C7 Energy Changes

Question 1

What are exothermic reactions

Answer 1

When a chemical reaction happens, energy is transferred to or from the surroundings. When energy is transferred to the surroundings, this is called an exothermic reaction, and the temperature of the surroundings increases. Examples of exothermic reactions include:

-combustion reactions
many
-oxidation reactions
most
-neutralisation
reactions

Question 2

What are endothermic reactions?

Answer 2

When energy is taken in from the surroundings, this is called an
endothermic
reaction and the temperature of the surroundings decreases. Examples of endothermic reactions include:

-thermal decomposition
reactions
-the reaction of citric acid and sodium hydrogen carbonate

Question 3

What are the differences in energy expressed in?

Answer 3

kilojoules per mole

Question 4

What reaction takes place in a hand warmer?

Answer 4

Exothermic.

Question 5

How are some hand warmers reusable?

Answer 5

They are based on the formation of crystal from solutions of a salt (often sodium ethanoate).A super saturated solution is prepared by dissolving as much of the salt as possible in hot water, then it cools. A small metal disc in the pack is used to start the exothermic reaction. When you press this a few times, small particles of metal are scaped off. These start the crystallisation and the crystals spread throughout the solution. Transferring energy to the surroundings in an exothermic change. To reuse the warmer you put the pack into boiling water to redissolve the crystals.

Question 6

How do cold packs work?

Answer 6

Ammonium nitrate and water are kept separate in the pack. When squeezed, the bag inside the water pack breaks releasing ammonium nitrate. They can only be used once. It is an endothermic reaction.

Question 7

What is an energy level diagram?

Answer 7

Chart showing the energy in the reactants and products, and the difference in energy between them.It shows whether a reaction is exothermic or
endothermic. It shows the energy in the reactants
and products, and the difference in energy between them.

Question 8

Describe an exothermic and endothermic energy level diagram?

Answer 8

The energy level decreases in an exothermic reaction. This is because energy is given out to the surroundings. The energy level increases in an endothermic reaction. This is because energy is taken in from the surroundings.

Question 9

What is an activation energy?

Answer 9

The minimum amount of energy that colliding particles must have for them to react.

Question 10

What is a reaction profile?

Answer 10

Chart showing how the energy of reactants and products changes during a reaction.

Question 11

What is the overall change in energy?

Answer 11

The overall change in energy in a reaction is the difference between the energy of the reactants and products.

Question 12

How to tell the difference between exothermic and endothermic reaction profiles?

Answer 12

In the profile for an exothermic reaction, the overall change is negative. You can tell this because the products have less energy than the reactants, and the arrow showing the overall change in energy points downwards. In the profile for an endothermic reaction, the overall change is positive. You can tell this because the products have more energy than the reactants, and the arrow showing the overall change in energy points upwards.

Question 13

What happens during a chemical reaction?

Answer 13

During a chemical reaction:

bonds in the
reactants are broken
new bonds are made in the
products

Question 14

How to tell the type of reaction from how it breaks its bonds

Answer 14

The difference between the energy needed to break bonds and the energy released when new bonds are made determines the type of reaction.

A reaction is:

exothermic
if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants

endothermic
if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants

Question 15

What is bond energy?

Answer 15

The amount of energy needed to break one mole of a particular covalent bond.

Question 16

How to calculate the energy change for a reaction?

Answer 16

To calculate an energy change for a reaction:

add together the bond energies for all the bonds in the
reactants
- this is the ‘energy in’
add together the bond energies for all the bonds in the
products
- this is the ‘energy out’
energy change = energy in - energy out

Question 17

What is the difference between non-rechargable and rechargable cells

Answer 17

In non-rechargeable cells, eg alkaline cells, a
voltage
is produced until one of the
reactants
is used up. When this happens, we say the battery ‘goes flat’.

In rechargeable cells and batteries, like the one used to power your mobile phone, the chemical reactions can be reversed when an
external circuit
is supplied.

Question 18

The greater the difference in reactivity the greater the ____

Answer 18

voltage produced.

Question 19

What does swapping the electrodes do?

Answer 19

Swapping the two electrodes means that the recorded voltage becomes negative.

Question 20

When two metals are dipped into a salt solution ad joined by a wire the more reactive metal will ___?

Answer 20

donate electrons to the less reactive metal forming a simple electrical cell.

Question 21

What is a fuel cell?

Answer 21

Fuel cells
work in a different way than chemical cells. Fuel cells produce a
voltage
continuously, as long as they are supplied with:

a constant supply of a suitable
fuel
oxygen, eg from the air
The fuel is
oxidised electrochemically, rather than being burned, so the reaction takes place at a lower temperature than if it was to be burned. Energy is released as electrical energy, not thermal energy
(heat).

Question 22

What is a hydrogen oxygen fuel cell?

Answer 22

Hydrogen-oxygen fuel cells are an alternative to rechargeable cells and batteries. In a hydrogen-oxygen fuel cell, hydrogen and oxygen are used to produce a voltage. Water is the only product. The overall reaction in a hydrogen-oxygen fuel cell is:

hydrogen + oxygen → water

Question 23

What are the half equations happening at the electrodes in a hydrogen fuel cell?

Answer 23

At the negative electrode: 2H2 + 4OH- → 4H2O + 4e-

At the positive electrode: O2 + 2H2O + 4e- → 4OH-

Question 24

What are the pros of different cells?

Answer 24

Alkaline-Cheaper to manufacture
Rechargeable-Can be recharged many times before being recycled, reducing the use of resources
Hydrogen fuel cell-Easy to maintain as there are no moving parts; small size; water is the only chemical product, doesn’t need to be electrically recharged.

Question 25

Explain which metal will be the negative terminal of a iron zinc electrical cell.

Answer 25

zinc, more reactive than iron,
greater tendency to lose electrons and form positive ions,
zinc atoms → Zn2+ ions at negative terminal, donating 2
electrons to iron(II) ions
Zn → Zn2+ + 2e−
Fe2+ ions → iron atoms at positive terminal
Fe2+(aq) + 2 e− → Fe(s)

Question 26

What is the reaction that happens in self heating cans?

Answer 26

CaO(s) + H2O(l) → Ca(OH)2(aq)

Question 27

Describe the chemical reaction in a hand warmer

Answer 27

hand warmer uses energy transferred to surroundings
in oxidation of iron,
forming hydrated iron(III) oxide in exothermic reaction,
NaCl catalyst