?

Answer 9

• From the surroundings to break bonds in the
  reactants to form separate atoms.
• To the surroundings from the reacting particles
  when bonds form between atoms.

Question 10

What is a reaction profile?

Answer 10

A chart that shows the energy involved in a chemical reaction.

Question 11

What is activation energy?

Answer 11

• Energy must be transferred to the reacting
  particles to start most chemical reactions.
• Activation energy is the minimum energy needed
  for a reaction to start.
• It is often provided by a flame or heating, and it
  breaks bonds the in reactants.

Question 12

Why do acid and alkalis react as soon as you mix them, without heating?

Answer 12

The reactions activation energy is so low that the reactants already have enough energy to react and just need to be mixed together.

Question 13

Why do acid and alkalis react as soon as you mix them, without heating?

Answer 13

The reactions activation energy is so low that the reactants already have enough energy to react and just need to be mixed together.

Question 14

Draw a reaction profile for an endothermic reaction.

Answer 14

Product line higher than reactants line.

Question 15

Draw a reaction profile for an exothermic reaction?

Answer 15

Product line lower than the reactant line.