Chapter 15 · Chemical Equilibrium
15.3The Idea of Dynamic Chemical Equilibrium
9 min · two checks
Predict
A closed flask of evaporating water stops losing mass. Have molecules stopped leaving the liquid?
The idea
- Describe dynamic equilibrium with a chemical example.
- Explain why concentrations become constant.
Start with only reactants and the forward rate is relatively fast. As products build up, the reverse rate grows. Equilibrium is the moment those rates match. After that, concentrations stay constant because each substance is made as fast as it is consumed. If you could paint product molecules, you would still see paint appear and disappear; the amount of paint would hold steady.
Equilibrium can be reached from the product side too. The same mixture of concentrations results, at a given temperature, whether you started with reactants or products. It is not necessary that reactants and products be equal. “Equilibrium” says the rates are equal, not the amounts. A reaction that sits at 99% products is still at equilibrium if the tiny reverse rate matches the tiny forward rate.
Keep these
- Equal rates, not equal concentrations.
- You can approach equilibrium from either side.
- Constant concentration does not mean frozen molecules.