A Level Chemistry Acids & Bases PDF

A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES ACIDS The Brønsted-Lowry Definition: An Acid is a Proton (H+) Donor Acid molecules dissociate in solution to release hydrogen ions. e.g....

A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES ACIDS The Brønsted-Lowry Definition: An Acid is a Proton (H+) Donor Acid molecules dissociate in solution to release hydrogen ions. e.g. HCl(aq) → H+(aq) + Cl-(aq) Acids are classified as strong or weak. This is a reflection of how much they dissociate like this in solution. STRONG ACIDS Strong acids fully dissociate in solution. This means every molecule releases its H+ ions. e.g. hydrochloric acid nitric acid HCl(aq) → H+(aq) + Cl-(aq) HNO3(aq) → H+(aq) + NO3-(aq) } Monoprotic acids Only release 1 H+ ion per molecule sulfuric acid H2SO4(aq) → 2H+(aq) + SO4-(aq) Diprotic acid Releases 2 H+ ions per molecule WEAK ACIDS Weak acids only partially dissociate in solution. This means only some of the molecules release H+ ions. They exist in an equilibrium. (More on these later!) e.g. ethanoic acid CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES HINTS | TIPS | HACKS Be careful! “Strong” and “Weak” DO NOT refer to concentration! Concentration refers to the number of molecules per unit volume. You can have a concentrated solution of a weak acid and a dilute solution of a strong acid. If you have strong acid and a weak acid of equal concentration, the strong acid will have a lower pH as it releases more H+ ions. Since strong acids fully dissociate, if you know the concentration of acid, you know the concentration of H+ ions. You need this to calculate pH! e.g. If you have a 0.02 mol.dm-3 solution of HCl(aq), the H+ concentration will also = 0.02 mol.dm-3 Watch out for diprotic strong acids! If you have a 0.02 mol.dm-3 solution of H2SO4(aq), the H+ concentration will = 0.04 mol.dm-3 Since weak acids exist in equilibrium, all the rules relating to le Chatelier’s Principle apply! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES BASES The Brønsted-Lowry Definition: A Base is a Proton Acceptor (H+) Bases accept protons (H+) from acids. Some bases are classified as “alkalis” as they are soluble. e.g. All Metal Oxides and hydroxides are bases Group 1 Hydroxides are also classed as alkalis as they are all soluble STRONG BASES Strong bases fully dissociate in solution. This means every molecule releases its OH- ions. These are what accept protons (more on this later). e.g. sodium hydroxide NaOH(aq) → Na+(aq) + OH-(aq) potassium hydroxide KOH(aq) → K+(aq) + OH-(aq) barium hydroxide Ba(OH)2(aq) → Ba2+(aq) + 2 Ba(OH)2(aq) WEAK BASES Like weak acids, weak bases only partially dissociate in solution. This means only some of the molecules accept H+ ions. They exist in an equilibrium. e.g. ammonia NH3(aq) + H+(aq) ⇌ NH4+(aq) HINTS | TIPS | HACKS If you have strong base and a weak base of equal concentration, the strong acid will have a higher pH as it accepts more H+ ions. You do not have to calculate the pH of a base / alkali (yet!) Watch out for diprotic strong bases! e.g. Group 2 hydroxides e.g. If you have a 0.02 mol.dm-3 solution of Ba(OH)2(aq), the OH- concentration will = 0.04 mol.dm-3 Since weak bases exist in equilibrium, all the rules relating to le Chatelier’s Principle apply! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES CALCULATING pH OF STRONG ACIDS pH is a measure of acidity / alkalinity of a solution 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Strong acids Weak Acids Neutral Weak Alkalis Strong Alkalis pH is calculated using concentration of H+ ions [H+] present in the solution pH is actually defined as this equation: pH = -log [H+] So, if you know the [H+] you can calculate pH! Likewise, if you know the pH, you can calculate the [H+] [H+] = 10-pH Here’s some context… Pure water has a pH of 7 and is considered neutral. The [H+] of pure water is 1x10-7 mol.dm-3. This is because water naturally dissociates! H2O(l) ⇌ H+(aq) + OH-(aq) So, when an acid is dissolved in water, it releases H+ ions, increasing the [H+] and lowers the pH. When an alkali is dissolved in water, it releases OH- ions, which combine with (neutralise) H+ ions. This lowers the [H+] and raises the pH. How To Calculate pH For Strong Acids AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES Kw - THE IONIC PRODUCT OF WATER As, mentioned previously, water naturally dissociates! H2O(l) ⇌ H+(aq) + OH-(aq) Since it is an equilibrium, we can write the Kc expression for it… [H+][OH-] Kc = [H2O] However, since H2O is a liquid, and has an essentially infinite concentration, it is not included in the expression. This new expression is known as the “ionic product of water”, Kw. Kw = [H+][OH-] Like any expression such as this, if you know two of the values, you can rearrange to calculate the third. Kw & TEMPERATURE At 298K, the value for Kw = 1.0 x10-14 mol2.dm-6 However, this value changes with temperature! endothermic H2O(l) ⇌ H+(aq) + OH-(aq) exothermic Since this is an equilibrium, it responds to changes in temperature according to le Chatelier’s principle. Increase in temp. = Shift to the right = increase in the value of Kw Decrease in temp. = Shift to the left = decrease in the value of Kw However, even though [H+] changes with temperature pure water is always considered neutral as the [H+] always = [OH-] AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES CALCULATING pH of STRONG BASES Soluble bases (alkalis) release OH- ions into solution. We can find the [OH-] for a strong base in the same way as we found the [H+] for strong acids as they fully dissociate in solution. e.g. sodium hydroxide NaOH(aq) → Na+(aq) + OH-(aq) A 1.0 mol.dm-3 solution of NaOH produces an [OH-] of 1.0 mol.dm-3. barium hydroxide Ba(OH)2(aq) → Ba2+(aq) + 2OH-(aq) A 1.0 mol.dm-3 solution of Ba(OH)2 produces an [OH-] of 2.0 mol.dm-3. Once you have deduced the [OH-], we need to use the Kw expression to calculate the pH in 2 steps. 1. Use the Kw expression to find [H+]… Kw Kw = [H+][OH-] So.. [H+] = [OH-] 2. Use the [H+] to calculate the pH using.. pH = -log [H+] How To Calculate pH For Strong Bases AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES CALCULATING pH of WEAK ACIDS (Ka) Weak acids only partially dissociate in solution. This means only some of the molecules release H+ ions. They exist in an equilibrium. e.g. ethanoic acid CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq) Since it is an equilibrium, we can write the Kc expression for it, but this time, we call it Ka, the acid dissociation constant. [H+] [CH3COO-] Ka = [CH3COOH] I order to calculate Ka, you need to know two things: 1. The [H+] or pH (you can calculate [H+] from the pH if it is given in the question) 2. The [acid] Important! When calculating Ka for a weak acid, the two values at the top of the equation, [H+] and [CH3COO-]are assumed to be equal. This is because, when the acid dissociates, one cannot be produced without the other. So this gives you the expression… [H+]2 Ka = [CH3COOH] HINTS | TIPS | HACKS You will only ever be asked to work with monoprotic weak acids You may be asked to calculate Ka, or rearrange the expression to calculate the pH (via finding How To Work with the [H+]), or the [acid] Ka Expression Like Kw , Ka is also affected by temperature The greater the value of Ka, the more the weak acid dissociates, so the “stronger” it is. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES pKa pKa is a value that we can calculate from Ka. Quite simply, by using a logarithmic scale, the numbers become a bit nicer to work with (like pH compared to [H+]) You can calculate pKa from Ka and vice versa. pKa = -log [Ka] Notice how these are the same expressions used to convert [H+] to Ka= 10-pKa pH an device versa! The lower the pKa value for a weak acid, the more it dissociates and the “stronger” the weak acid is (and vice versa) In terms of calculations, you need to know how to convert between the two values. The exam paper may give you the value for pKa for a weak acid for you to first convert it to Ka, then use Ka in the expression to find the pH or the [acid]. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES pH CURVES pH curves are generated experimentally. They are essentially just titrations, but rather than using an indicator to find the end point, we use a pH probe to closely monitor the changes in pH of the reaction mixture as an alkali is added to an acid. A detailed description of this practical is given in the Required practicals book. Here's a general overview of a pH curve that would be generated from slowly adding an alkali to an acid. e.g. 0.1 mol.dm-3 NaOH(aq) being added to 25cm3 of mol.dm-3 HCl(aq) pH ends high 14 The “vertical” region is pH where neutralisation 7 occurs. The middle of this line is known as the equivalence point 0 0 25 pH starts low Vol. NaOH(aq) (cm3) Depending on the combination of strong / weak acids and bases, we produce 4 distinct, different pH curves: Strong acid Vs Strong Base (above) Strong acid Vs Weak Base Weak Acid Vs Strong Base Weak Acid Vs Weak Base AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES TYPES OF pH CURVE Strong Acid Vs Strong Base Strong Acid Vs Weak Base pH pH * Vol. Base Vol. Base - Large, sudden change in pH - Sharp increase of pH into equivalence point - Large equivalence point (approx. pH 7) - Small equivalence point (below pH 7) - *Buffer region after neutralisation Weak Acid Vs Strong Base Weak Acid Vs Weak Base * pH pH * * Vol. Base Vol. Base - Small, initial increase in pH - Gradual increase of pH into equivalence point - Gradual increase of pH into equivalence point - Little to no equivalence point about pH 7 - *Buffer region after neutralisation - *Buffer region before and after neutralisation - Small equivalence point (above pH 7) The *Buffer Regions are caused by the fact that a “buffer” is created by the reaction which resists changes in pH. More on these later. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES pH CURVES & INDICATORS You may be asked to “select an appropriate indicator” for a pH curve. The “range” of the indicator must lie within the range of the equivalence point (vertical section) on the curve. The question will provide a list of indicators with their ranges to select from. You do not need to learn these! Strong Acid Vs Strong Base Strong Acid Vs Weak Base pH pH Vol. Base Vol. Base Weak Acid Vs Strong Base Weak Acid Vs Weak Base pH pH * Vol. Base Vol. Base Use the Y axis on the pH curve to find the range of the equivalence point (red lines). So long as the range of the indicator is within this, it is suitable for use in that titration. * Weak Acid Vs Weak base has little to no equivalence point, so indicators are not useful for this titration. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.12 ACIDS & BASES pH CURVES & Ka A pH curve for a Weak Acid Vs Strong Base can be used to find the Ka for the weak acid. e.g. CH3COOH Vs NaOH Weak Acid Vs Strong Base pH 3. 1. 2. Vol. NaOH 1. Use the curve to find the volume of strong base that is required to neutralise the acid. 2. Halve that volume and mark it on the x axis. 3. Read the pH of the mixture at “half-volume” 4. At this point pH = pKa 5. Calculate Ka using Ka= 10-pKa Why this works… [H+] [CH3COO-] The expression for Ka is… Ka = [CH3COOH] When the acid is half neutralised, the [CH3COO-] and the [CH3COOH] are equal So this means they cancel out in the expression, leaving… Ka = [H+] If we take the -log of both… pKa = pH AQA www.chemistrycoach.co.uk © scidekick ltd 2024