Titrations, Chemical Cells and Fuel Cells
Titrations
A titration is a practical technique used to find the precise volume of an acid or alkali needed to neutralise a known volume of the other. It is a required practical for AQA Separate Chemistry and a very commonly tested topic.
In a titration, the solution in the burette is added to the solution in the conical flask until the indicator shows the end point — the exact point at which neutralisation is complete.
Titration Method
- Use a pipette to measure an exact volume of alkali (e.g. 25.0 cm³ of sodium hydroxide) into a conical flask.
- Add a few drops of indicator (e.g. phenolphthalein or methyl orange).
- Fill a burette with the acid solution and record the starting volume.
- Slowly add acid from the burette to the flask, swirling continuously.
- Near the end point, add the acid drop by drop until the indicator permanently changes colour.
- Record the final burette reading. The volume of acid used = final reading − initial reading.
- Repeat the titration until you have concordant results (two or more results within 0.10 cm³ of each other). Calculate a mean from the concordant results only, ignoring any anomalous results.
Once you have the titre (volume of acid used), you can calculate the unknown concentration:
Step 1: Calculate moles of the known solution: \(\text{moles} = \text{concentration} \times \text{volume (dm}^3\text{)}\)
Step 2: Use the balanced equation to find the mole ratio and calculate moles of the unknown.
Step 3: Calculate the unknown concentration: \(\text{concentration} = \frac{\text{moles}}{\text{volume (dm}^3\text{)}}\)
Example: 25.0 cm³ of 0.10 mol/dm³ NaOH was neutralised by 20.0 cm³ of HCl.
Moles of NaOH = 0.10 × 0.025 = 0.0025 mol
From \(\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}\), the ratio is 1:1, so moles of HCl = 0.0025 mol.
Concentration of HCl = 0.0025 ÷ 0.020 = 0.125 mol/dm³
Chemical Cells (Batteries)
A chemical cell (battery) produces electricity from chemical reactions. When two different metals are placed in an electrolyte (a conducting solution), a voltage is produced because the metals have different reactivities.
The voltage produced by a chemical cell depends on the difference in reactivity between the two metals. The greater the difference in reactivity, the higher the voltage. For example, zinc and copper produce a higher voltage than iron and copper because zinc is more reactive than iron.
In a chemical cell, the more reactive metal is oxidised (loses electrons) at the negative electrode, and the less reactive metal (or its ions) is reduced (gains electrons) at the positive electrode. Electrons flow through the external circuit from the more reactive to the less reactive metal.
Non-rechargeable cells: The chemical reactions are irreversible. Once the reactants are used up, the cell can no longer produce electricity and must be disposed of.
Rechargeable cells: The chemical reactions are reversible. Passing an electrical current through the cell reverses the reactions, regenerating the original reactants. Examples include lithium-ion batteries used in phones and laptops.
Fuel Cells
A fuel cell produces electricity from the reaction of a fuel (usually hydrogen) with oxygen. Unlike batteries, fuel cells do not run out — they produce electricity continuously as long as fuel and oxygen are supplied.
In a hydrogen fuel cell, the overall reaction is: \(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\). The only waste product is water, making fuel cells a clean energy source. At the negative electrode: \(\text{H}_2 \rightarrow 2\text{H}^+ + 2e^-\) (oxidation). At the positive electrode: \(\text{O}_2 + 4\text{H}^+ + 4e^- \rightarrow 2\text{H}_2\text{O}\) (reduction).
Advantages of hydrogen fuel cells: the only waste is water (no carbon dioxide), they are very efficient, and they can power vehicles. Disadvantages: hydrogen is difficult and expensive to store and transport, most hydrogen is currently produced from fossil fuels, and fuel cell technology is expensive.