chemistry module 5 buffers

what is a buffer solution and what is it used for

A buffer solution is a solution which resists changes in pH when small amounts of acids or alkalis are added

A buffer solution is used to keep the pH almost constant by maintaining an almost constant concentration of hydrogen and hydroxide ions in a solution

how are acidic buffers made

a weak acid and a soluble salt of the acid

explain the example of ethanoic acid and sodium ethanoate

In solution, ethanoic acid partially ionises:

CH3COOH (aq) ⇌ H+ (aq) + CH3COO- (aq)

ethanoic acid ⇌ ethanoate

high conc ⇌ low conc

Sodium ethanoate is a salt which fully dissociates in solution:

CH3COONa + aq → Na+ (aq) + CH3COO- (aq)

sodium ethanoate → ethanoate ion

low conc. → high conc.

This results in a buffer solution with high concentrations of CH3COOH and CH3COO-

The establishes the equilibrium with hydrogen and ethanoate ions:

CH3COOH (aq) ⇌ H+ (aq) + CH3COO- (aq)

high conc. high conc.

what happens when H+ ions are added

The extra H⁺ ions react with CH3COO- to form CH3COOH

The equilibrium shifts left to reduce [H+]

This limits any drop in pH

Since there's a large supply of CH3COO-, its concentration doesn’t change much

So, the pH remains almost constant

what happens when OH- ions are added

The OH- ions react with H+ to form water:

OH- (aq) + H+ (aq) → H2O (l)

The [H+] decreases, shifting the equilibrium right

CH3COOH (aq) → H+ (aq) + CH3COO- (aq)

More CH3COOH dissociates to replace H+ ions

So, the pH stays nearly constant due to the buffer action

The buffer works because it contains a reservoir of both the weak acid and its conjugate base, which can react with added H⁺ or OH⁻ to restore balance.

how do you calculate the pH of a buffer solution

The Ka of the weak acid

The equilibrium concentration of the weak acid and its conjugate base (salt)

equilibrium expression of Ka to find the pH

[H+] = Ka x [acid] / [salt]

what is the henderson-hasselbach equation

pH = pK_a + log₁₀ x [salt] / [acid]

in humans, what is the buffer of the blood

HCO3- ions act as a buffer to keep the blood pH between 7.35 and 7.45

equation for how HCO3- ions are made

CO₂ (g) + H₂O (l) ⇌ H⁺ (aq) + HCO₃^- (aq)

what happens if the H+ conc isnt regulated

the blood pH would drop and cause ‘acidosis’

Acidosis refers to a condition in which there is too much acid in the body fluids such as blood

This could cause body malfunctioning and eventually lead to coma

what would happen if there was an increase in H+ ions

The equilibrium position shifts to the left until equilibrium is restored

This reduces the concentration of H+ and keeps the pH of the blood constant

what would happen if there was a decrease in H+ ions

The equilibrium position shifts to the right until equilibrium is restored

This increases the concentration of H+ and keeps the pH of the blood constant

what is a pH titration curve

A pH curve is a graph showing how the pH of a solution changes as the acid (or base) is added

The result is characteristically shaped graph which can yield useful information about how the particular acid and alkali react together with stoichiometric information

example of a titration curve

features of a pH curve

All pH curves show an s-shape curve and the midpoint of the inflection is called the equivalence or stochiometric point

From the curves you can:

Determine the pH of the acid by looking where the curve starts on the y-axis

Find the pH at the equivalence point

Find volume of base at the equivalence point

Obtain the range of pH at the vertical section of the curve

4 types of acid-base titration

strong acid + strong base

weak acid + strong base

strong acid + weak base

weak acid + weak base

titration graph for strong acid and weak base, and strong acid and strong base

titration graphs for weak acid and strong base and weak acid and weak base

what is an acid base indicator

An acid-base indicator is a weak acid which dissociates to give an anion of a different colour

Consider a weak acid HIn:

HIn (aq) + H2O (l) ⇌ H3O+ (aq) + In- (aq)

colour 1 colour 2

what does the colour of the solution depend on

the relative concentrations of the two species

what happens if the solution is more acidic

the above equilibrium will be shifted to the left and more HIn will be present

Colour 1 will thus dominate

what happens if the solution is alkaline

If the solution is alkaline, the above equilibrium will shift to the right and more In- will be present

Colour 2 will thus dominate

what does the colour of the indicator depend on

he pH of the solution

The colour will not change suddenly at a certain pH, but will change gradually over a pH range

The colour of the indicator depends on the ratio of [HIn] to [In-]

The pH at which these transitions will occur depends on the Ka of the indicator