Energy Changes

Question 1

When chemical reaction occurs, what happens to energy?

Answer 1

It’s transferred to or from the surroundings

Question 2

What is an exothermic reaction?

Answer 2

One that transfers energy to the surroundings, usually by heating

Question 3

How is an exothermic reaction shown?

Answer 3

By a rise in temperature

Question 4

What is an endothermic reaction?

Answer 4

One which takes in energy from the surroundings

Question 5

How is an endothermic reaction shown?

Answer 5

By a fall in temperature

Question 6

Give three examples of an exothermic reaction

Answer 6

1. Combustion 2. Neutralisation reactions 3. Oxidation reactions

Question 7

Give 2 everyday uses of exothermic reactions

Answer 7

1. Hand warmers 2. Self-heating cans

Question 8

Explain how hand warmers use exothermic reactions

Answer 8

Use exothermic oxidation of iron in air (with a salt solution catalyst) to release energy

Question 9

What do self-heating cans rely on?

Answer 9

Exothermic reactions between chemicals in their bases

Question 10

Give an example of an endothermic reaction

Answer 10

Thermal decomposition

Question 11

Give a everyday use of an endothermic reaction

Answer 11

In some sports injury packs

Question 12

Explain how sports injury packs work

Answer 12

Chemical (endothermic) reaction allows pack to become instantly cooler without having to put it in the freezer

Question 13

Give an example of a reversible reaction (that has the forward endothermic reaction and backwards exothermic reaction)

Answer 13

Hydrated copper sulfate (blue) ⇌ Anhydrous copper sulfate (white) + water

Question 14

How can you measure the energy released when a substance is burnt?

Answer 14

Via simple calorimetry

Question 15

Why is calorimetry used?

Answer 15

Used to compare amounts of energy released

Question 16

What is the bad thing about calorimetry and why?

Answer 16

Not accurate because of energy losses

Question 17

What equation can you use to calculate and compare amount of energy released by different fuels?

Answer 17

Q = mc ΔT

Question 18

How is energy measured?

Answer 18

In J or kJ for a given mass/mole e.g. J/mol

Question 19

How can the amount of energy produced by a chemical reaction in solution be calculated + when can this method be used?

Answer 19

Can be calculated from measured temperature change of the solution when the reagents are mixed in an insulated container Method can be used for reactions of solids with water or for neutralisation reactions

Question 20

When doing bond energy calculations, what are the 3 things you need to remember?

Answer 20

1. Reactants − Products 2. − = exothermic 3. + = endothermic

Question 21

In endothermic reactions why must energy must be supplied?

Answer 21

To break existing bonds

Question 22

In exothermic reactions when is energy released?

Answer 22

When new bonds are formed

Question 23

In endothermic reactions: (comment on energy + bonds = comparison)

Answer 23

Energy used to break bonds > energy released by forming them

Question 24

In exothermic reactions: (comment on energy + bonds = comparison)

Answer 24

Energy released by forming bonds > energy used to break them

Question 25

In a energy level diagram, what does the difference in height of (the reactants and products) represent?

Answer 25

Overall energy change (energy given out/taken in) in reaction per mole

Question 26

In a energy level diagram, what does the initial rise represent?

Answer 26

Activation energy (energy needed to break old bonds and start reaction)

Question 27

What is activation energy?

Answer 27

Minimum amount of energy needed for reactants to collide with each other and react

Question 28

What does it mean if a substance has a large activation energy?

Answer 28

Large amount of energy needed to start the reaction (needed to be supplied e.g. via heating)

Question 29

In a chemical reaction, what must happen?

Answer 29

Old bonds are broken & new bonds are formed

Question 30

What does a chemical cell do?

Answer 30

Produces a potential difference until the reactants are used up

Question 31

What does a fuel cell do?

Answer 31

Supplied with fuel and oxygen (or air) & uses energy from reaction between them = to produce electrical energy efficiently

Question 32

What happens to fuel when it enters a fuel cell?

Answer 32

Becomes oxidised & sets up a potential difference within cell

Question 33

What is an example of a fuel cell?

Answer 33

Hydrogen-oxygen fuel cell

Question 34

What does a hydrogen-oxygen fuel cell do?

Answer 34

Combines hydrogen and oxygen to produce clean water + energy

Question 35

What can a hydrogen-oxygen fuel cell be used for?

Answer 35

Can be used in vehicles

Question 36

Name 3 pros of hydrogen-oxygen fuel cells

Answer 36

1. Don’t need to be electrically recharged
2. Don’t produce as many pollutants as other fuels (by-products are water & heat)
3. Can be designed in a range of sizes for different uses

Question 37

Name 4 cons of hydrogen-oxygen fuel cells

Answer 37

1. Hydrogen is a gas = takes up more space store than liquid fuels
2. Hard to store safely
3. Not actually that environmentally friendly
4. Constant supply of hydrogen is needed to run fuel cell

Question 38

Why are hydrogen-oxygen fuel cell hard to store safely?

Answer 38

Hydrogen is explosive when mixed with air

Question 39

Why aren’t hydrogen-oxygen fuel cell 100% environmentally friendly?

Answer 39

Hydrogen fuel is made from hydrocarbons (from fossil fuels) or by electrolysis of water which uses electricity (generated by fossil fuels)

Question 40

How can we reduce the amount of space hydrogen takes up?

Answer 40

Can use high-pressure cylinders but have safety problems in crashes

Question 41

What is the downside of using high-pressure cylinders?

Answer 41

Have safety problems in crashes

Question 42

How do you work out enthalpy change?

Answer 42

ΔH = mcΔT

n

Question 43

What does the letters stand for in this formula?

ΔH = mcΔT

n

Answer 43

ΔH - enthalpy change (aka energy change per mole)

m - mass of water used

c - specific heat capacity of water

ΔT - change in temperature

n - moles of fuel used

Question 44

Draw a energy level diagram for endothermic reaction

Answer 44

Question 45

Draw a energy level diagram for exothermic reaction

Answer 45

Question 46

Illustrate the effect of a catalyst on a energy level diagram for exothermic reaction

Answer 46