Chapter 21 BUFFERS & NEUTRALISATION

• What is a buffer?

  A chemical that resists the change in pH when small amounts of acid or base are added. They do not stop the change in pH but they just resist is
• What are the 2 types of buffers?

  Acidic or basic buffers
• What are acidic buffers and what are they made up of?

  Acidic buffers resist the change in pH in order to keep solution below pH7 and it is made of a weak acid and its salt.

  For example:

  Ethanoic acid (CH3COOH) and sodium ethanoate (CH3COO-Na+)
• What is always in a buffer solution?

  2 equilibrium reactions in a buffer solution and they co-exist in the same beaker.
• What equilibrium equations occur in a buffer of ethanoic acid and sodium ethanoate?

  $CH_3COOH\$⇌ $CH_3COO^-+$$H^+$

  [HIGH] [LOW] [LOW]

  • weak acid dissociates weakly so equilibrium lies well over to the left

  $CH_3COO^-Na^+$⇌ $CH_3COO^-+$$Na^+$

  [LOW] [HIGH] [HIGH]

  • Salts dissociate strongly so equilibrium lies well over to the right
• In a buffer of ethanoic acid and sodium ethanoate, what happens when H+ is added?

  H+ ions react with $CH_3COO^-$ions in solution (there is high concentration of these from the salt) so more $CH_3COOH$ is made, shifting the equilibrium to the left.
• In a buffer of ethanoic acid and sodium ethanoate, what happens when OH- is added?

  OH- react with H+ ions in solution (there is a low concentration of these but they are reproduced from a high concentration of $CH_3COOH$ due to Le Chatlier's Principle) to the equilibrium shifts to the right to replace the H+ ions
• How do you calculate the pH of a buffer solution?

  1. Write the equation and the Ka expression (in buffers [H+]$\ne$[A-] and you must use equilibrium concentrations not the initial concentrations. Assume that salts dissociate fully and weak acids dissociate poorly so [salt]=[A-] and [HAstart]=[HAequilibrium])
  2. Rearrange Ka expression to make [H+] the subject (use concentration so convert to concentration if moles was given)
  3. Calculate [H+] using the rearranged equation and calculate pH from [H+] using pH = -log[H+]
• How do you calculate the pH change of a buffer solution?

  When you add a small amount of acid or base to a buffer, the pH changes but so does the concentration of the weak acid. The concentration of the weak acid needs to be calculated using the equation: New concentration = Old concentration x Old volume/New volume.

  1. Calculate the pH of the original weak acid
  2. Calculate the pH of the new concentration of the weak acid
  3. Then calculate pH of the buffer like normal
• What are the uses of buffers in the blood?

  Buffers are used in the blood to keep the pH as close to pH 7.4 as possible.

  Carbonic acid $\left(H_2CO_3\right)$ and hydrogen carbonate $\left(HCO_3^-\right)$ system exists in the blood.
• What equilibrium equations are present in the blood?

  $H_2CO_3$⇌ $H^+$$+$$HCO_3^-$

  $H_2CO_3$⇌$\ H_2O$$\ +$$CO_2$
• How is carbonic acid levels controlled in the blood?

  Carbonic acid is controlled by respiration in cells. When CO2 is exhaled, levels of carbonic acid in the blood reduces and equilibrium shifts right to replace the CO2 lost