The Balancing Act

A reaction that runs both ways

Not every reaction goes one way and stops. In a reversible reaction the products can turn back into the reactants, and in a sealed container the two directions settle into a balance called dynamic equilibrium. This module works through reversible reactions, how the energy change flips between the two directions, what equilibrium really means, and how it responds when conditions change.

Equilibrium words to know

Learn these before you find the balance.

One way against the other

The energy change flips when a reversible reaction runs backwards.

Describe the balance precisely

In the exam, be precise about equilibrium. A reversible change runs both ways, and inside a sealed container it settles into a dynamic balance: the two opposite reactions run at matching speeds, so the amounts hold steady even though nothing has actually stopped. Whatever heat one direction gives out, the other takes back in. And if you alter a condition, the balance moves so as to push against your change.

Match the term

Match the fact

What is true at equilibrium?

What is true at dynamic equilibrium?

• The forward and reverse rates are equal
• Both reactions have completely stopped
• Only the forward reaction still happens
• The concentrations are always exactly equal

True about reversible reactions

Select the TWO true statements about reversible reactions and equilibrium.

• A reversible reaction can go in both directions
• If the forward reaction is exothermic, the reverse is endothermic
• At equilibrium both reactions have stopped
• Equilibrium can be reached in an open container

Add the equal rates

At equilibrium the forward reaction makes 6 molecules of product each second, and the reverse breaks down the same number each second. Add the forward and reverse counts to find the total molecules changing each second. What is the answer?

Order the road to equilibrium

Put the stages of reaching equilibrium in order, earliest first.

• The forward reaction starts quickly with plenty of reactants
• Products build up and the reverse reaction speeds up
• The forward and reverse rates become equal
• The system reaches dynamic equilibrium

Complete the rules

A reaction whose products can re-form the reactants is _____. When the forward and reverse rates are equal, the system is at dynamic _____. This can only happen in a _____ system. If the forward reaction is exothermic, the reverse is _____.

Spot the true equilibrium facts

Tap the TWO true statements about dynamic equilibrium.

• the forward and reverse rates are equal
• both reactions are still carrying on
• the reactions have completely stopped
• it happens in an open container

Read the reaction

Read each case and choose the best answer.

• In a sealed flask a reaction reaches a point where the forward and reverse rates are equal. What is this?
• The forward reaction gives out heat. What is the reverse reaction?
• More reactant is added to a system at equilibrium. Which way does the balance shift?

Explain reversible reactions

Explain reversible reactions and what happens at dynamic equilibrium.

• Explain what a reversible reaction is
• Explain what dynamic equilibrium is and why a closed system is needed
• Explain how the energy change reverses between the forward and reverse reactions