What a learner can do afterwards
- Writes the expression for Kc from a balanced equation, products over reactants, each raised to its coefficient
- Works out the units of Kc for a given equation
- Calculates Kc from equilibrium amounts and the volume of the container
- Reads a very large or very small value of Kc as a statement about where the position lies
1 · Read
The equilibrium constant Kc says how far a reaction goes. Write product concentrations on top and reactant concentrations below, raising each to its balancing number. That products over reactants pattern never changes. A huge Kc means the mixture sits almost all as products, while a tiny one means it barely reacts. Kc says nothing about speed, only about the final position.
To calculate Kc, turn every equilibrium amount into concentration by dividing moles by the container volume in cubic decimetres. Slot the concentrations into the expression and crunch the numbers. Units come from the expression, so they differ from one equation to the next. The classic slip is forgetting to divide by the volume.
Try 0.4, 0.4, 0.2 and 0.2 mol at equilibrium in a 2 cubic decimetre vessel, all powers one. Concentrations are 0.2, 0.2, 0.1 and 0.1 mol per cubic decimetre. Then Kc is 0.1 times 0.1 over 0.2 times 0.2, which is 0.25 with no units.
Equilibrium here is dynamic: both reactions keep running at equal speeds while concentrations sit flat, inside a closed system. And Kc belongs to one temperature only. Shift the temperature and the constant itself changes.
Divide by the volume, write products over reactants with powers, and read the size for the position.
2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.