A Level Chemistry Buffers PDF

A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES BUFFERS A buffer is a solution that maintains an approximately constant pH, even when small amounts of acid or alkali are added to it. IN other words, they keep the [H+]...

A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES BUFFERS A buffer is a solution that maintains an approximately constant pH, even when small amounts of acid or alkali are added to it. IN other words, they keep the [H+] constant! They are particularly useful in biology & chemistry. There are two type of buffer that work in similar ways, acid buffers and basic buffers. You need to understand how both work, but all calculations will be based on acid buffers. HOW ACID BUFFERS WORK An acid buffer is made up of a mixture of a weak acid + the salt of the weak acid, and the equilibrium between them. e.g. CH3COOH + CH3COO-Na+ How a buffer works is rooted in le Chatelier’s Principle, so make sure you understand how changes in concentration affect the position of an equilibrium. The weak acid exists in an equilibrium: CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq) If an acid is added, this increases the [H+]. This causes the equilibrium to shift to the left. The H+ combines with the CH3COO- to bring the [H+] back down again and maintain the pH. Extra CH3COO- (salt) is added to the solution when making a buffer as, naturally, a weak acid only produces a very small amount. The extra is needed for situations like this. If an alkali is added, this decreases the [H+], as the OH- from the alkali reacts / neutralises the H+ ions. This causes the equilibrium to shift to the right. More CH3COOH molecules dissociate to bring the [H+] back up again and maintain the pH. If water is added, this decreases the [H+]. The volume has increased, so this dilutes the solution. This causes the equilibrium to shift to the right. More CH3COOH molecules dissociate to bring the [H+] back up again and maintain the pH. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES HOW BASIC BUFFERS WORK An basic buffer is made up of a mixture of a weak base + the salt of the weak base, and the equilibrium between them. e.g. NH3 + NH4+Cl- How a buffer works is rooted in le Chatelier’s Principle, so make sure you understand how changes in concentration affect the position of an equilibrium. The weak acid exists in an equilibrium: NH3(aq) + H2O ⇌ NH4+(aq) + OH-(aq) If an acid is added, this decreases the [OH-], as the H+ from the acid reacts / neutralises the OH- ions. This causes the equilibrium to shift to the right. More NH3 molecules react with H2O to bring the [OH-] back up again and maintain the pH. If an alkali is added, this increases the [OH-]. This causes the equilibrium to shift to the left. The OH- reacts with the NH4+ to bring the [OH-] back down again and maintain the pH. If water is added, this decreases the [OH-]. The volume has increased, so this dilutes the solution.This causes the equilibrium to shift to the right. More NH3 molecules react with H2O to bring the [OH-] back up again and maintain the pH. APPLICATIONS OF BUFFERS Agriculture - to maintain soil pH Industrial processes - to maintain pH of paints, dyes,, inks etc Preservatives - to maintain pH of food products Household products - to maintain the pH of laundry detergents, shampoos etc Blood - The body has a natural buffer system to maintain the pH of our blood AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES BUFFER REGIONS IN pH CURVES Now you know how buffers work, we can revisit “Buffer Regions” that occur in pH curves. These occur when we are reacting a.. Weak Acid Vs Strong Base or Strong Acid Vs Weak Base * pH pH * Vol. Base Vol. Base As the strong base is added to the weak As the weak base is added to the strong acid, it produces the salt of the weak acid, it produces the salt of the weak acid as a product. base as a product. An acid buffer is a mixture of a weak A basic buffer is a mixture of a weak acid and its salt. So, what has happened base and its salt. So, what has here is that we have created an acid happened here is that we have created buffer as part of the reaction. a basic buffer as part of the reaction. The strong base should cause the pH to The buffer solution does its job and increase sooner (you can see the sharp resists the change in pH after increase at the very beginning) but then neutralisation. This is why we get the the buffer solution does its job and gradual increase*. resists the change in pH. This is why we Note how the basic buffer causes the get the gradual increase*. buffer region in the basic part of the Note how the acid buffer causes the curve (after the equivalence point). buffer region in the acidic part of the curve (before the equivalence point). AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES BUFFER CALCULATIONS All buffer calculations in A level chemistry are based on acid buffers, and are based on the Ka expression. Take the following weak acid in equilibrium: weak HA(aq) ⇌ H+(aq) + A-(aq) “salt” of the weak acid acid [H+] [A-] The Ka expression: Ka = [HA] Important! Unlike weak acid Ka calculations, we cannot assume that [H+] = [A-]. When we make a buffer, extra “salt” is added. As a consequence, these two values will not be equal. In exam questions, you may be asked to calculate: [H+] [A-] 1. the concentration of the acid [HA] = Ka [HA] [A-] 2. the pH of the buffer [H+] = Ka [HA] [H+] 3. The concentration of the salt - [A ] = Ka How To Smash Buffer Calculations There are 3 different types of buffer questions in exams, with a specific technique to each. AQA www.chemistrycoach.co.uk © scidekick ltd 2024