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Unit 7
7–9% of examMany reactions don't just run one way and stop. They run forward and backward at the same time until the two rates match, a balance called dynamic equilibrium. In this unit you describe that balance with the equilibrium constant K, predict which way a reaction will shift using Q and Le Châtelier's principle, and use the same ideas to figure out how much of a salt dissolves.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Lots of processes can go both ways, like evaporation and condensation or dissolving and precipitating. When the forward and reverse processes run at the same rate, the amounts of everything stop changing even though particles keep reacting: that's dynamic equilibrium.
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Whichever direction runs faster wins for the moment: if the forward rate is bigger, reactants turn into products overall, and if the reverse rate is bigger, products turn back into reactants. Equilibrium is reached once the two rates become equal.
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The reaction quotient Q is products over reactants, each raised to the power of its coefficient in the balanced equation, and at equilibrium Q equals K. You can write it with molar concentrations (Qc and Kc) or, for gases, with partial pressures (Qp and Kp), and you leave out pure solids and liquids.
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If you know the concentrations or partial pressures of every species once the reaction has reached equilibrium, plug them into the K expression to calculate K. When you're given starting amounts and only one equilibrium value, an ICE table (Initial, Change, Equilibrium) fills in the rest.
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A very large K means the equilibrium mixture is mostly products, so the reaction essentially goes to completion. A very small K means the mixture is mostly reactants and the reaction barely happens.
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If you reverse a reaction, the new K is 1/K. If you multiply all the coefficients by a number n, K is raised to the power n, and if you add reactions together, you multiply their K values to get the overall K.
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Start by comparing Q with K: if Q < K the reaction makes more products, if Q > K it makes more reactants, and if Q = K it's already at equilibrium. Then use an ICE table with the starting amounts and K to work out the equilibrium concentrations or pressures.
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Particle diagrams can show a mixture before and at equilibrium. By counting reactant and product particles in a diagram, you can tell whether the system has reached equilibrium and estimate the value of K.
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Le Châtelier's principle says that when you disturb a system at equilibrium, it shifts in the direction that partly counteracts the change. Adding or removing a substance, changing the temperature, changing the volume of a gas mixture, or diluting a solution can all cause a shift you might see as a change in color, temperature or pH.
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A disturbance pushes Q away from K, and the reaction then runs in whichever direction brings Q back to K. Adding or removing a substance changes Q but leaves K the same, while changing the temperature changes K itself.
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Dissolving a salt in water is a reversible process, and its equilibrium constant is the solubility product, Ksp. You can calculate a salt's molar solubility from its Ksp, or work backward from a measured solubility to find Ksp.
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A salt dissolves less in a solution that already contains one of its ions. For example, AgCl is less soluble in a NaCl solution than in pure water, because the extra Cl⁻ pushes the dissolving equilibrium back toward the solid.
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