Energy Changes

Question 1

Energy is conserved in chemical reactions. The amount of energy in
the universe at the end of a chemical reaction is the same as before
the reaction takes place. If a reaction transfers energy to the
surroundings the product molecules must have less energy than the
reactants, by the amount transferred.

Question 2

Exothermic reaction examples

Answer 2

An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.

—>Exothermic reactions include combustion, many oxidation reactions
and neutralisation.

—>Everyday uses of exothermic reactions include self-heating cans
and hand warmers.

Question 3

Endothermic reaction examples

Answer 3

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.

—>Endothermic reactions include thermal decompositions and the
reaction of citric acid and sodium hydrogencarbonate.

—>Some sports injury packs are based on endothermic reactions.

Question 4

• evaluate uses and applications of exothermic and endothermic reactions

Question 5

2 things during a chemical reaction:

Answer 5

During a chemical reaction:
• energy must be supplied to break bonds in the reactants
• energy is released when bonds in the products are formed.

Answer 6

Bond energies

Answer 7

The difference between the sum of the energy needed to break
bonds in the reactants and the sum of the energy released when
bonds in the products are formed is the overall energy change of
the reaction.
In an exothermic reaction, the energy released from forming new
bonds is greater than the energy needed to break existing bonds.
In an endothermic reaction, the energy needed to break existing
bonds is greater than the energy released from forming new bonds.

Question 8

Cells

Answer 8

Cells contain chemicals which react to produce electricity
— voltage produced depend on electrode and electrolyte
— simple cell make by connecting 2 diff metals in contact with electrolyte

Question 9

Difference between non rechargeable and rechargeable batteries

Answer 9

Non rechargeable: chemical reactions stop when one of the reactants are used up (alkaline batteries)

Rechargeable: can be recharged because the chemical reactions are reversed when an external electrical current in supplied

Question 10

Rechargeable vs (hydrogen) fuel cells

Answer 10

Fuel cells- supplied by External source of fuel (hydrogen) and oxygen/air. Fuel is oxidised electrochemically within the cell to provide potential difference/voltage
—oxidation of hydrogen to produce water