Bond Energies, Cells and Fuel Cells
Bond energies, cells and fuel cells are three linked ideas in energy changes: calculating the overall energy change of a reaction from bond energies, using the rule that breaking bonds requires energy and forming bonds releases energy; how chemical cells and batteries use a reaction between chemicals to produce a voltage, including the difference between rechargeable and non-rechargeable cells; and how a hydrogen fuel cell produces electricity from the reaction between hydrogen and oxygen, producing only water as a waste product.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the rule that breaking bonds in the reactants requires (absorbs) energy, and forming new bonds in the products releases energy; whether the overall reaction is exothermic or endothermic depends on the balance between these two energy transfers.
- Calculate the overall energy change using: energy change = total energy needed to break bonds in the reactants - total energy released forming bonds in the products. The bond energy values needed will always be given in the question.
- If the energy needed to break bonds is less than the energy released forming new bonds, the overall energy change is negative and the reaction is exothermic; if it is greater, the overall energy change is positive and the reaction is endothermic.
- When working through a bond energy calculation, first list every bond in every reactant molecule and every bond in every product molecule, using the number of each type of bond shown by the balanced symbol equation, so that no bond is missed or double counted.
- For chemical cells, learn that a cell contains chemicals that react to produce a voltage, that the size of the voltage depends on factors including the type of electrode and the electrolyte used, and that connecting cells together in series (a battery) produces a larger combined voltage.
- Distinguish non-rechargeable cells, where the chemical reaction stops once a reactant is used up, from rechargeable cells, where passing an external electric current through the cell reverses the chemical reaction, restoring the original chemicals.
- For fuel cells, learn that a hydrogen fuel cell is continuously supplied with hydrogen fuel and oxygen from outside the cell, and that the overall reaction, hydrogen + oxygen -> water, produces a voltage without needing to be recharged, as long as fuel keeps being supplied.
Worked example
Hydrogen burns in oxygen: 2H2 + O2 -> 2H2O. Use the bond energies given to calculate the overall energy change for this reaction, and state whether the reaction is exothermic or endothermic. Bond energies: H-H = 436 kJ/mol, O=O = 498 kJ/mol, O-H = 464 kJ/mol.
- List the bonds broken in the reactants: 2 mol of H-H bonds (in 2H2) and 1 mol of O=O bonds (in O2).
- Calculate the energy to break these bonds: (2 x 436) + (1 x 498) = 872 + 498 = 1370 kJ.
- List the bonds formed in the products: each H2O molecule has 2 O-H bonds, and there are 2 H2O molecules, so 4 mol of O-H bonds are formed.
- Calculate the energy released forming these bonds: 4 x 464 = 1856 kJ.
- Calculate the overall energy change: energy to break bonds - energy released forming bonds = 1370 - 1856 = -486 kJ.
- Final answer: the overall energy change is -486 kJ; since this is negative, more energy is released forming bonds than is used breaking them, so the reaction is exothermic.
Practice questions
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Q1State whether breaking a chemical bond requires energy or releases energy.Show answer
Answer: It requires (absorbs) energy.
Q2State whether forming a new chemical bond requires energy or releases energy.Show answer
Answer: It releases energy.
Q3Hydrogen reacts with chlorine to form hydrogen chloride: H2 + Cl2 -> 2HCl. Use the bond energies given to calculate the overall energy change for this reaction. Bond energies: H-H = 436 kJ/mol, Cl-Cl = 243 kJ/mol, H-Cl = 432 kJ/mol.Show answer
Answer: -185 kJ, exothermic (energy to break bonds = 436 + 243 = 679 kJ; energy released forming bonds = 2 x 432 = 864 kJ; overall energy change = 679 - 864 = -185 kJ).
Q4State one factor that affects the size of the voltage produced by a chemical cell.Show answer
Answer: The type of electrode used (accept: the type of electrolyte used).
Q5State the difference between a rechargeable cell and a non-rechargeable cell.Show answer
Answer: In a rechargeable cell, the chemical reaction can be reversed by passing an external electric current through the cell, restoring the original chemicals; in a non-rechargeable cell, the reaction stops permanently once a reactant is used up.
Q6State the two substances that react inside a hydrogen fuel cell.Show answer
Answer: Hydrogen and oxygen.
Q7State the only substance produced when a hydrogen fuel cell reacts pure hydrogen with pure oxygen.Show answer
Answer: Water.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student is given bond energy data for a displacement reaction. Breaking the bonds in the reactants requires 945 kJ per mole of reaction, and forming the bonds in the products releases 1180 kJ per mole of reaction. (a) Calculate the overall energy change for the reaction. (b) State, with a reason, whether the reaction is exothermic or endothermic.
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A car manufacturer is comparing a rechargeable lithium-ion battery with a hydrogen fuel cell as ways of powering an electric car. Describe one similarity and two differences between how a rechargeable battery and a hydrogen fuel cell produce electrical energy.
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Some car manufacturers are developing cars powered by hydrogen fuel cells, as an alternative to cars with petrol engines or cars with rechargeable batteries. Evaluate the advantages and disadvantages of using hydrogen fuel cells to power cars.
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Free printable worksheet
Want more practice on paper? Download the bond energies, cells and fuel cells worksheet pack - 20 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 23 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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