C5 - Energy Changes

Question 1

Energy is ______ in chemical reactions. If a reaction transfers energy to the surroundings, the product molecules must have _____ energy than the reactants.

Answer 1

Conserved
Less

Question 2

What is an exothermic reaction?

Answer 2

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

Question 3

What is an endothermic reaction?

Answer 3

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

Question 4

Give two every day uses of exothermic reactions

Answer 4

Self-heating cans
Hand warmers

Question 5

Give two examples of endothermic reactions

Answer 5

Thermal decompositions

Reaction of citric acid and sodium hydrogencarbonate

Question 6

Give an everyday use of endothermic reactions

Answer 6

Sport injury packs

Question 7

Students should be able to evaluate uses and applications of exothermic and endothermic reactions given appropriate information

Question 8

AT 5: Opportunity to measure temperature changes when substances react or dissolve in water

Question 9

RP4: Design an investigation for the variables that affect temperature changes in reacting solutions such as:

• Acid + metals
• Acid + carbonates
• Neutralisations
• Displacement of metals

Question 10

When can chemical reactions only occur?

Answer 10

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy

Question 11

What is the activation energy?

Answer 11

The minimum amount of energy that particles must have to react is called the activation energy

Question 12

What three things can reaction profiles be used to show?

Answer 12

• Relative energies of reactants and products
• The activation energy
• Overall energy change of the reaction

Question 13

Students need to explain that the activation energy is the energy needed for a reaction to occur.

This could be a ‘suggest why the reaction did not occur/start’ question

Question 14

During a chemical reaction:

Energy must be supplied to ______ bonds in the reactants

Energy is released when bonds in the ____ are formed

Answer 14

Break
Products

Question 15

How can the energy needed to break bonds and the energy released when bonds are formed be calculated?

Answer 15

By bond energies

Question 16

In an exothermic reaction, the energy released from forming _______ is greater than the energy needed to break _______ bonds

Answer 16

New bonds
Break

Question 17

In an endothermic reaction, the energy needed to break ______ bonds is greater than the energy released from forming new bonds

Answer 17

Existing

Question 18

Cells contain ____ which react to produce _____

Answer 18

Chemicals
Electricity

Question 19

What is the voltage produced by a cell dependant on?

Answer 19

Type of electrode
Type of electrolyte

Question 20

How is a simple cell made?

Answer 20

A simple cell can be made by connecting two different different metals in contact with Electrolyte

Question 21

How is a battery made?

Answer 21

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

Question 22

How can the voltage of a simple cell and battery be increased?

Answer 22

Increase the reactivity difference between the two metals

Question 23

Breaking bonds in reactants is an ____ process

Answer 23

Endothermic process

Question 24

Making bonds in products is an _____ process

Answer 24

Exothermic

Question 25

When one of the reactants has been used up, the chemical reactions ___ in non-rechargeable ___ and ____

Answer 25

Stop
Cells
Batteries

Question 26

Are alkaline batteries non-rechargeable or rechargeable?

Answer 26

Alkaline batteries are non-rechargeable

Question 27

Rechargeable cells and batteries can be ______ because the chemical reactions are _____ when an external ______ ______ is supplied

Answer 27

Recharged
Reversed
Electrical current

Question 28

Students should be able to interpret data for relative reactivity of different metals and evaluate the use of cells

Question 29

How are fuel cells supplied?

Answer 29

Fuel cells are supplied by an external source of fuel such as hydrogen and oxygen or air

Question 30

How is a potential difference produced in a fuel cell?

Answer 30

The fuel is oxidised electrochemically within the fuel cell to produce a potential difference

Question 31

Advantages of using hydrogen fuel cells in comparison with rechargeable cells and batteries?

Answer 31

Hydrogen can be renewable
Produces a constant voltage
No toxic chemicals released after disposal

Question 32

What do hydrogen fuel cells offer?

Answer 32

Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries

Question 33

Suggest why using hydrogen as a fuel could help reduce the human impact on global warming

Answer 33

The reaction does not produce carbon dioxide is a greenhouse gas

Question 34

Write two half equations that show what happens to the hydrogen and oxygen gases in a hydrogen fuel cell

Answer 34

2H2 (g) + 4OH- (aq) —> 4H20 (l) + 4e-
Hydrogen gas is supplied as a fuel to the negative electrode and it diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to an external circuit

O2 (g) + 2H20 (l)+ 4e- —-> 4OH- (aq)
Oxygen gas is supplied to the positive electrode. It diffuses through the graphite and reacts to form hydroxide ions, accepting electrons from the external circuit

Question 35

What are three disadvantages of hydrogen fuel cells?

Answer 35

Hydrogen is highly flammable
Hydrogen is difficult to store
Hydrogen is sometimes produced for the cell by nonrenewable sources

Question 36

Suggest why hydrogen fuel cells are useful

Answer 36

Offer a potential alternative to rechargeable cells and batteries

Question 37

What is the overall reaction in a hydrogen fuel cell?

Answer 37

The oxidation of hydrogen to form water

Water is the only product, so this is useful! No separation! Better alternative as no toxic products!