2.4 - Chemical reactions and energy

Question 1

What is an exothermic reaction?

Answer 1

Reactions that transfer energy to the surroundings so the temperature of the surroundings increases

Question 2

What are some examples of exothermic reactions?

Answer 2

Combustion (burning)

Neutralisation reactions between acids/alkalis

Question 3

What is an endothermic reaction?

Answer 3

Reactions that take in energy from its surroundings so the temperature of the surroundings decreases

Question 4

What are some examples of endothermic reactions?

Answer 4

Electrolysis

Thermal decomposition of calcium carbonate in blast furnace

Question 5

Bond-breaking is an _______process

Answer 5

Endothermic

Energy is absorbed to break bonds

Question 6

Bond-making is an ______ process

Answer 6

Exothermic

Energy is released when new bonds form

Question 7

During a chemical reaction:

bonds in the _____ are broken
new bonds are made in the _____

(reactants/products)

Answer 7

reactants

products

Question 8

Remember:

Answer 8

If more heat energy is released when making the bonds than was taken in when breaking bonds, the reaction is exothermic. If more heat energy was taken in when breaking the bonds than was released when making the bonds, the reaction is endothermic

Question 9

in exothermic reactions the reactants are ____ than the products
in endothermic reactions the reactants are ____ than the products

(higher/lower)

Answer 9

higher

lower

Question 10

What is an enthalpy change (∆H)?

Answer 10

The difference between the energy of the reactants and the energy of the products

Question 11

In an exothermic reaction enthalpy change is always _____

In an endothermic reaction enthalpy change is always _____

Answer 11

negative

positive

Question 12

What is the activation energy?

Answer 12

The minimum amount of energy required for a reaction to take place.

Question 13

What is bond energy?

Answer 13

Bond energy is the amount of energy needed to break one mole of a particular bond.

Question 14

How do you calculate bond energy?

Answer 14

1) Add together the bond energies for all the bonds in the reactants – this is the ‘energy in’.
2) Add together the bond energies for all the bonds in the products – this is the ‘energy out’.
3) Calculate the energy change = energy in – energy out.

If the overall change is negative then reaction is exothermic
If the overall change is positive then reaction is endothermic