C7- Energy Changes

Question 1

Explain what an Endothermic Reaction is?

What are some examples of endothermic reactions?

Answer 1

1) An Endothermic Reaction is when energy is transferred to the reacting substances.
2) Examples of endothermic reactions are thermal decompositions, the reaction of citric acid and sodium hydrogencarbonate. Sports injury packs are based on endothermic reactions.

Question 2

Explain what an Exothermic Reaction is.

Give some everyday examples of exothermic reactions.

Answer 2

1) An Exothermic Reaction is a reaction where energy is transferred from the reacting substances to their surroundings.
2) Some examples of exothermic reactions are combustion, oxidation reactions and neutralisation.

Question 3

How is energy conserved?

Answer 3

1) Energy is conserved in chemical reactions.

Question 4

What is Activation Energy?

Answer 4

1) Activation Energy is the minimum amount of energy that particles must have to react.

Question 5

When can chemical reactions only occur?

Answer 5

1) Chemical reactions can only occur when the reacting particles collide with each other and with sufficient energy.

Question 6

What do reaction profiles show?

Answer 6

1) They show the relative energies of reactants and products , they show the activation energy and the overall energy change of a reaction.

Question 7

What must happen in a chemical reaction?

Answer 7

1) Energy must be supplied to break bonds in the reactants.

2) Energy is released when the bonds in the products are formed.

Question 8

How do you calculate the overall energy change of a reaction?

Answer 8

1) The difference between the sum of energy needed to break bonds in the reactants and the sum of energy released when bonds in the products are formed.

Question 9

What is the energy released from forming new bonds in an exothermic reaction greater than?

Answer 9

1) The energy released from forming new bonds is greater than the energy needed to break existing bonds.

Question 10

What is the energy needed to break existing bonds in an endothermic reaction greater than?

Answer 10

1) The energy needed to break existing bonds is greater than the energy released from forming new bonds.

Question 11

What is the voltage produced by a cell dependent upon?

Answer 11

1) The voltage is dependent on a number of factors including the type of electrode and electrolyte.

Question 12

How can a simple cell be made?

Answer 12

1) A simple cell can be made by connecting two different metals in contact with an electrolyte.

Question 13

What do batteries consist of?

Answer 13

1) Batteries consist of two or more cells connected together in series to provide a greater voltage.

Question 14

Why do non-rechargeable cells and batteries die?

Give an example of a non-rechargeable battery.

Answer 14

1) In non-rechargeable cells and batteries, the chemical reactions stop because one of the reactants has been used up.
2) An alkaline battery in non-rechargeable.

Question 15

Why can rechargeable cells and batteries be recharged?

Answer 15

1) Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied (eg: phone charger charging the phone battery)

Question 16

How are fuel cells supplied?

Answer 16

1) Fuel cells are supplied by an external source of fuel (such as hydrogen) and oxygen or air
2) The fuel is the electrochemically oxidised within the fuel cell to produce a potential difference.

Question 17

What are some advantages and disadvantages of hydrogen fuel cells?

Answer 17

Advantages
1) They do not need to be electrically recharged.
2) Can be a range of sizes for different uses.
3) Only waste product is water, no pollutants are produced.
Disadvantages
1) Hydrogen is highly flammable
2) Hydrogen is sometimes produced for a fuel cells using non renewable sources.
3) Hydrogen is difficult to store.

Question 18

What is the half equation for the reaction at the negative electrode in a hydrogen fuel cell

Answer 18

1) 2H2 —-> 2H(+) +2e(-)

Question 19

What is the half equation for the reaction that takes place at the positive electrode a hydrogen fuel cell?

Answer 19

1) O2+4H(+) + 4E(-) —–> 2H20

Question 20

How are fuel cells supplied?

Answer 20

1) Fuel cells are supplied by an external source of fuel and O2 or air.

Question 21

What is the overall equation for a hydrogen fuel cell?

Answer 21

1) 2H2 + 2e(-) —-> 2H20

Question 22

Does the temperature increase or decrease in an Exothermic Reaction?

Answer 22

1) The Temperature increases.

Question 23

Does the temperature increase or decrease in an Endothermic Reaction?

Answer 23

1) The temperature decreases