5. Energy Changes

Question 1

What is an endothermic reaction and give an example?

Answer 1

A reactions where energy enters - heat is taken from the surroundings and transferred to the object

E.g. melting, boiling, thermal decomposition reactions

Question 2

What is an exothermic reaction and give an example?

Answer 2

A reaction where energy exits - heat is transferred from the object to the surroundings and makes it feel warmer

E.g. a hand warmer, condensing, freezing, combustion reactions

Question 3

What is a reaction profile?

Answer 3

A diagram that shows whether a reaction is endo or exothermic, and shows the energy of the reactants, products, activation energy

Question 4

What does the reaction profile for an exothermic reaction look like?

Answer 4

The reactants have more energy than the products

• The overall energy change is negative

Question 5

What does the reaction profile for an endothermic reaction look like?

Answer 5

The products have more energy than the reactants

• The overall energy change is positive

Question 6

What is activation energy?

Answer 6

The minimum amount if energy needed to start a reaction

Question 7

In a reaction profile for an exothermic reaction, which part is the activation energy?

Answer 7

From the reactants to the top of the hump

Question 8

In a reaction profile for an endothermic reaction, which part is the activation energy?

Answer 8

From the reactants (the lowest part) to the top of the hump

Question 9

Is bond breaking endo or exothermic?
And forming bonds?

Answer 9

Breaking bonds: endothermic
Forming bonds: exothermic

Question 10

If more heat energy is released in making bonds in the products than is taking in when breaking bonds in the reactants, is the reaction endo or exothermic ?

Answer 10

Exothermic as the total energy change is negative

Question 11

How could the energy change be calculated?

NOT METHOD

Answer 11

By using bond energies - the amount of energy needed to break one mole of a particular covalent bond

Question 12

How would you calculate the energy change of a reaction and work out if it is exo or endothermic?

Answer 12

1. Add together the bond energies for all the bonds in the reactants
2. Add together the bond energies for all the bonds in the products
3. Energy change = bonds in reactants - bonds in products

If the energy change is negative, the reaction is exothermic

If the energy change is positive, the reaction is endothermic