Mistake Master
Energy diagrams
An energy diagram is a one-picture summary of a reaction's energetics: where the reactants start, where the products end, and whether energy came out or went in. Read the vertical gap and its direction.
§1
Reading the diagram.
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An energy diagram plots enthalpy (energy) on the vertical axis against reaction progress on the horizontal axis. Reactants sit at one level and products at another.
The vertical difference between reactants and products is ΔH. If products are lower than reactants, energy was released — exothermic (ΔH < 0). If products are higher, energy was absorbed — endothermic (ΔH > 0).
So the diagram encodes both the size of the energy change (the gap) and its direction (which level is higher). Reading it correctly is just identifying reactants, products, and comparing their heights.
§2
Extracting ΔH from a diagram.
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Locate the two levels and compare.
- Identify reactants and products. Reactants are on the left (start), products on the right (end).
- Read their energy levels. Note the height of each on the enthalpy axis.
- Compute ΔH. ΔH = products − reactants (a downhill result is negative).
- Classify the reaction. Products lower → exothermic; products higher → endothermic.
§3
The pieces you'll meet.
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A diagram shows the size and direction of the energy change.
§4
Worked example: read a diagram.
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Diagram. Reactants are at 80 kJ; products are at 30 kJ.
Compute ΔH. ΔH = products − reactants = 30 − 80 = −50 kJ.
Classify. Products are lower than reactants and ΔH is negative, so the reaction is exothermic — energy was released.
Reverse it. The reverse reaction (products → reactants) would climb from 30 to 80 kJ, so its ΔH = +50 kJ, endothermic. Reversing a reaction flips the sign of ΔH.
§5
Mistakes that cost real points.
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"If the products are lower, the reaction is endothermic."
Products lower than reactants means energy was released — that is exothermic (ΔH < 0), not endothermic. Endothermic has products higher. Reading the diagram's direction backward flips the classification.
Fix. Products below reactants → exothermic (ΔH < 0); products above → endothermic (ΔH > 0).
"ΔH is the height of the reactants (or products) above the axis."
ΔH is the difference between the reactant and product levels, not the absolute height of either above the axis (which is arbitrary). Only the gap between the two levels matters.
Fix. Read ΔH as products minus reactants — the vertical gap — not as any single level's height.
"Reversing the reaction keeps ΔH the same."
Reversing a reaction reverses the energy flow, so it flips the sign of ΔH. If the forward reaction is exothermic (ΔH < 0), the reverse is endothermic (ΔH > 0) by the same magnitude.
Fix. Flip the sign of ΔH when you reverse a reaction; the magnitude stays the same.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.