Buffers and neutralisation

Question 1

What is a buffer solution

Answer 1

A system that minimizes pH changes when small amounts of an acid or base is added
Contain weak acid and conjugate base to remove added acid or alkali
Once all of 1 component is used up, loses buffering ability

Question 2

Preparing weak acid buffer solutions

Answer 2

Based on weak acid and salt solution (conjugate base)
Weak acids partially dissociate, amount of ethanoate ions is v small
Use Chateliers principle to work

Question 3

Preparing by partial neutralization of the weak acid

Answer 3

Add alkali to excess of weak acid
Some weak acid is left unreacted
Weak acid is partially neutralized forming the conjugate base
Uses Chateliers principle to work

Question 4

How to choose the components for a buffer solution

Answer 4

Most effective when concs or both weak acid and conjugate base are equal
When this happens
The pH of the buffer solution is the same as the pKa value of HA
Operating pH is typically over 2 pH units centered around pKa
Ratio of concs can be adjusted to fine tune the pH of the buffer solution

Question 5

Calculating the pH of a buffer solution

Answer 5

[H+aq]=Ka x [HAaq]/[A-aq]

Question 6

How to calculate the pH of a partial neutralization buffer?

Answer 6

Work out the moles of the conjugate base
Moles of NaOH = Moles of HCOO-
Work out moles of weak acid used
Work out the remaining weak acid moles
Moles of weak acid = moles of weak acid used - moles of strong alkali used
Use the formula

Question 7

Why is it important to control blood pH

Answer 7

Enzyme function

Question 8

Blood pH

Answer 8

7.35-7.45

Question 9

What buffer controls blood pH

Answer 9

Carbonic acid and hydrogen carbonate buffer

Question 10

What happens if the blood pH is out of the healthy range

Answer 10

Acidosis, death
Alkalosis, nausea, muscle spasms
Tiny changes can have a significant effect

Question 11

Calculating the ration between the 2 components in a buffer

Answer 11

Take the existing equation, rearrange it so it matches the ratio that you want
Calculate it

Question 12

How does the pH calculation curve change as base is added to acid

Answer 12

Excess of acid, pH increases slowly as basic solution added
Vertical section, pH increases rapidly on addition of a very small volume of base, acid and base concs are similar
Excess of base, pH increases slowly as basic solution added

Question 13

What is the equivalence point

Answer 13

When the volume of 1 solution exactly reacts with the volume of the other solution

Question 14

What is the end point

Answer 14

When the volume of 1 solution is close to reacting exactly with the volume of the other solution
1 drop out

Question 15

Indicator colour changes and equation

Answer 15

Weak acid, 2 different colors with similar function to buffer
HA(aq) <=> A-(aq) + H+(aq)

Question 16

Sensitivity of the end point

Answer 16

Ka=[H+] and pKa=pH

Most indicators change over a range of 2 units

Question 17

How to choose an indicator

Answer 17

Colour change must coincide with a vertical section of the titration curve
Ideally, the end points and equivalent point would coincide

Question 18

Would methyl orange be suitable for strong acids?

Answer 18

Yes

Question 19

Would methyl orange be suitable for weak acids?

Answer 19

No

Question 20

Would phenolphthalein be suitable for strong bases?

Answer 20

Yes

Question 21

Would phenolphthalein be suitable for weak bases?

Answer 21

No