Chapter 5: Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

A reaction in which thermal energy is transferred from the chemicals to the surroundings, increasing their temperature

Question 2

What are some uses of exothermic reactions?

Answer 2

Hand warmers, self-heating cans

Question 3

What are some examples of exothermic reactions?

Answer 3

Oxidation reactions, neutralisation reactions, combustion reactions

Question 4

What is an endothermic reaction?

Answer 4

A reaction in which thermal energy is transferred from the surroundings to the chemicals, increasing their temperature

Question 5

What are some uses of endothermic reactions?

Answer 5

Cold packs

Question 6

What are some examples of endothermic reactions?

Answer 6

Decomposition, reaction of acids with hydrogencarbonates

Question 7

What is activation energy?

Answer 7

The minimum energy particles must have to react with each other

Question 8

What is a reaction profile diagram?

Answer 8

A graph of energy and progress of reaction

Question 9

What are bond energies?

Answer 9

The amount of energy it takes to make/break a bond

Question 10

How is energy used/released?

Answer 10

Breaking bonds uses up energy, making bonds releases energy (BENDO MEXO)

Question 11

How do you calculate the total energy change?

Answer 11

(Energy needed breaking bonds in reactants) - (Energy released making bonds in products)

Question 12

What is an electrolyte?

Answer 12

A liquid that conducts electricity. Ions carry the electric charge through the liquid

Question 13

What is a cell?

Answer 13

2 electrodes and an electrolyte, used to generate electricity

Question 14

What is a battery?

Answer 14

2 or more chemical cells connected together

Question 15

How do chemical cells work?

Answer 15

Electrons flow from the more reactive metal to the less reactive metal because there is a potential difference (voltage) between them, The ions in the electrolyte carry the charge

Question 16

What is the voltage produced by many chemical cells?

Answer 16

1.5 volts, so multiple cells may need to be connected together in a series to provide a higher voltage

Question 17

What does it mean when a chemical cell is flat?

Answer 17

The reactants in the cell have been used up and the battery no longer works

Question 18

How do rechargeable cells work?

Answer 18

Passing an electric current through the cell reverses the reaction

Question 19

What are the advantages of using chemical cells?

Answer 19

Cheap, convenient, portable source of electricity, some are rechargeable

Question 20

What are the disadvantages of chemical cells?

Answer 20

Many cells contain environmentally-harmful chemicals, some go flat and must be recharged

Question 21

How are fuel cells different from chemical cells?

Answer 21

There is a constant supply of chemicals (usually oxygen) into the cell, as well as a fuel (usually hydrogen)

Question 22

What happens inside a fuel cell?

Answer 22

Hydrogen gas enters the cell at the anode, where a chemical reaction strips it of its electrons and turns it into H+ ions. The electrons pass through a circuit which things can be connected to. The hydrogen ions pass through a proton exchange electrolyte and combine with the returning electrons and oxygen at the cathode

Question 23

What are the advantages of fuel cells?

Answer 23

Only waste product is water, produce constant supply of electricity

Question 24

What are the disadvantages of fuel cells?

Answer 24

Expensive, contain toxic chemicals in electrodes and electrolytes, use flammable hydrogen gas, hydrogen is sometimes made in a process that uses fossil fuels

Question 25

What is a decomposition reaction?

Answer 25

When substances break down into simpler substances (e.g. * carbonate => * oxide + carbon dioxide). These are always endothermic

Question 26

What is an redox reaction?

Answer 26

When a substance reacts with oxygen and loses electrons, the substance is oxidised and the oxygen is reduced. These are exothermic

Question 27

What is a combustion reaction?

Answer 27

An exothermic reaction where a substance reacts with oxygen and catches fire

Question 28

What is a neutralisation reaction?

Answer 28

Exothermic reactions when acids react with bases to form water and a salt

Question 29

What happens when acids react with metal hydrogencarbonates?

Answer 29

The temperature drops

Question 30

What is the potential difference of a chemical cell?

Answer 30

How easy it is for charge to flow between the two electrodes

Question 31

How do you calculate the potential difference?

Answer 31

The difference in how reactive the metals are

Question 32

What is the overall reaction in a hydrogen cell?

Answer 32

2H2 + O2 -> 2H2O

Question 33

What is the half equation at the anode?

Answer 33

2H2 -> 4H+ + 4e-

Question 34

What is the half equation at the cathode?

Answer 34

O2 + 4H+ 4e- -> 2H2O