Buffers and Neutralisation Word Stimulants

Question 1

Buffer Solution Purpose

Answer 1

Minimise pH change in solution on addition of acid or base.

Question 2

Buffer Solution Equation

Answer 2

HA(aq) <=> H+ +A-
(HA=weak acid - removes added alkali)
(A- = Conjugate Base - removed added acid)

Question 3

Preparing Solution

Answer 3

Contains large conc HA and A-
Mix : Weak Acid solution + Salt of weak acid solution
Mix: Excess of weak acid solution + Alkali solution
(Weak acid partially neutralised -> Conjugate base+unreacted acid)

Question 4

Conjugate Base removes Acid

Answer 4

Addition of H+
React with A-
Equilibrium shifts left (removes H+)

Question 5

Weak Acid removes alkali

Answer 5

Addition of OH-
Small conc H+ reacts
HA dissociates - Restores most H+
Equilibrium shifts right (HA dissociates, restores H+)

Question 6

Calculating Ka

Answer 6

[H+][A-] / [HA]

Question 7

Calculating [H+]

Answer 7

Ka x ([HA]/[A-]

Question 8

Difference to weak acid

Answer 8

[H+] not = to [A-]

Question 9

pH in the Body

Answer 9

Narrow pH ranges

Controlled by buffer solutions

Question 10

Blood plasma

Answer 10

7.35-7.45 pH

Carbonic acid- hydrogen carbonate buffer system

Question 11

Carbonic acid- hydrogen carbonate buffer system

Answer 11

H2CO3 : Weak Acid; removes excess alkali
HCO3- : Conjugate Base; removes excess acid
H2CO3 <=> HCO3- + H+

Question 12

H2CO3 <=> HCO3- + H+ Equilibrium

Answer 12

H2CO3 <=> HCO3- + H+
Added Alkali ->
Added Acid

Question 13

pH titration curve

Answer 13

Acid-base titration
y = pH
x = vol of base added
Gradual increase in pH (excess acid)
Vertical section
Equivalence/End point (middle of vertical)
Gradual increase in pH (excess base)

Question 14

Vertical section (pH curve)

Answer 14

pH increases rapidly
Increase of small vol of base
Acid and base conc similar

Question 15

pH meter

Answer 15

Connected to electrode
pH readings (titration curve)

Question 16

Indicators

Answer 16

Weak Acid (HA)
Different colour to conjugate base (A-)

Question 17

Indicator End Point

Answer 17

Equal conc of HA + A-
Colour between two extremes
Eg methyl orange (HA red <=> A- yellow)

Question 18

Excess

Answer 18

Change in colour when equilibrium shifts
Alkali : shifts right (OH- reacts with H+, HA dissociates to replace H+)
Acid : shifts left (H+ reacts with OH-)

Question 19

Selecting Indicator

Answer 19

Changes colour at different pH ranges
Midpoint = kPa of indicator
End point must coincide with vertical section on pH curve