Lesson 1 of 4

Reversible reactions.

About 20 minutes · 5 practice questions

Read the full article: Chemical equilibrium

A reaction that runs both ways and never reaches an end: a brown gas that pairs up and splits back, until the mixture stops changing.

Step 1 of 5 · Both ways

Step 1 of 5

Both ways

Until now every reaction ran forwards until one reactant was used up. But some reactions make a product that can break back down to the reactants. Such a reaction is called reversible, and its equation has a double arrow, $\rightleftharpoons$ ⁠.

The flask holds nitrogen dioxide, $\mathrm{NO_2}$ ⁠: a brown, poisonous gas that engines make and that gives city smog its colour. Two $\mathrm{NO_2}$ ⁠ molecules can join into one molecule of $\mathrm{N_2O_4}$ ⁠, which is colourless, and that can split again:

$\mathrm{2NO_2 \rightleftharpoons N_2O_4}$ ⁠

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Step 2 of 5

Watching the colour

The flask’s colour shows how much $\mathrm{NO_2}$ ⁠ it holds: the more of the nitrogen is in $\mathrm{N_2O_4}$ ⁠, the paler the flask. Time runs from Start, the moment the flask is sealed. The charts beside the Scene draw the rates and the concentrations over time.

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Step 3 of 5

Counting the nitrogen

Nitrogen is neither lost nor made: it only moves between $\mathrm{NO_2}$ ⁠ and $\mathrm{N_2O_4}$ ⁠. So at every moment $[\mathrm{NO_2}] + 2[\mathrm{N_2O_4}]$ ⁠ equals what was sealed in. Square brackets mean a concentration, in mol/L or mmol/L.

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Step 4 of 5

More in, more out

NO₂ sealed in sets how much gas the flask holds. Time is now fixed at equilibrium.

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Step 5 of 5

From the other side

You can reach equilibrium from the products’ side too.

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Still of the Chemical equilibrium 3D scene

Interactive scene

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Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.

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Practice · Choose

You seal brown $\mathrm{NO_2}$ ⁠ gas in a flask. $\mathrm{NO_2}$ ⁠ molecules pair up into $\mathrm{N_2O_4}$ ⁠, a colourless gas, and each pair can also split back: $\mathrm{2NO_2 \rightleftharpoons N_2O_4}$ ⁠. What will the flask hold after an hour?

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Practice · Predict, then look

Time is at Start, just sealed: the flask is all brown. What will happen to the colour as time runs on?

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Practice · Calculate

You sealed in 40 mmol/L of $\mathrm{NO_2}$ ⁠. At equilibrium the chart shows about 15.2 mmol/L of $\mathrm{N_2O_4}$ ⁠. How much $\mathrm{NO_2}$ ⁠ is left?

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Practice · Reach the target

Change NO₂ sealed in so that at equilibrium the flask holds 24 mmol/L of $\mathrm{N_2O_4}$ ⁠, then press Check.

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Practice · Choose

Instead of 40 mmol/L of $\mathrm{NO_2}$ ⁠ you seal in, at the same temperature, 20 mmol/L of pure $\mathrm{N_2O_4}$ ⁠: the same nitrogen. What will the flask hold at equilibrium?