All courses › Chemistry 2 › Reaction quotient

Reaction quotient

The reaction quotient Q is calculated with the same fraction as K, but with the concentrations right now, not necessarily at equilibrium. If Q is smaller than K, there is too little product, and the reaction goes to the right. If Q is larger than K, it goes to the left. If Q equals K, the system is at equilibrium. This is the numerical version of Le Chatelier's principle: the system counteracts a change by using up what you added. Temperature is the only thing that changes K itself.

Q=[HI]2[H2][I2]Q = \frac{[\mathrm{HI}]^2}{[\mathrm{H}_2][\mathrm{I}_2]}the same fraction as K, with current values

Symbols

QQreaction quotient
KKequilibrium constant

Example

Now [H2] = 0.20, [I2] = 0.40 and [HI] = 0.40 mol/L, and K = 8 for the reaction.

Q=0.402/(0.20⋅0.40)=2Q = 0.40^2 / (0.20 \cdot 0.40) = 2. Since 2 is less than 8, the reaction goes towards HI.

Always compare Q with K for the same temperature.
Practise chemical equilibrium for free →

← The equilibrium constant · An unknown concentration →

Part of Chemistry 2: Chemical equilibrium.