Reactions That Go Both Ways
- Chemistry
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Some reactions are reversible and reach equilibrium.
- At equilibrium both directions continue at equal rates.
- Changing conditions shifts the position of equilibrium.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Not every reaction finishes
Some reactions run to completion and stop — burn a log and you cannot un-burn it. Others run in both directions at the same time, with products turning back into reactants as fast as they are formed. Chemists draw these with a double arrow rather than a single one. A reaction like this never truly finishes. That can seem odd, because the reactions you meet first usually do finish. But reversible reactions are very common, and a great deal of industrial chemistry consists of learning to work with them rather than against them.
Busy, but not changing
At equilibrium the amounts of each substance stop changing. That does not mean the reaction has stopped — both directions are still running, just at the same speed. Picture an up escalator and a down escalator moving at equal rates: the crowd on each floor stays the same size even though every single person is in motion. That is why it is called dynamic equilibrium, dynamic meaning moving. There is one trap worth flagging. Equal rates does not mean equal amounts. A system can sit at equilibrium with far more product than reactant, or the reverse; what has to be equal is the speed of the two directions, not the quantities.
Pushing the balance
You can push an equilibrium in either direction. Add more of one substance and the system shifts to use it up. Remove something and it shifts to replace it. Change the temperature and it shifts again. In every case the system responds so as to partly undo the change you made. This is what makes reversible reactions useful rather than merely frustrating. A plant cannot force such a reaction to completion, but it can continuously draw off the product as it forms — and the system keeps shifting to replace what was removed, producing more each time. Across a production run that converts an apparently stubborn reaction into a high-yielding one.
Curriculum links: NGSS HS-PS1-6