Calculating the Equilibrium Constant
Introduced| English |
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| ICE table/aɪs ˈteɪbl/ |
Plugging in the numbers
- Once a reaction settles, measure what is left.
- Drop those amounts into the ratio and compute.
- Out pops a single number that describes the balance.
- The same number returns no matter how you started.
Using equilibrium concentrations
- Measure the concentrations once the system reaches equilibrium.
- Put them into $K = [\text{products}]^{\text{coeffs}} / [\text{reactants}]^{\text{coeffs}}$.
- The result is $K$ for those conditions.
Which concentrations do you put into the $K$ expression?
$K$ uses the equilibrium concentrations, not the initial ones.
The ICE table
- An ICE table 三段式表 tracks Initial, Change, and Equilibrium amounts.
- Start with the initial values, apply the change, then read the equilibrium row.
- Compute $K$ from that last row.
Order the rows of an ICE table.
Initial, then Change, then Equilibrium (I-C-E).
You read the ____ row of the ICE table to calculate $K$.
The equilibrium row holds the concentrations for $K$.
Concentrations or pressures
- $K_c$ uses concentrations; $K_p$ uses gas pressures.
- Both describe the same equilibrium in different units.
- Pick the one that matches your data.
Build an ICE table
An ICE table organises the amounts before and at equilibrium.
$K_p$ is written using...
$K_p$ uses partial pressures; $K_c$ uses concentrations.
For the same reaction, $K_c$ and $K_p$ are generally different numbers.
They describe the same equilibrium but in different units.
At equilibrium, $[\text{products}] = 4$ and $[\text{reactants}] = 2$, all coefficients 1.
- $K = 4 / 2 = 2$.
- The products are favoured a little.
At equilibrium $[\text{product}] = 6$ and $[\text{reactant}] = 3$ (coefficients 1). Find $K$.
$K = 6/3 = 2$.
Use equilibrium concentrations in $K$, not the initial ones -- that is what the ICE table's bottom row is for. $K$ is written without units by convention. And $K_c$ (concentrations) and $K_p$ (pressures) are different numbers for the same reaction.
To find $K$, put the equilibrium concentrations into the products-over-reactants ratio. An ICE table organizes the Initial, Change, and Equilibrium amounts so you can read that final row. Use $K_c$ for concentrations or $K_p$ for gas pressures.