acids and bases

Question 1

explain why enthalpy change to neutralise 1 mol of CH3COOH is less -ve than for HCl

Answer 1

CH3COOH is a weak acid so doesn’t fully dissociate; energy taken in to break the O-H bond

Question 2

explain why the Ka of CH2OHCOOH is greater than CH3COOH

Answer 2

O from OH attracts electron density; so anion formed is more stable; so its yield is bigger for equilibrium

Question 3

how to find the pH of a buffer solution

Answer 3

-log( Ka [ HA ] / [ A- ] ); make sure to use values after reaction if there was one

Question 4

what is an acid

Answer 4

proton donor

Question 5

what makes a weak acid weak

Answer 5

it does not fully dissociate

Question 6

when does pH = pKa

Answer 6

when [ A- ] = [ HA ] (so Ka = [ H+ ] x 1)

Question 7

explain how a buffer resists pH change when a small amount of acid is added

Answer 7

acid reacts with A-, converting it to HA; change in ratio [ HA ] / [ A- ] is very small due to high concentrations

Question 8

explain how a buffer resists pH change when a small amount of base is added

Answer 8

base reacts with HA, converting it to A-; change in ratio [ HA ] / [ A- ] is very small due to high concentrations

Question 9

H2O <=> OH - + H + for water dissociation; reaction is endothermic; use this info to explain why the pH of pure water isn’t always 7.00

Answer 9

temp increase => higher [ H+ ] => lower pH; so pH of water depends on temperature

Question 10

what happens in a titration curve when you start with a weak base / weak acid

Answer 10

buffer solution formed; so there is a horizontal region at the start