Edexcel IGCSE Chemistry · Spec 3.7C, 3.18
Bond-Energy Calculations and Reversible Heating (Paper 2)
Covers calculate enthalpy change from bond energies, Interpret the sign of the answer and Explain heating ammonium chloride.
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Bond-Energy Calculations and Reversible Heating (Paper 2)
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Explained
Bond energies, and what the sign of your answer means
Breaking a bond always needs energy put in. Making a bond always gives energy out. Every calculation in this topic is a comparison between those two totals, and the sign of the answer tells you which one won.
The calculation
The energy change equals the total energy needed to break all the bonds in the reactants minus the total energy released when all the bonds in the products form.
Work through it in the same order every time. List the bonds in the reactants with their energies and add them up. List the bonds in the products and add those up. Then subtract the second total from the first.
The most common slip is counting the bonds wrongly. Methane has four carbon to hydrogen bonds, not one. Oxygen gas is O equals O, a double bond, and it counts once. Draw the displayed formulae if you are unsure, because counting from a drawing is far more reliable than counting from a formula.
Reading the sign
A negative answer means the reaction is exothermic. Less energy was needed to break the old bonds than was released making the new ones, so energy has left the chemicals and gone to the surroundings. The surroundings get warmer.
A positive answer means the reaction is endothermic. More energy was needed to break the old bonds than was released making the new ones, so energy has been taken in from the surroundings and they get colder.
Combustion is always exothermic, so if you calculate a positive value for burning something, check your arithmetic before you write the conclusion.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Chemistry is unusually specific about drawing the profile. It records that double headed arrows are not accepted, that the arrow for the energy change needs to point downwards while the activation energy arrow points upwards, and that arrows drawn too short did not gain credit.
It also records a response where the arrow was in the right place but unlabelled, so the examiner could not tell whether it was meant to be the energy change or the activation energy, and no mark could be given. The report notes that where a question states the energy change must be labelled, an unlabelled arrow does not score.
One more detail from the same report. The word exothermic was printed in bold in the question stem, and a candidate who drew an endothermic profile lost the mark for it. Read the stem before you draw.
Heating ammonium chloride
Heated ammonium chloride breaks down into ammonia and hydrogen chloride, both gases. That takes energy in, so it is endothermic. When the gases cool further along the tube they recombine into solid ammonium chloride, which releases energy, so it is exothermic. A white solid appears at the cool end.
The two directions are the same reaction run forwards and backwards, so the energy change has the same size and the opposite sign. That is the general rule for any reversible reaction, and this experiment is the standard way of demonstrating it.
Spec 3.7C, 3.18
What you need to know
- Calculate enthalpy change from bond energies
- Interpret the sign of the answer
- Explain heating ammonium chloride
Active recall
Quick check
Answer each question before opening the answer.
What is the bond-energy equation?
Energy to break bonds minus energy released making bonds
What does a negative delta H mean?
The reaction is exothermic
Why does ammonium chloride appear on cooler glass?
Ammonia and hydrogen chloride recombine as they cool
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