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Equillibrium/ Buffer Solutions

medapplicant2015

What would happen to the equillibrium in you addded a buffer solution?

Also how do you find the concentration of a coloured ion using colorimetry?

Reply 1

mollyvet

For the first one - when there is a buffer solution in with a reaction mixture, basically what happens is that the buffer solution changes the equilibrium in order to balance the reaction out in terms of pH.

So say you have a buffer solution of ethanoic acid and ethanoate ions (CH3COOH/CH3COO-). This just kind of hangs around in the solution of reactants until equilibrium is disturbed, and occurs because ethanoic acid is a weak acid and thus dissociates in water to give ethanoate ions and H+ (protons).

CH3COOH <--> CH3COO- + H+

1. If H+ ions (eg. acid) are added, then CH3COO- "mops up" these H+ ions by forming more ethanoic acid. This disturbs the equilibrium, because there are less CH3COO- ions in the solution and so, according to Le Chateliers principle, equilibrium moves to the right to oppose the change and give more ethanoate ions.

2. If OH- ions (eg. alkali) are added, then CH3COOH dissociates to give H+ ions, which "mop up" the OH- ions. However, this disturbs the equilibrium, because there are less CH3COOH molecules in the solution, so equilibrium moves to the left to oppose the change (according to Le Chat) and this gives more ethanoic acid molecules.

These things happen separately ie. you won't be asked what will happen if both an alkali and acid are added at the same time, so it's better to practice writing these answers out so you have them prepared.

Afraid I don't know about the second question, but I hope that helped!

Reply 2

oversizedcarrot

Original post by medapplicant2015

What would happen to the equillibrium in you addded a buffer solution?

Also how do you find the concentration of a coloured ion using colorimetry?

2. Colorimetry can be used to find the concentration of a coloured ion by making a series of solutions at different known concentrations of the coloured ion you would like to find the concentration of.

You can then use a colorimeter to measure the absorbency of each known concentration alongside your own unknown concentration solution to find out what the unknown concentration is.

Plot a graph of concentration (x) against Absorbance (y), plot and draw a line of best fit. Draw a line from the absorbance of your unknown and you should find the corresponding concentration from your line of best fit.