AQA GCSE Combined Science (8464), Higher tier · Chemistry › Energy changes › Exothermic and endothermic reactions
Practise The energy change of reactions (HT only). 15 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Higher tier only. Bond energies explain why a reaction is exothermic or endothermic: breaking bonds takes in energy and making bonds releases it. You need to calculate the overall energy change of a reaction from bond energies given in the question (usually 3 to 4 marks) and explain the result in terms of bonds.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
5
State energy changes in breaking and making bondsBreaking bonds needs energy to be supplied; making bonds releases energy.
6
Count the bonds in displayed formulaeFind how many of each type of bond there are, e.g. H–O–H has two O–H bonds and O=C=O has two C=O bonds.
6
Explain exothermic or endothermic using bond energiesCompare the energy needed to break the bonds with the energy released when the new bonds form.
7
Calculate the energy change for a simple reactionOverall energy change = energy to break bonds − energy released making bonds, e.g. for H2 + Cl2 → 2HCl.
8
Calculate energy changes involving many bondsMultiply each bond energy by the number of bonds in a molecule and by the balancing number, e.g. for a combustion reaction.
9
Calculate an unknown bond energyPut the known values into overall energy change = bonds broken − bonds made and rearrange to find the missing bond energy.
Notes
Breaking and making bonds
During a reaction, bonds in the reactants break and new bonds form in the products.
Breaking bonds needs energy: energy must be supplied (an endothermic process).
Making bonds releases energy (an exothermic process).
A bond energy is the energy needed to break one mole of a particular bond, in kJ/mol. The same amount of energy is released when one mole of that bond forms.
Exothermic or endothermic?
Exothermic: the energy released forming new bonds is greater than the energy needed to break the existing bonds.
Endothermic: the energy needed to break the existing bonds is greater than the energy released forming new bonds.
Calculating the overall energy change
Overall energy change = total energy needed to break the bonds in the reactants − total energy released forming the bonds in the products.
A negative answer means the reaction is exothermic. A positive answer means it is endothermic.
Step 1: write out the displayed formula of every molecule, so you can see every bond.
Step 2: count each type of bond, multiplying by the balancing numbers in the equation.
Step 3: add up the energy for all the bonds broken, then for all the bonds made.
Step 4: subtract: bonds broken − bonds made. Give the sign and the unit kJ/mol.
Use the right bond: C=O (double) and C–O (single) have different energies. Oxygen, O=O, has a double bond and nitrogen, N≡N, has a triple bond.
Example: H2 + Cl2 → 2HCl, with H–H = 436, Cl–Cl = 243 and H–Cl = 432 kJ/mol. Bonds broken = 436 + 243 = 679 kJ/mol. Bonds made = 2 × 432 = 864 kJ/mol. Overall energy change = 679 − 864 = −185 kJ/mol, so the reaction is exothermic.
Exothermic: energy released making bonds > energy needed to break bonds
Endothermic: energy needed to break bonds > energy released making bonds
H2O: two O–H bonds. CO2: two C=O bonds. CH4: four C–H bonds
O2: one O=O bond. N2: one N≡N bond
Bond energies are in kJ/mol
How to answer each type of question
Explain, in terms of bonds, why a reaction is exothermic or endothermic
1 to 2 marks6
Decide which is bigger: the energy needed to break the bonds in the reactants, or the energy released when the bonds in the products form.
Exothermic: more energy is released making bonds than is needed to break bonds.
Endothermic: more energy is needed to break bonds than is released making bonds.
If numbers are given, quote them in your answer.
Example. For the reaction 2HI → H2 + I2: energy needed to break the bonds in the reactants = 598 kJ/mol energy released when the bonds in the products form = 587 kJ/mol Explain, in terms of bonds, whether the reaction is exothermic or endothermic. (2)
Show the model answer
Endothermic (1). More energy is needed to break the H–I bonds (598 kJ/mol) than is released when the H–H and I–I bonds form (587 kJ/mol) (1).
Calculate the overall energy change (simple reaction)
3 marks7
List every bond broken in the reactants and every bond made in the products, multiplied by the number of molecules.
Add up the energy for the bonds broken and for the bonds made separately.
Overall energy change = bonds broken − bonds made.
Give the sign and the unit, kJ/mol.
Example. Hydrogen reacts with fluorine to produce hydrogen fluoride. H–H + F–F → 2 H–F Bond energies in kJ/mol: H–H = 436, F–F = 158, H–F = 568 Calculate the overall energy change for this reaction. (3)
Put in all the values you know, and write the unknown bond energy as a letter (e.g. x) multiplied by the number of those bonds.
Rearrange carefully, keeping track of the negative sign.
Divide by the number of those bonds to find the energy of one bond.
Example. Nitrogen reacts with hydrogen to produce ammonia. N≡N + 3 H–H → 2 NH3 (each NH3 molecule has three N–H bonds) The overall energy change for the reaction is −93 kJ/mol. Bond energies in kJ/mol: N≡N = 945, H–H = 436 Calculate the bond energy of the N–H bond. (4)
'Break in, make out': breaking bonds takes energy in, making bonds gives energy out.
Always 'broken minus made'. If the bonds made total is bigger, the answer must be negative (exothermic).
Sense check: combustion and neutralisation are exothermic, so a positive answer for them means you have made a mistake.
Check your displayed formulae: carbon makes 4 bonds, nitrogen 3, oxygen 2, and hydrogen and the halogens 1 each.
Write the bond counts in a small table (bond, number, energy, total) for reactants and products. It makes it easy to spot a missed bond.
Where marks are lost
Working out bonds made − bonds broken, which gives the right number with the wrong sign.
Forgetting the balancing numbers, e.g. counting one O=O bond for 3O2 instead of three.
Counting one O–H bond per water molecule instead of two, or one C=O bond per carbon dioxide molecule instead of two.
Using the C–C or C–O value where the molecule has C=C or C=O.
Leaving out the negative sign or the unit (kJ/mol) in the final answer.
Writing 'breaking bonds releases energy'. It is the other way round: breaking bonds needs energy.
Exam technique
Set out your working as two clear lines, 'bonds broken = ...' and 'bonds made = ...'. Each line can earn a mark even if your final answer is wrong.
Always give the sign and the unit, e.g. −542 kJ/mol. The negative sign tells the examiner the reaction is exothermic.
If the equation uses molecular formulae (e.g. CH4), sketch the displayed formulae in the margin before you count.
For 'explain in terms of bonds', compare the two amounts of energy using 'more' or 'less'. Just writing 'bonds are broken and made' does not score.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What does the bond energy of a covalent bond measure?
The energy needed to break one mole of that bond.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Energy changes in chemical reactions can be explained using ideas about chemical bonds.
(a) Which statement about bond breaking and bond making is correct? Tick (✓) one box.[1]
Breaking bonds releases energy and making bonds needs energy.
Breaking bonds needs energy and making bonds releases energy.
Breaking bonds and making bonds both need energy.
Breaking bonds and making bonds both release energy.
(b) For one reaction:
energy needed to break the bonds in the reactants = 2650 kJ/mol
energy released when the bonds in the products form = 3120 kJ/mol.
Calculate the overall energy change for the reaction.[2]
(c) Explain, in terms of bonds, why this reaction is exothermic.[1]
Show the answer and mark scheme
(a)Answer: Breaking bonds needs energy and making bonds releases energy.
(b)Answer: −470 kJ/mol
2650 − 3120
−470 (kJ/mol)
(c)Answer: More energy is released when the new bonds form than is needed to break the old bonds.
more energy is released when (new) bonds form than is needed to break the (old) bonds
Question 2Medium2 marks
(a) Which equation is used to calculate the overall energy change of a reaction from bond energies? Tick (✓) one box.[1]
overall energy change = energy needed to break bonds in the reactants − energy released making bonds in the products
overall energy change = energy released making bonds in the products − energy needed to break bonds in the reactants
overall energy change = energy needed to break bonds in the reactants + energy released making bonds in the products
overall energy change = energy released making bonds in the reactants − energy needed to break bonds in the products
(b) What does the bond energy of a covalent bond measure?[1]
Show the answer and mark scheme
(a)Answer: overall energy change = energy needed to break bonds in the reactants − energy released making bonds in the products
(b)Answer: The energy needed to break one mole of that bond.
the energy needed to break (one mole of) the bond
Question 3Hard6 marks
Hydrogen reacts with iodine vapour in a reversible reaction. H–H + I–I ⇌ 2 H–I The overall energy change for the forward reaction is −9 kJ/mol. Bond energy of H–H = 436 kJ/mol Bond energy of I–I = 151 kJ/mol
(a) Calculate the bond energy of the H–I bond.[4]
(b) The bond energy of H–Cl is 432 kJ/mol. Use your answer to suggest what this shows about the strength of the H–I bond compared with the H–Cl bond. Suggest a reason for this, based on the position of iodine and chlorine in the periodic table.[2]
Show the answer and mark scheme
(a)Answer: 298 kJ/mol
bonds broken: 436 + 151 = 587 (kJ/mol)
let the H–I bond energy be X: 587 − 2X = −9
2X = 587 + 9 = 596
X = 298 (kJ/mol)
(b)Answer: The H–I bond (298 kJ/mol) is weaker than the H–Cl bond (432 kJ/mol); iodine atoms are much bigger than chlorine atoms, so the bonding pair of electrons is further from the iodine nucleus and less strongly attracted.
the H–I bond is weaker than the H–Cl bond
iodine is below chlorine in Group 7, so iodine atoms are larger, meaning the shared (bonding) electrons are further from the nucleus and less strongly attracted