C7.4 - Bond energy calculations

Question 1

What is there always a balance between?

Answer 1

There is always a balance between the energy:

1. Needed to break bonds
2. Released when new bonds are made in a reaction

Question 2

There is always a balance between the energy needed to break bonds and the energy released when new bonds are made in a reaction.
This is what decides what?

Answer 2

This is what decides whether the reaction is:
1. Endothermic
Or,
2. Exothermic

Question 3

In an exothermic reaction, the energy released when new bonds are formed, as the products are made, is more than what?

Answer 3

In an exothermic reaction, the energy released when new bonds are formed, as the products are made, is more than the energy needed to break the bonds in the reactants

Question 4

In an endothermic reaction, the energy needed to break the bonds in the reactants is more than what?

Answer 4

In an endothermic reaction, the energy needed to break the bonds in the reactants is more than the energy released when new bonds are formed in the products

Question 5

Bond energy

Answer 5

Bond energy is the energy required to break a specific chemical bond

Question 6

What are bond energies measured in?

Answer 6

Bond energies are measured in kJ/mol

Question 7

The bond energy for the O = O bond

Answer 7

The bond energy for the O = O bond is 498 kJ/mol

Question 8

The bond energy for the N ≡ N bond

Answer 8

The bond energy for the N ≡ N bond is 945 kJ/mol

Question 9

What does breaking and making a particular bond always involve?

Answer 9

1. Breaking
2. Making
   a particular bond always involves the same amount of energy

Question 10

To calculate the energy change for a chemical reaction, what do you need to do?

Answer 10

To calculate the energy change for a chemical reaction, you need to work out how much energy is:

1. Needed to break the chemical bonds in the reactants
2. Released when the new bonds are formed in the products

Question 11

The bond energy for the C - C bond is 347 kJ/mol taken in from the surroundings, an endothermic change.
What does this mean?

Answer 11

This means that the energy released forming a C - C bond is 347 kJ/mol transferred to the surroundings, an exothermic change

Question 12

When calculating bond energy, what do you always look at?

Answer 12

When calculating bond energy, you always look at the balanced symbol equation

Question 13

When calculating bond energy, why do you always look at the balanced symbol equation?

Answer 13

When calculating bond energy, you always look at the balanced symbol equation, because in the formation of ammonia, the energy released, 93 kJ, is from the formation of 2 moles of ammonia, as shown in the balanced equation

Question 14

In an exothermic reaction, the energy released when new bonds are formed, as the products are made, is more than the energy needed to break the bonds in the reactants.
These exothermic reactions transfer energy to what?

Answer 14

These exothermic reactions transfer energy to the surroundings

Question 15

In an endothermic reaction, the energy needed to break the bonds in the reactants is more than the energy released when new bonds are formed in the products
These endothermic reactions transfer energy from what to what?

Answer 15

These endothermic reactions transfer energy from the surroundings to the reacting chemicals

Question 16

When calculating bond energy, you always look at the balanced symbol equation, because in the formation of ammonia, the energy released, 93 kJ, is from the formation of 2 moles of ammonia, as shown in the balanced equation.
So if you wanted to know the energy change for the reaction per mole of ammonia formed, you would have to do what?

Answer 16

If you wanted to know the energy change for the reaction per mole of ammonia formed, you would have to half this