GCSE · Chemistry · AQA · Spec 8462

Energy change calculations from bond energies (HT)

Some reactions give energy out and some take it in, and you can predict which with a few multiplications and one subtraction.

The energy account

Where do the products end up?

The reactants start at 0 on this energy line (kJ/mol). Reaction A: breaking all its reactant bonds takes 1400, and forming its product bonds releases 1700. Reaction B: breaking takes 1000, and forming releases 700. For each reaction, place the level once every bond is broken, then where the products finish.

A up — Reaction A: every reactant bond brokenA end — Reaction A: products formedB up — Reaction B: every reactant bond brokenB end — Reaction B: products formed

Before any maths

Where does the energy come from?

A reaction gives out energy overall. During the reaction, the bonds in the reactants break and new bonds form in the products.

Which of these is closest to what you think right now?

The energy comes from the bonds breaking. Snap a bond and the energy stored in it pings out.Breaking bonds takes energy, and making new bonds takes energy too. Building something always costs energy.Breaking bonds gives out energy and making bonds gives out energy too, so the reaction gets hot twice.Breaking the old bonds takes energy in, and forming the new bonds gives energy out. Here, forming gave out more than breaking took in.

How sure are you?

I'm sureI think soI'm guessing

The energy comes from the bonds breaking. Snap a bond and the energy stored in it pings out.

This is a common misconception

This is the most common mix-up in the whole topic, so you're in good company. But a bond is what holds two atoms together. To pull them apart you have to overcome that attraction, and that takes energy. So breaking a bond always needs energy put in. The energy that comes out comes from the other half of the reaction: new bonds forming.

Breaking bonds takes energy, and making new bonds takes energy too. Building something always costs energy.

This is a common misconception

You've got the first half exactly right. The second half is the trap: forming a bond is the opposite of breaking one. When atoms join and a bond forms, energy is released. If both halves took energy in, no reaction could ever give energy out.

Breaking bonds gives out energy and making bonds gives out energy too, so the reaction gets hot twice.

This is a common misconception

Only one of those halves gives energy out. Breaking and forming are opposites, so they can't both do the same thing to energy. Breaking bonds takes energy in; forming bonds releases it. A reaction's overall energy change is the gap between those two amounts.

Breaking the old bonds takes energy in, and forming the new bonds gives energy out. Here, forming gave out more than breaking took in.

This matches the evidence

Exactly right. Breaking needs energy supplied; forming releases energy. This reaction gives out energy overall because the energy released by forming the new bonds is bigger than the energy needed to break the old ones. That's what exothermic means in terms of bonds.

Breaking bonds takes energy in. Forming bonds gives energy out. Whichever is bigger decides whether the reaction gives out energy or takes it in overall.

Count before you calculate

Tap each molecule to see its bonds. The count is always: bonds in one molecule × the number in front of its formula.

Tap a molecule to see what it does in the reaction.

Watch it done once

Problem

Hydrogen reacts with oxygen to make water: 2H₂ + O₂ → 2H₂O. Bond energies: H–H = 436 kJ/mol, O=O = 498 kJ/mol, O–H = 464 kJ/mol. Calculate the overall energy change and say whether the reaction is exothermic or endothermic.

Your turn — with a safety net

This one comes out the other way

Ammonia can be broken down into nitrogen and hydrogen: 2NH₃ → N₂ + 3H₂. Bond energies: N–H = 391 kJ/mol, N≡N = 945 kJ/mol, H–H = 436 kJ/mol. Calculate the overall energy change.

Draw the bonds: each NH₃ molecule has three N–H bonds, N₂ is one N≡N triple bond, and each H₂ is one H–H bond.
missing step

Which line is step 2?

Bonds broken: 3 N–H bonds, so 3 × 391 = 1173 kJ/mol.Bonds broken: 2 molecules × 3 N–H = 6 N–H bonds, so 6 × 391 = 2346 kJ/mol.Bonds broken: 2 N–H bonds, so 2 × 391 = 782 kJ/mol.

Spot the slip

One line loses the marks

Hydrogen reacts with chlorine: H₂ + Cl₂ → 2HCl. Bond energies: H–H = 436 kJ/mol, Cl–Cl = 243 kJ/mol, H–Cl = 432 kJ/mol. Calculate the overall energy change.

A student's answer — which line goes wrong?

Bonds broken: one H–H bond and one Cl–Cl bond.
Energy needed to break bonds = 436 + 243 = 679 kJ/mol
Bonds formed: one H–Cl bond.
Energy released forming bonds = 432 kJ/mol
Difference = 679 − 432 = 247 kJ/mol
Breaking the bonds needs more energy than forming them releases, so the reaction is endothermic.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Every reaction breaks bonds and makes new ones. Add up the energy going in, add up the energy coming out, and the difference tells you the whole story.

What you need to know

Energy must be supplied to break the bonds in the reactants.
Energy is released when the bonds in the products are formed.
Overall energy change = the difference between the total energy needed to break bonds and the total energy released forming bonds.
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.

The big picture

In a reaction, energy has to be supplied to break the bonds in the reactants, and energy is released when the bonds in the products form. Bond energies let you calculate both totals. The overall energy change is the difference between them. If forming the new bonds releases more energy than breaking the old ones needed, the reaction is exothermic; if breaking needs more, it is endothermic.

Key points

1Breaking takes energy in; forming gives energy out.

2Count every bond: bonds in one molecule × the number in front of its formula.

3Total the bonds broken and the bonds formed separately, using the bond energies you are given.

4Find the difference between the two totals: that's the overall energy change.

5Forming releases more than breaking needs: exothermic. Breaking needs more than forming releases: endothermic.

6Energy changes worked out this way are given in kJ/mol.

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Worked example

Problem

Methane burns in oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies: C–H = 413 kJ/mol, O=O = 498 kJ/mol, C=O = 805 kJ/mol, O–H = 464 kJ/mol. Calculate the overall energy change and say whether the reaction is exothermic or endothermic.

⚠ Watch out

Counting the bonds in one molecule and forgetting the number in front of its formula. In 2HCl there are two H–Cl bonds, not one, and missing that can flip your answer from exothermic to endothermic.

🧠

Memory hook

Break takes, make gives. Then compare: if make gives back more than break took, the reaction gives energy away.

✓

Check yourself

A reaction needs 1200 kJ/mol to break its bonds and releases 1450 kJ/mol forming new ones. Without calculating, is it exothermic or endothermic? Explain using the words 'break' and 'form'.

Flashcards

(11)

What has to happen to energy for a bond to break?

Energy must be supplied (taken in). A bond holds atoms together, so pulling them apart takes energy.

What happens to energy when a new bond forms?

Energy is released (given out).

How do you work out the overall energy change of a reaction from bond energies?

The difference between the total energy needed to break the bonds in the reactants and the total energy released forming the bonds in the products.

In bond terms, what makes a reaction exothermic?

The energy released forming the new bonds is greater than the energy needed to break the existing bonds.

Breaking a reaction's bonds needs more energy than forming its new bonds releases. Exothermic or endothermic?

Endothermic: more energy is taken in than given out.

You have the two totals. How do you decide if the reaction is exothermic or endothermic?

Compare them. More energy released forming bonds than needed to break them: exothermic. More needed to break bonds than released forming them: endothermic.

How do you find how many of one type of bond there are in a reaction?

Bonds of that type in one molecule × the number in front of that molecule's formula in the balanced equation.

Why do you subtract the two totals instead of adding them?

One total is energy going in and the other is energy coming out, so they act in opposite directions.

What unit is the energy change from bond energies usually given in?

kJ/mol (kilojoules per mole).

A molecule has a double bond, such as O=O. How do you count it?

As one bond of that type, using the bond energy given for the double bond (O=O), not the single bond value.

On an energy line starting at 0 for the reactants, where do the products finish in an exothermic reaction?

Below 0, because the energy released forming bonds was more than the energy put in to break them.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Where do the products end up?

Where does the energy come from?

This one comes out the other way

One line loses the marks

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