Making a Buffer Example

[Yajing Feng 2J]

In Friday (1/14) lecture, Dr. Lavelle showed us an example of a weak acid and its salt (HNO2 and KNO2), making a buffer solution. When he was drawing out the ICE table, the reactant's change row was "-x" while the product's change row was "+x" when there was both reactants and product present from the beginning. How do we know initially which side is the plus x and which side is the minus x?

[Jiayi_Cao_3E]

Hello!

It depends on the value of Q, which is calculated by [P]/[R] when the system is not at equilibrium. When you add a reactant, Q will be smaller than K and since more concentration of reactants will increase the rate of products being formed, the reaction will shift right toward the product, which will result in -x for reactants and +x for products.

When you add a reactant to a system that is already at equilibrium, you will always have an initial Q that is smaller than K.
When you add a product instead, Q will be initially larger than K and the reaction will shift left toward the reactants.

Hope this helps!

[Amy Wong 2F]

Hi!
Like Jiayi said, it depends on the values of Q and K. You'd use the initial values to calculate Q, and if Q is less than K, then the system would need to produce products to establish equilibrium. If Q is greater than K, then the system would need to produce reactants to establish equilibrium.