This is the sequel to the post Acids & Bases: a Guide to Tutorial 9 Part 1.
See below for hints and tips and things you should know for questions 9.17-9.36.
Questions 9.17, 9.18, 9.19, 9.20, 9.21
Using equilibrium expressions for pKa to calculate pH and vice versa
- Understand how to write equations for the dissociation of acids and bases with water. The general chemical equations are as follows:General equation of dissociation of an acid, HA in water:HA + H20 → A− + H3O+ = KaGeneral equation of dissociation of a base, B in water:
B + H20 → BH+ + OH− = Kb
- Know how to construct the equilibrium expressions for the above dissociation equations: Understand that the concentrations of the species that we plug into the above expressions are the equilibrium concentrations after dissociation has occurred (not initial concentrations of acid or base)
- Know or be able to derive the following equations:pH = -log[H+] and rearrange to:
[H+] = 10-pH
pOH = -log[OH–] and rearrange to:
[OH–] = 10-pOH[H+][OH–] = 1 x 10-14 and rearrange to:
pH + pOH = 14
- Be able to construct an equilibrium table (sometimes known as an ICE table or RICE chart, see here for an explanation) to show how concentrations change from initial to equilibrium concentrations
- Understand the weak acid/base approximation-if you are told that the acid or base used is weak, then it will hardly dissociate at all. The initial concentration of acid or base will be pretty much the same at equilibrium. So we can approximate and make our calculations easier.
- Equation used to calculate the percent ionisation of an acid or base:
Step by step guide:
1. Write out the equation for the dissociation of the acid or base with water.
2. Ascertain what variable you need to calculate based on which of the following equilibrium expressions you will use:
for example, you might be given the pH and therefore can calculate the H+ concentration, or you may be given the pKa and will need to convert this to the Ka.
3. Construct an ICE table in order to work out equilibrium concentrations.
4. Plug these values into one of the equilbrium expression equations above, you may need to rearrange and/or do a weak acid approximation in order to arrive at the value you need to answer the question.
5. Calcuate percent ionization if required using the equation shown earlier.
Questions 9.22 and 9.24
Acid strength as a result of molecular structure
- Carboxylic acids are organic acids and are weak acids.
- Be familiar with the 6 common strong acids: hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid (HBr), perchloric acid (HClO4), nitric acid (HNO3) and sulfuric acid (H2SO4). These acids all ionize 100% in water. Know that anything that you come across that is not one of these strong acids is most likely a weak acid.
- Understand how molecular structure affects acid strength. A general understanding of electronegativity and periodic trends would help here.
- Benzoic acid is an organic acid with the carboxylic acid functional group, COOH. Boron and Fluorine are in the same period, so their acids, boric acid and hydrofluoric acid can be compared on the basis of the relative electronegativities of Fluorine and Boron. Fluorine is much more electronegative than Boron, so is a much stronger acid (although both are classed as weak acids).
- Hydrofluoric acid, HF and iodic acid, HI are hydrogen halides, and their relative strength can be illustrated by the trend of acidity within the group 17 halogens. Unlike the trend for acidity across a period, the acidity down group 17 does not follow electronegativity. Iodic acid is the stronger acid of the two based on the poor orbital overlap due to size mismatch of the two atoms meaning that the H-X bond is weaker. This trend is discussed in the lecture notes for this topic.
Questions 9.23 and 9.25 – 9.28 and 9.30-9.32
Buffers and the Henderson-Hasselbalch equation
- Know what a buffer is.
- Be able to recognise and distinguish between acids, bases and their salt forms.
- Understand how to use the Henderson-Hasselbalch equation:
Know that sometimes A– is replaced with “base” and HA is replaced with “acid”
Keep in mind that when equal concentrations of acid and base are used, that it results in the entire last variable in the equation being “log 1” which is = 0.
Questions 9.29 and 9.33 – 9.36
Acid Base Titrations
- Know the terms: equivalence point, half equivalence point, end point and be able to label them on a titration curve.
- Know how to construct a titration curve (plot mL vs pH) and know roughly the different shapes depending on whether the titration involves a strong acid/strong base, strong acid/weak base or weak acid/strong base
Check out the videos below from Crash Course and Khan Academy that summarise the concepts required for this tutorial.
Titration curves and acid-base indicators:
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