C5 - Energy Changes

In chemical reactions, energy is __________.

conserved

What happens in a chemical reaction (in terms of energy)?

Energy is transferred either to or from the surroundings.

What happens in an exothermic reaction?

Energy is transferred to the surroundings.

What happens to the temperature of the reaction in an exothermic reaction?

It increases

What happens in an endothermic reacton?

Energy is transferred to the reaction.

What happens to the temperature of the reaction in an endothermic reaction?

It decreases

What are examples of exothermic reactions? (3)

• combustion reactions

• many oxidation reactions

• most neutralisation reactions

What are everyday uses of exothermic reactions? (2)

• Self-heating cans

• Hand warmers

What are examples of endothermic reactions? (2)

• thermal decomposition reactions

• the reaction of citric acid and sodium hydrogencarbonate

What are everyday uses of endothermic reactions? (1)

• instant ice packs

What is the equation of an endothermic neutralisation reaction?

ethanoic acid + sodium carbonate → sodium ethanoate + water + CO2

What is the required practical for this topic?

Temperature changes in reacting solutions

What can be used to display energy changes in a reaction?

• A reaction profile

• An energy level diagram

Exothermic reaction - energy level diagram

Endothermic reaction - energy level diagram

Exothermic - reaction profile

Endothermic - reaction profile

Which is typically more useful in displaying energy changes, an energy level diagram or a reaction profile? Why?

A reaction profile, because it includes the activation energy

activation energy

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

How can you tell from a reaction profile that a reaction is exothermic?

The overall change is negative, as the products have less energy than the reactants.

How can you tell from a reaction profile that a reaction is endothermic?

The overall change is positive, as the products have more energy than the reactants.

When is chemical energy transferred?

When bonds are broken or formed.

Breaking bonds is exothermic/endothermic.

endothermic

Breaking bonds takes/gives heat energy in/out.

Breaking bonds takes heat energy in.

Forming bonds is exothermic/endothermic.

exothermic

Forming bonds takes/gives heat energy in/out.

Forming bonds gives heat energy out.

What happens during a chemical reaction?

• Bonds in the reactants are broken

• New bonds are made in the products

What determines the type of reaction?

The difference between…

Energy needed to break bonds - energy released when new bonds are formed

broken - formed = final

In an exothermic reaction, what happens? (in terms of bond energies)

More heat energy is released in making the bonds in the products than is taken in when reading bonds in the reactants.

In an endothermic reaction, what happens? (in terms of bond energies)

Less heat energy is released in making the bonds in the products than is taken in when breaking the bonds in the reactants.

What is a bond energy>

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

How can you calculate an energy change for a reaction? (step-by-step method)

1. Add together all reactant bonds’ bond energies.

   This is energy in.

2. Add together all product bonds’ energies.

   This is energy out.

3. energy change = energy in - energy out

Work out this question.

Work out this question.

Work out this question.

What is a chemical cell?

A store of internal energy that can be transferred as an electric current in a circuit.

Chemical cells use ____________________ to transfer energy by ___________.

Chemical cells use chemical reactions to transfer energy by electricity.

What does the voltage of a cell depend upon?

A number of factors, including…

• What the electrolytes are made from

• The substance used as the electrolyte

In a cell, which is the positive electrode?

Cathode

In a cell, which is the negative electrode?

Anode

How can a simple cell be made?

By connecting 2 different metals in contact with an electrolyte.

How can a battery be made?

By connecting a number of cells in series.

A battery has a ______ voltage than a single cell.

A battery has a higher voltage than a single cell.

What are the 2 types of batteries?

• non-rechargeable (alkaline) batteries

• rechargeable batteries

How do non-rechargeable batteries work?

A voltage is produced until one of the reactants becomes limiting (is used up), and the battery ‘goes flat’.

How do rechargeable batteries work?

The chemical reactions can be reversed when an external circuit is supplied.

How can you achieve a cell with the highest voltage?

By using electrodes of the largest difference in reactivity.

Finish this table.

Swapping the 2 electrodes means that the recorded voltage becomes __________.

negative

To achieve a positive voltage, which electrode should go on which side?

• Left: more reactive

• Right: less reactive

What is a fuel cell?

Device that produces a voltage continuously when supplied with a fuel and oxygen.

Is the fuel cell reaction economically efficient?

Yes

Why is the fuel cell reaction economically efficient?

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.

Which is a very common fuel cell example?

The hydrogen-oxygen fuel cell

What is the word equation of hydrogen-oxygen fuel cells?

hydrogen + oxygen → water

Hydrogen-oxygen fuel cell

Electrode half equation - anode

Anode is negative.

2H₂ + 4OH^- → 4H₂O + 4e^-

Hydrogen-oxygen fuel cell

Electrode half equation - cathode

Cathode is positive.

O₂ + 2H₂O + 4e^- → 4OH^-

Hydrogen-oxygen fuel cell

Overall equation

2H₂ + O₂ → 2H₂O

Non-rechargeable cells are also known as…

…alkaline cells.

Alkaline cells - Advantages (1)

• Cheaper to manufacture

Alkaline cells - Disadvantages (2)

• May end up in landfill sites once fully discharged

• Recyclable, but it is expensive

Rechargeable cells - Advantages (2)

• Can be recharged many times before being recycled

• Reduces use of resources

Rechargeable cells - Disadvantages (1)

• Costs more to manufacture

Hydrogen fuel cell - Advantages (3)

• Easy to maintain as there are no moving parts

• Small size

• Water is the only chemical product

Hydrogen fuel cell - Disadvantages (3)

• Very expensive to manufacture

• Needs a constant supply of hydrogen fuel

• Hydrogen is a flammable gas

Where are fuel cells often used?

in…

• spacecraft

• vehicles

What may be a problem with using hydrogen fuel cells in space?

Much be supplied with hydrogen fuel and oxygen, which aren’t present in space.

How is the problem of no oxygen in space overcome?

Spacecraft in orbit, eg. ISS, have solar cells to convert light into electricity, so the hydrogen and oxygen are replaced by the electrolysis of water.

Why are fuel cells still used, instead of just the electrolysis of water alone?

Solar cells only work when they are in light, so the fuel cells allow electricity to be produced even when the spacecraft is in the dark.

What is the ISS?

International Space Station