C5- Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

A reaction where energy is transferred to the

surroundings.

Question 2

Give examples of exothermic reactions.

Answer 2

Combustion, respiration and many neutralisation reactions

Question 3

What happens to the temperature of the

surroundings during an exothermic reaction?

Answer 3

They increase

Question 4

What is an endothermic reaction?

Answer 4

A reaction where energy is transferred from the

surroundings.

Question 5

Give examples of endothermic reactions.

Answer 5

Thermal decomposition reactions, the reaction between

citric acid and sodium hydrogencarbonate and respiration

Question 6

What happens to the temperature of the

surroundings during an endothermic reaction?

Answer 6

They decrease

Question 7

State two uses of exothermic reactions

Answer 7

Self-heating cans, hand warmers

Question 8

State two uses of exothermic reactions

Answer 8

Some cooling sports injury packs

Question 9

What is a reaction profile?

Answer 9

A diagram which shows whether the reactants have more
or less energy than the products and the overall energy
change of a reaction.

Question 10

What is the activation energy?

Answer 10

The energy needed to break bonds to start a reaction

Question 11

Why does the reaction profile always go up at the start?

Answer 11

A reaction starts with bonds being broken

Question 12

Why does the reaction profile always go down at the end?

Answer 12

A reaction always ends with bonds being made

Question 13

If the reactants have more energy than the products,

what kind of a reaction must have taken place?

Answer 13

An exothermic one. Energy has been transferred to the surroundings.

Question 14

If the reactants have less energy than the products,

what kind of a reaction must have taken place?

Answer 14

An endothermic one. Energy has been taken in from the surroundings.

Question 15

Is breaking bonds endothermic or exothermic?

Answer 15

Exothermic. Energy is released when chemical bonds are formed (like how two
magnets move together when close and generate kinetic energy)

Question 16

How do we work out the overall energy change of a

reaction?

Answer 16

Work out the difference between the energy needed to break all the bonds in
the reactants and the energy released to form all the bonds in the products.

Question 17

How do we work out the energy change from a reaction profile?

Answer 17

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

Question 18

How do we work out the activation energy from a reaction

profile?

Answer 18

The difference between the energy of the reactants and the highest point on
the profile

Question 19

What is meant by bond energy?

Answer 19

The amount of energy required to break a particular type of bond.

Question 20

What is a chemical cell?

Answer 20

A unit which contains chemicals which produce electricity

through a reaction

Question 21

What are the main components of a chemical cell?

Answer 21

An anode, a cathode (made of 2 different metals) and an

electrolyte

Question 22

What is an electrolyte?

Answer 22

A solution containing ions

Question 23

What does the voltage of a cell depend on?

Answer 23

The type of electrode and the electrolyte.

Question 24

What is a battery?

Answer 24

Two or more cells connected in series

Question 25

What happens to the electrodes in a chemical cell?

Answer 25

The more reactive metal is used up and the less reactive | one increases in size.

Question 26

In non-rechargeable cells, why do the chemical | reactions stop over time?

Answer 26

One of the reactants becomes used up.

Question 27

How can certain cells be recharged?

Answer 27

Applying an external electric current

Question 28

How does the reactivity of the metal electrodes | affect the size of the potential difference?

Answer 28

The greater the difference in reactivity, the greater the | voltage

Question 29

Which way do electrons move in a cell?

Answer 29

From the more reactive metal to the less reactive metal

Question 30

What is a fuel cell?

Answer 30

A cell which uses a fuel and oxygen (or | air) to generate electricity.

Question 31

What are the products in a hydrogen | fuel cell?

Answer 31

Water

Question 32

State advantages of hydrogen fuel | cells.

Answer 32

Do not need to be recharged, no | pollutants are produced

Question 33

State three disadvantages of | hydrogen fuel cells.

Answer 33

Hydrogen is highly flammable, hydrogen is often made from fossil fuels, hydrogen is difficult to store

Question 34

What reaction occurs at the negative | electrode of a hydrogen fuel cell?

Answer 34

H2 → 2H + 2e⁻

Question 35

Which reaction occurs at the positive | electrode of a hydrogen fuel cell?

Answer 35

O2 + 4H+ + 4e- → 2H2O

Question 36

What must be supplied for a bond to break

Answer 36

Energy

Question 37

What is the conservation of energy principle?

Answer 37

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

Question 38

What is an exothermic reaction? Give examples

Answer 38

A reaction where energy is transferred to the surroundings so that the surroundings temperature increases – combustion, oxidation reactions and neutralisation (acid + alkali) reactions. Negative sign of energy change.

Question 39

What is an endothermic reaction? Give examples

Answer 39

A reaction where energy is taken in from the surroundings so the surroundings temperature decreases – thermal decomposition, reaction of citric acid and sodium hydrogencarbonate. Negative sign of energy change.

Question 40

What is activation energy?

Answer 40

Minimum amount of energy that particles need to react

Question 41

What is a reaction profile?

Answer 41

Reaction profile is a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction.

Question 42

What occurs in a chemical reaction in terms of bond energies? Describe exothermic and endothermic reactions in terms of bond breaking/forming.

Answer 42

Energy is supplied to break bonds and energy is released when bonds are made; exothermic – energy released from forming bonds is greater than that needed to break the bonds; endothermic – energy needed to break bonds is greater than energy released making them

Question 43

What is the equation to find enthalpy change in terms of bond energies?

Answer 43

Energy of reaction = sum of bonds broken – sum of bonds made

Question 44

What is a cell?

Answer 44

A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.

Question 45

What is a battery

Answer 45

A battery consists of two or more cells connected in series.

Question 46

What determines the voltage obtained from a | cell?

Answer 46

Identities of metals used as electrodes and the identity and concentration of an electrolyte.

Question 47

State the advantages and disadvantages of | using cells and batteries

Answer 47

(+) more or less cheap, some are rechargeable, a convenient source of electrical energy (-) harmful chemicals

Question 48

Describe rechargeable and non-rechargeable cells

Answer 48

Rechargeable – chemical reactions are reversed when an external current is supplied Non-rechargeable – reactants are used up, cannot be recharged

Question 49

What is a fuel cell? What is the overall reaction in a hydrogen fuel cell? What are the half equations?

Answer 49

Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.

Question 50

What is the overall reaction in a hydrogen fuel cell? What are the half equations?

Answer 50

Cathode: 2 H2 → 4 H+ + 4 e− Anode: O2 + 4 H+ + 4 e−→ 2 H2O Overall: → 2 H2 + O2 → 2 H2O

Question 51

What are the advantages and disadvantages of hydrogen fuel | cells?

Answer 51

``` Advantages: no pollutants, no recharging Disadvantages: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis ```