GCSE · Chemistry · AQA · Spec 8462

Energy changes in reversible reactions

Walk down a flight of stairs, then back up. You drop, then climb, exactly the same height. Reversible reactions do the same thing with energy.

One reaction, one energy gap, two directions

Progress of the forward reaction →: 5. Energy of the substances (kJ): 85. Gap between the two levels: 90 kJ

Drag the point left to the reactants' level (130 kJ), then down to the products' level (40 kJ). Now drag it all the way back up. Same gap both times: which way are you crossing it?

Watch out: The sloping line just joins the two levels. This diagram only compares where the reaction starts and where it finishes, so read the flat levels at each end, not the slope in between.

What do you think?

Now run it backwards

In the example on the graph, the forward reaction A + B → C + D transfers 90 kJ to the surroundings. Now the reaction runs the other way: C + D → A + B.

Which is closest to what you think happens to the energy in the reverse reaction?

It's exothermic too. It's the same reaction, so it gives out another 90 kJ.It's endothermic, but it takes in less than 90 kJ, because some energy gets lost on the way back.It's endothermic, and it takes in exactly 90 kJ from the surroundings.There's no energy change at all. It just undoes the forward reaction.

How sure are you?

I'm sureI think soI'm guessing

It's exothermic too. It's the same reaction, so it gives out another 90 kJ.

This is a common misconception

Tempting, because it's the same reaction. But look at the graph again: going backwards means climbing from the products' level (40 kJ) up to the reactants' level (130 kJ). Climbing up needs energy, so the reverse reaction takes energy in from the surroundings. That makes it endothermic, not exothermic.

It's endothermic, but it takes in less than 90 kJ, because some energy gets lost on the way back.

This is a common misconception

You've got the direction right, so that's half the idea. The catch is the size. The reverse reaction goes between exactly the same two levels, 40 kJ and 130 kJ, so the gap is still 90 kJ. Energy doesn't leak away somewhere: exactly 90 kJ is taken in, the same amount the forward reaction gave out.

It's endothermic, and it takes in exactly 90 kJ from the surroundings.

This matches the evidence

Spot on. Same gap, opposite direction: the forward reaction transfers 90 kJ to the surroundings, and the reverse reaction takes exactly 90 kJ back in.

There's no energy change at all. It just undoes the forward reaction.

This is a common misconception

It does undo the forward reaction, but undoing it is an energy change of its own. The forward reaction sent 90 kJ out into the surroundings. To turn C + D back into A + B, the substances have to take those 90 kJ back in. Undoing an exothermic change means running an endothermic one.

Reverse the reaction and you reverse the energy change: exothermic becomes endothermic (and endothermic becomes exothermic), and the amount stays exactly the same.

Why exactly the same amount?

Reason it through

Why must the reverse reaction transfer exactly the same amount of energy as the forward reaction?

Link 1 of 4

First link · your turn

Run A + B → C + D forwards, then C + D → A + B backwards. What have you got at the end?

Locked — reveal the link above first

Your turn

Flip it: what does the reverse reaction do?

Each card describes the FORWARD direction of a different reversible reaction. Sort each one by what its REVERSE reaction does.

Still to sort

Reverse is endothermic (0)

The reverse reaction takes energy in from the surroundings.

Where the line is: Decide which way the energy goes in the forward direction first, then flip it.

Reverse is exothermic (0)

The reverse reaction transfers energy to the surroundings.

Where the line is: Decide which way the energy goes in the forward direction first, then flip it.

4 of 4 still to sort.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Exothermic means energy is transferred to the surroundings; endothermic means energy is taken in from the surroundings.
If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.
The same amount of energy is transferred in each direction; only the direction of the transfer changes.
The amount has to be the same because energy cannot be created or destroyed: forwards then backwards returns the starting substances.

The big picture

If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction, and the same amount of energy is transferred in each case. So from the forward energy change you can give the reverse one: keep the amount, swap the direction.

Key points

1Forward exothermic → reverse endothermic. Forward endothermic → reverse exothermic.

2Keep the number, swap the direction: 90 kJ given out one way means 90 kJ taken in the other way.

3On an energy-level diagram, both directions cross the same gap between the reactants' and products' levels.

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Worked example

Problem

A reversible reaction transfers 46 kJ to the surroundings when it runs forwards. Give the energy change for the reverse reaction.

⚠ Watch out

Saying the reverse reaction is 'also exothermic', or that it transfers 'less energy on the way back'. Reverse the direction, but keep the amount exactly the same.

🧠

Memory hook

Down the stairs, back up the stairs: same height, opposite way.

✓

Check yourself

A reversible reaction takes in 30 kJ from the surroundings when it runs forwards. Without looking back, say whether the reverse reaction is exothermic or endothermic, and how much energy it transfers.

Flashcards

(6)

Exothermic vs endothermic: what's the difference?

Exothermic: energy is transferred to the surroundings. Endothermic: energy is taken in from the surroundings.

A reversible reaction is exothermic forwards. What is the reverse reaction?

Endothermic.

A reversible reaction is endothermic forwards. What is the reverse reaction?

Exothermic.

How does the amount of energy transferred compare in the two directions of a reversible reaction?

It is exactly the same. Only the direction of the transfer changes.

Why must the forward and reverse amounts be equal?

Forwards then backwards gives back the starting substances, and energy can't be created or destroyed, so what went out must come back in.

On an energy-level diagram, what does the gap between the reactants' and products' levels show?

The amount of energy transferred: the same size whichever way the reaction goes.

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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One reaction, one energy gap, two directions

Now run it backwards

Flip it: what does the reverse reaction do?

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