Buffers

Question 1

What is a buffer?

Answer 1

A solution that resists change in pH when small amounts of acid or alkali are added to it

Question 2

Do buffers fully stop changes in pH?

Answer 2

No just resists change in pH when you add acid or alkali, but if you add too much will eventually change pH

Question 3

What are the 2 types of buffer?

Answer 3

Acidic buffer
Basic buffer

Question 4

What is an acidic buffer?

Answer 4

Formed from a week acid (carboxylic acid) and its salt (carboxylate salt)

Question 5

What pH do acidic buffers aim to keep?

Answer 5

Under 7

Question 6

What is a basic buffer made of?

Answer 6

A weak base + the salt it can form eg NH3 and NH4+

Question 7

What pH do basic buffers aim to keep?

Answer 7

Over 7

Question 8

What type of buffer does Edexcel require knowledge of?

Answer 8

Acidic buffers

Question 9

What equations occur in an acidic buffer?

Answer 9

RCOOH <——> RCOO- + H+
RCOO-Na+ <——> RCOO- + Na+

Question 10

How does a a buffer work if we add an acid?

Answer 10

We already have a large reservoir of carboxylate ions due to the dissociation of the weak acid’s salt + the weak acid
Adding acid = Increases the concentration of H+ ions
These react with carboxylate ions and shifts POE of first equation left to remove H+, until original is restored

Question 11

How long is the position of equilibrium shifted left when an acid is added to an acidic buffer?

Answer 11

Until the original concentration of H+ is restored

Question 12

How does a buffer work if we add an alkali?

Answer 12

Adding it Increases the concentration of OH-
This will react with H+ ions which decreases pH
Causes a shift in equilibrium to the right so carboxylic acid dissociates more and adds H+ ions to its original level

Question 13

What are the important aspects of an acidic buffer solution we must have?

Answer 13

High concentration of carboxylate ion so it can react with additional H+ ions to resist pH change by removing H+
High concentration of ethanoic acid to dissociate to replace lost H+ ions

Question 14

2 methods of making a buffer solution

Answer 14

Mixing a weak acid and its salt
Mixing an excess of weak acid and a small volume of strong base

Question 15

How does making a buffer from weak acid + strong base work?

Answer 15

ALLL the strong base reacts so mol of strong base added = mol of salt formed
all Mol of salt formed dissociated into the carboxylate ion, so this forms the buffer
So amount of base added = amount of salt formed = amount of carboxylate (A-) ion

Question 16

Assumption when calculating pH of a weak base made from strong base and weak acid

Answer 16

Mol of strong base reacts = mol of carboxylate ion A-

Question 17

Calculating the pH of a buffer made of weak acid + strong base

Answer 17

Find start Mol of weak acid
Find mol of strong base = mol of carboxylate ion
Find left over mole of weak acid: original mole - mole of strong base
Calculate total volume and convert all moles to conc
Find H+ using the Ka expression using above concentrations

Question 18

Calculating pH of a buffer made of weak acid + weak acid salt assumption

Answer 18

Weak acid salt dissociates fully so the moles of the weak acids salt = moles of carboxylate ion

Question 19

Calculating pH of a buffer made of