Chemical Cells & Fuel Cells Flashcards

All 19 cards in this deck

What is needed to make a simple chemical cell?

Two different metals as electrodes, dipped in an electrolyte and connected by wires in a complete circuit.

What is a battery, and what voltage does it give?

e.g. four 1.5 V cells joined together

Two or more cells connected in series; their voltages add to give a greater voltage.

e.g. $4 \times 1.5 = 6$ V

Why do non-rechargeable cells and batteries stop producing electricity?

The chemical reactions stop when one of the reactants has been used up.

True or false? A rechargeable cell can be recharged because the chemical reactions in it are reversed when an external electrical current is supplied.

True. Alkaline batteries cannot be recharged in this way — they are non-rechargeable.

What are the steps to deduce an order of reactivity from the voltages of cells made from different metals?

e.g. metals each paired with copper give Mg 2.7 V, Zn 1.1 V, Ag 0.5 V

1. Compare the voltages: the higher the positive voltage, the more reactive the metal.
2. Order from highest to lowest voltage: Mg, Zn, Ag (most to least reactive).
3. Justify: the greater the difference in reactivity of the two metals, the greater the voltage.

What is a fuel cell?

A cell supplied by an external source of fuel (e.g. hydrogen) and oxygen or air, in which the fuel is oxidised electrochemically to produce a potential difference.

What is the overall equation for the reaction in a hydrogen fuel cell?

$2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$

How does a hydrogen fuel cell produce a potential difference?

Hydrogen is oxidised electrochemically (it loses electrons) to produce water.

Give two advantages of hydrogen fuel cells compared with rechargeable cells and batteries.

No toxic chemicals to dispose of at the end of the cell's life; they take less time to refuel and travel further before refuelling; no loss of efficiency over time.

Give two disadvantages of hydrogen fuel cells compared with rechargeable cells and batteries.

Hydrogen is a highly flammable gas that is difficult to store and there are few refuelling stations; energy is needed to produce the hydrogen fuel.

True or false? A hydrogen fuel cell must be recharged from an external electrical supply once its reactants run out.

False. It keeps producing a potential difference as long as hydrogen and oxygen are supplied from outside the cell.

At which electrode of a hydrogen fuel cell is hydrogen oxidised?

The negative electrode, where hydrogen loses electrons.

Write the half equation for the reaction at the negative electrode of an acidic hydrogen fuel cell.

$\mathrm{H_2} \rightarrow 2\mathrm{H^+} + 2\mathrm{e^-}$

Write the half equation for the reaction at the positive electrode of an acidic hydrogen fuel cell.

$\mathrm{O_2} + 4\mathrm{H^+} + 4\mathrm{e^-} \rightarrow 2\mathrm{H_2O}$

Write the half equation for the reaction at the negative electrode of an alkaline hydrogen fuel cell.

$\mathrm{H_2} + 2\mathrm{OH^-} \rightarrow 2\mathrm{H_2O} + 2\mathrm{e^-}$

Write the half equation for the reaction at the positive electrode of an alkaline hydrogen fuel cell.

$\mathrm{O_2} + 2\mathrm{H_2O} + 4\mathrm{e^-} \rightarrow 4\mathrm{OH^-}$

True or false? In a hydrogen fuel cell, hydrogen is reduced at the negative electrode.

False. Hydrogen is oxidised at the negative electrode — it loses electrons.

Name two factors that affect the voltage produced by a chemical cell.

The type of electrode (the metals used) and the type of electrolyte.

True or false? Changing the electrolyte in a simple cell has no effect on the voltage produced.

False. The voltage depends on the type of electrolyte as well as the type of electrode.