Energy changes

What are the two types of energy changes ?

• Exothermic

• Endothermic

What is an Exothermic reaction and state examples ?

• A reactionf that transfers energy to its surrounding

• The temperature of the surroundings increase

  Examples include

• Combustion

• Oxidation

• Neutralisation

What are the everyday uses of exothermic reactions ?

• Self-heating cans

• Hand warmers

What is an endothermic reaction and what are some examples?

• A reaction that takes in energy from the surroundings

• The temperature of the surroundings decreases

  Examples include:

• Thermal decomposition

• The reaction of citric acid and sodium hydrogencarbonate

What are the everyday uses of endothermic reactions?

• Some sports injury packs are based on endothermic reaction

What does the energy profile of an exothermic reaction look like?

• Products have lower energy than reactants

• Energy change for an exothermic reaction is negative

What is the activation energy ?

• The minimum amount of energy that particles must have to react is called the activation energy

How do we know a reaction is an exothermic reaction of the overall energy change is a negative number?

This is an exothermic reaction because the energy released from forming new bonds

is greater than the energy needed to break existing bonds.

How do we know a reaction is an endothermic reaction of the overall energy change is a postive number?

This is an endothermic reaction as the energy needed to break existing bonds is

greater than the energy released from forming new bonds.

How do cells produce chemicals?

• Cells contain chemicals which react to produce electricity

How can a simple cell be made?

• A simple cell can be made by connecting two different metals in contact with an electrolyte.

• The two electrodes are metals as they conduct electricity delocalised electrons which carry electrical charge.

• The electrolyte is a solution that conducts electricity and contains ions which are free to move and carry electrical charge.

How can batteries produce a greater voltage?

• Two or more cells connected in series produce a larger voltage

What is the voltage produced by the cell dependent on?

• Type of electrode

• Type of electrolyte

In non rechargeable batteries when does it stop working?

• When one of the reactants been used up

Are alkaline batteries rechargeable or non rechargeable?

• Non rechargeable

If there is a larger difference in reactivity, what does this tell us about the voltage ?

• Greater voltage

What type of ions do alkaline batteries include and what are the uses of this battery?

• OH- ions (hydroxide ions)

• Used in remote controls

Why can rechargeable batteries recharge?

• Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.

What are the difference between the processes in electrolysis and in a chemical cell?

o Electrolysis uses electricity to produce a chemical reaction

o Cells use a chemical reaction to produce electricity

When will the voltage be zero in a cell?

• The electrodes are made of the same metal

• The reactivity of the metals is the same

What is a fuel cell?

• Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air.

• The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.

What does the overall reaction in a hydrogen fuel cell involve ?

• The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water.

How do fuel cells produce electricity?

• Fuel and oxygen to produce electrical energy

How do hydrogen-oxygen fuel cells work?

• Electrolyte is often a solution of phosphoric acid and the electrodes are often porous carbon with a catalyst

• Hydrogen gas goes into the anode and oxygen into the cathode

• At the anode the hydrogen loses electrons to form hydrogen ions

• Hydrogen ions in the electrolyte moves to the cathode

• At the cathode, oxygen gains electrons from the cathode and reacts with hydrogen ions to make water

• The electrons flow through an external circuit from the anode to the cathode. This is the electric current.

What are the products of a hydrogen-oxygen fuel cell?

• Water and energy

As hydrogen-oxygen fuel cells involve a redox reaction what occurs at the anode and cathode + half equations?

• Anode - Oxidation

• Hydrogen loses electrons to form hydrogen ions

  H2 → 2H+ + 2e-

• Cathode - Reduction

• Oxygen gains electrons from the cathode and reacts with hydrogen ions to make water

  O2 + 4H+ 4e- → 2H2O

What is the overall equation for a hydrogen-oxygen fuel cell?

What are the advantages and disadvantages of Hydrogen fuel cells?

Hydrogen Fuel cells

Advantages

• No pollutants-Produces only water

• Do not need recharging

• Hydrogen can be made renewable if made by electrolysis using renewable energy.

Disadvantages

• Hydrogen is highly flammable

• Hydrogen is difficult to store/takes up a lot of space

• Hydrogen not renewable if produced using fossil fuels

• Not many hydrogen filling stations

What are the advantages and disadvantages of rechargeable batteries?

Rechargeable batteries

Advantages

• Charging points more widely available

• Rechargable

Disadvantages

• May release toxic chemicals on disposal

• Rechargeable batteries have finite life-time

• Can catch fire

• Store less energy than fuel cells

What is a potential alternative to rechargeable batteries and cells?

Hydrogen fuel cells