Chapter 21 Buffers

Question 1

What is a buffer solution?

Answer 1

A solution which minimises pH changes when small amounts of acid/base are added

Question 1

What are two methods to prepare a buffer solution?

Answer 1

Mixing a weak acid and its salt
Partial neutralisation- excess of weak acid leaving some salt also

Question 2

How does a buffer minimise the addition of an acid?

Answer 2

Increased concentration of H+
H+ + A- —/// HA
Conjugate base reacts with the protons to minimise their addition, minimise change to pH
Shifts position of equilibrium to the left as decreasing A- and increasing acid

Question 3

How does a buffer minimise the addition of a base?

Answer 3

OH- + HA /// H2O + A-
Minimise the loss of protons
Pushes the position of equilibrium to the right as we decrease the acid and increase the conjugate base p

Question 4

Why do buffers work in general?

Answer 4

The concentration acid and conjugate base are not contributing the pH
If there concentration increases or decreases, the position of equilibrium will change, but the amount of protons would not, so pH is the same
It is purely the amount of (free) protons that affects pH

Question 5

When do buffers work best?

Answer 5

When [A-] = [HA]
Therefore pH=pKa

Question 6

What are the assumptions of buffer calculations?

Answer 6

The conjugate base only comes from the salt
The concentration of the acid is the same at the start as equilibrium

Question 7

How do you calculate the pH of a buffer from partial neutralisation?

Answer 7

Calculate the number of moles of acid left over, and amount of salt produced
Work out new concentration
Use pH calculations with the concentration of salt as conjugate base, and remaining acid conc for acid

Question 8

Why are buffers important for biology?

Answer 8

Enzymes have small pH ranges, cannot function if this changes as the interactions, such as hydrogen bonds, in the tertiary, secondary, and quartenary structure would be disrupted
Small pH changes have a tenfold impact

Question 9

What buffers are in the body? What pH is the blood plasma?

Answer 9

H2CO3/HCO3- buffer
7.35 and 7.45

Question 10

What happens if the pH of the blood goes outside it’s normal range?

Answer 10

Acidosis- pH too low, fatigue shock and death
Alkalosis- pH too high, muscles spasms and nausea

Question 11

What is the equations for the blood plasma buffer?

Answer 11

H2CO3 =//= HCO3- + H+

Question 12

How do use a pH meter when titrating?

Answer 12

pH probe and data logger needed

Add acid to a conical flask
Add the pH probe into the flask and record the pH
Add the base in 1cm3 intervals using a burette
Repeat until the pH changes rapidly
Then add the base till it is in excess
Graph

Question 13

What does a titration pH graph look like?

Answer 13

An S with a vertical section
First Acid in excess, then rapid change, the excess base

Question 14

What is the difference between the end point and equivalence point of a pH graph?

Answer 14

End point= equal amounts of indicator conjugate base and acid
Equivalence point = centre of vertical section of the graph

Question 15

How acid-base indicators work?

Answer 15

HA =//= H+ + A-
Red. Blue.

Indicators are weak acids
In excess acid, the position of equilibrium shifts to the left as the concentration of the products, H+, increase. This means a red colour.
When OH is added, the H+ decrease, pushing the position of equilibrium to the right of the weak acid
When A- = HA, a green colour is seen, this is the end point
As OH- is kept on added, the H+ decreases further, so in response, the position of equilibrium shifts to the right, producing a blue colour.

Question 16

What must be considered when choosing an indicator?

Answer 16

The pH range of the indictors must align with the vertical section of the acid-base graph
Otherwise, there will be no colour change when the pH changes from acidic to neutral, only before and after, so would not accurately show the end point

Question 17

How do you find the original concentration of salt added?

Answer 17

Useful rearrange with Ka
But the first conc you calculate would be the equilibrium concentration, with the new volume
Find moles then divide by original volume for original conc

Question 18

If you add an acid to a buffer system, how do you calculate the pH?

Answer 18

Calc original number of moles for both the salt and weak acid
Subtract the number of moles of strong acid from the salt
Add the number of moles of strong acid to the weak acid as more forms
Then new concs, pH calcs