Acids and Bases

Question 1

Explain why the expression for Kw does not include the concentration of water. ​(2)

Answer 1

• [H2O] is very high (compared with [H+] and [OH−])
• [H2O] is (effectively) constant

Question 2

Explain why the value of Kw increases as the temperature increases. (2)

Answer 2

• (Dissociation OR breaking bonds) is endothermic
• ∴ Equilibrium moves to RHS (at higher T) to lower T & oppose increase

Question 3

What is meant by a buffer solution? (2)

Answer 3

• solution that resists change in pH
• despite addition of small amounts of acid/base (or dilution)

Question 4

Equivalence point:

Answer 4

point in chemical reaction where equivalent quantities of both reactants have been mixed

Question 5

End point

Answer 5

stage in a titration where there is a color change

Question 6

For a titration to be effective:

Answer 6

equivalence point and end point need to coincide

Question 7

How do you tell if an base is strong or weak by the formula?

Answer 7

All strong bases are OH– compounds. So a base based on some other mechanism, such as NH3 (which does not contain OH− ions as part of its formula), will be a weak base.

Question 8

Effect on moles when excess OH- added:

Answer 8

• n(A-) increases
• n(HA) decreases

Question 9

Effect on moles when extra H+ added:

Answer 9

• n(A-) decreases
• n(HA) increases