Ch 2 · Hydrogen–Oxygen Fuel Cells · O Level Chemistry 5070
Topic 2 of 2 in Electrochemistry — O Level Chemistry 5070 — 38 lessons.
Introduction
Electrolysis uses electricity to drive a chemical reaction. A fuel cell does the reverse: it uses a chemical reaction to produce electricity directly, without burning the fuel. The hydrogen–oxygen fuel cell already powers buses, cars and spacecraft, and questions on it usually combine a short equation with an evaluation of its advantages and disadvantages compared with a petrol engine.
Key ideas — What a hydrogen–oxygen fuel cell does
A hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only chemical product.
🧑🏫 Teacher's explanation
Imagine a water slide where the water is constantly being pumped back to the top. In a normal battery, you have a limited amount of 'fuel' inside the casing; once it's used, the battery is dead. A fuel cell is different because it is an open system. As long as you keep feeding it hydrogen gas at one end and oxygen gas at the other, the 'slide' keeps running, producing electricity continuously.
📖 Definitions
🔍 Worked example — Easy
Write the balanced chemical equation for the reaction occurring in a hydrogen-oxygen fuel cell, including state symbols.
🔍 Worked example — Standard
A fuel cell produces 0.50 moles of water per second. Calculate the total volume of hydrogen gas required per second, assuming the gas is at STP (molar volume = 22.4 dm^3/mol).
🔍 Worked example — Exam-style
Compare the environmental impact of a hydrogen fuel cell to a petrol engine. Which is more sustainable and why?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — The overall equation
The overall reaction is the same as burning hydrogen in oxygen:
🧑🏫 Teacher's explanation
Think of a fuel cell like a battery that never runs out, as long as you keep feeding it fuel. In a normal battery, the chemicals inside get used up and the battery 'dies'. In a hydrogen-oxygen fuel cell, you are constantly pumping in hydrogen gas and oxygen gas. As long as the flow continues, the electricity keeps flowing.
📖 Definitions
🔍 Worked example — Easy
Write the balanced chemical equation for the reaction occurring in a hydrogen-oxygen fuel cell, including state symbols for gases.
🔍 Worked example — Standard
Calculate the mass of water produced when 4.0 ext g of hydrogen gas reacts completely in a fuel cell with excess oxygen.
🔍 Worked example — Exam-style
A fuel cell produces 1.8 ext g of water. How many moles of oxygen gas were consumed in this process? (Assume H=1, O=16)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Fuel cells compared with petrol engines
advantages and disadvantages. Examiners expect a balanced comparison with a gasoline (petrol) engine in a vehicle.
🧑🏫 Teacher's explanation
Think of a fuel cell like a high-tech battery that never runs out as long as you keep feeding it fuel. In a normal battery, the chemicals inside get used up and you have to recycle the battery. In a fuel cell, the 'fuel' (hydrogen) and the 'oxidizer' (oxygen) are pumped in continuously from an external tank.
📖 Definitions
🔍 Worked example — Easy
Write the balanced chemical equation for the reaction occurring in a hydrogen-oxygen fuel cell. Include state symbols.
🔍 Worked example — Standard
A fuel cell produces 0.5 moles of water. Calculate the volume of hydrogen gas required at RTP (Room Temperature and Pressure), assuming 1 mole of any gas occupies 24 dm^3.
🔍 Worked example — Exam-style
Compare the efficiency of a fuel cell to a petrol engine. If a petrol engine converts 25\% of the energy in fuel into useful work, and a fuel cell converts 60\% of the energy in fuel into electricity, what is the ratio of fuel cell efficiency to engine efficiency?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. A bus company is considering replacing its diesel and petrol vehicles with buses powered by hydrogen–oxygen fuel cells. (a) Write the overall equation for the reaction in the fuel cell. [1] (b) State the only chemical product and explain why this makes the buses attractive in a city centre. [2] (c) Give two disadvantages the company should consider. [2]
🔍 Worked example — Easy
Write the balanced chemical equation for the reaction occurring in a hydrogen-oxygen fuel cell, including state symbols.
🔍 Worked example — Standard
A fuel cell produces 4.0 moles of water. Calculate the total volume of oxygen gas required at RTP (Room Temperature and Pressure), assuming 1 mole of any gas occupies 24.0 dm^3.
🔍 Worked example — Exam-style
Compare a hydrogen fuel cell to a petrol engine. State one environmental advantage and one practical disadvantage of the fuel cell.
⚠️ Common errors
Common mistakes
Practice
Answers Uses hydrogen and oxygen to produce electricity [B1]; water is the only chemical product [B1]. 2H_2(g) + O_2(g) arrow 2H_2O(l): correct balanced equation [B1]; correct state symbols [B1]. The chemical energy is converted directly into electrical energy [B1]; a petrol engine loses a large proportion of the energy as heat to the surroundings [B1]. True at the point of use, because the only product of the cell is water [B1]; but hydrogen is usually made from natural gas or with electricity from fossil-fuel power stations, which does release carbon dioxide [B1]; so the statement is only correct if the hydrogen is produced using renewable energy [B1].