AQA Chemistry C5 (Part 2) - Chemical Cells and Fuel Cells

define electrolyte

A substance which, when molten or in solution, will conduct an electric current e.g. a molten ionic solution.

how is a charge difference/voltage created?

due to the reactions between the electrodes and the electrolyte.

factors that affect voltage:

→ type of electrode/electrolyte

→ difference in reactivity of electrodes

→ concentration of electrolyte

→ temperature of electrolyte

→ surface area of electrode

→ distance between electrodes

how would you know the charge of each electrode?

the most reactive metal is always the negative electrode as they give up electrons the easiest.

how would you create a 12V battery out of 1.5V cells?

by connecting 8, 1.5V cells in series to one another.

oxidation always occurs at …?

the anode

reduction always occurs at …?

the cathode

oxidation is the …

loss of electrons

reduction is the …

gain of electrons

non-rechargeable cells are also called …?

primary cells

rechargeable cells are also called …?

secondary cells

what happens to primary cells overtime?

they stop working due to the reacting particles (ions within the electrolyte and the ones in the electrode) being completely used up and turned into products that can’t be changed back, therefore no current is produced.

secondary cells involve a …

reversible reaction that is only possible due to an external electrical current being supplied to it, to change the products back into reactants.

fuel cells produce a voltage continuously as long as they are supplied with:

* a constant supply of a suitable fuel
* oxygen e.g from air, pure oxygen

state the half-equation for the reaction at the anode (-ve ) of a hydrogen-oxygen fuel cell. (acid electrolyte)

H2→2H^+ **+** 2e^-

state the half-equation for the reaction at the cathode (+ve electrode) of a hydrogen-oxygen fuel cell. (acid electrolyte)

O2 + 4H^+ **+** 4e^- → 2H2O

state the half-equation for the reaction at the anode (-ve ) of a hydrogen-oxygen fuel cell. (alkaline electrolyte)

H2 + 2OH^- → 2H2O + 2e^-

state the half-equation for the reaction at the cathode (+ve ) of a hydrogen-oxygen fuel cell. (alkaline electrolyte)

O2 + 2H2O + 4e^- → 4OH^-

state the overall equation for the reaction of a hydrogen-oxygen fuel cell.

2H2 + O2 → 2H2O

what creates the potential difference across a fuel cell?

the reaction between the fuel and oxygen (which is an electrochemical reaction rather than combustion, and releases electrical energy)

what reactants enter which compartment?

hydrogen enters the anode (-ve) compartment, oxygen enters the cathode (+ve) compartment

what are the electrodes of a fuel cell usually made of?

porous carbon and a catalyst

advantages of hydrogen fuel cells (compared to batteries)

* the toxic metal compounds batteries are composed of can contaminate when incorrectly disposed of
* their reactants are abundant with no greenhouse gases released as products
* high-energy storage
* they don’t get less efficient the longer they run
* you can travel a further distance with fuel cells without refuelling

disadvantages of hydrogen fuel cells (compared to batteries)

* hydrogen is a gas and is therefore less easy to store
* the process of obtaining pure hydrogen is expensive and uses lots of energy from fossil fuels unless the electrolysis is done with a renewable source of energy
* highly flammable compared to batteries (lithium-ion batteries are not flammable at all!!!)

acids produce ____ when a current flows through

H^+ ions

alkalines produce ____ when a current flows through

OH^- ions