Neutralisation and making saltsIB MYP Chemistry: Revision notes
Section 1
Acids and the salts they make
A salt is formed when the hydrogen in an acid is replaced by a metal (or by ammonium). The first part of the name comes from the metal or base; the second part comes from the acid:
- hydrochloric acid makes chlorides
- sulfuric acid makes sulfates
- nitric acid makes nitrates
For example, magnesium and sulfuric acid make magnesium sulfate, and zinc oxide and hydrochloric acid make zinc chloride.
Section 2
Acid + base (neutralisation)
A base is a substance that reacts with an acid to neutralise it. Metal oxides and metal hydroxides are bases. A base that dissolves in water is called an alkali.
acid + base → salt + water
Example: hydrochloric acid + sodium hydroxide → sodium chloride + water
Example: sulfuric acid + copper(II) oxide → copper(II) sulfate + water
The reaction is called neutralisation because the pH moves towards 7.
Section 3
Acid + metal and acid + carbonate
acid + metal → salt + hydrogen
Example: magnesium + sulfuric acid → magnesium sulfate + hydrogen. Only metals above hydrogen in the reactivity series react safely. The gas is tested with a lit splint: hydrogen gives a squeaky pop.
acid + carbonate → salt + water + carbon dioxide
Example: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide. The mixture fizzes. Carbon dioxide turns limewater cloudy.
Remember the three patterns: base gives salt + water; metal gives salt + hydrogen; carbonate gives salt + water + carbon dioxide.
Section 4
Making a soluble salt from an insoluble reactant
To make crystals of a soluble salt such as copper(II) sulfate:
- Warm the acid in a beaker.
- Add the insoluble solid (metal, metal oxide or carbonate) a little at a time, stirring, until excess solid remains. This makes sure all the acid has reacted.
- Filter to remove the excess solid. The salt solution is the filtrate.
- Heat the filtrate to evaporate some of the water until it is saturated (crystals form on a glass rod).
- Leave it to cool so that crystals form. This is crystallisation.
- Filter off the crystals and dry them between filter paper.
Do not evaporate all the water. Heating to dryness can break down the salt and does not give neat crystals.
Section 5
Titration (introductory)
If both the acid and the alkali are soluble, there is no excess solid to filter off, so we use a titration to find the exact volumes that react.
- A pipette measures a fixed volume (for example 25.0 cm³) of alkali into a conical flask.
- Add a few drops of indicator, such as phenolphthalein (pink in alkali, colourless in acid).
- Add acid from a burette a little at a time, swirling, until the indicator just changes colour. This is the end-point.
- The titre is the final burette reading minus the starting reading.
- Repeat until you have concordant results (within 0.2 cm³) and take their mean.
To get pure crystals, repeat with the same volumes but no indicator, then crystallise.
Ignore a rough first titration when you calculate the mean titre. Use only concordant results.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Neutralisation and making salts
- A school technician in Dubai prepares blue crystals of copper(II) sulfate for a Year 9 class. She warms 50 cm³ of dilute sulfuric acid and adds black copper(II) oxide a spatula at a time, stirring, until some black solid remains at the bottom of the beaker. She then filters the mixture and heats the blue filtrate in an evaporating basin until crystals begin to form at the edge.Explain why the mixture must be filtered before the filtrate is heated.2 marks
- A student in Nairobi investigates how the mass of calcium carbonate affects the volume of carbon dioxide made. Each time, she adds a different mass of calcium carbonate chips to 50 cm³ of 1.0 mol dm⁻³ hydrochloric acid in a flask connected to a gas syringe, and records the total volume of gas collected once the fizzing stops. Masses of 0.5 g, 1.0 g, 1.5 g, 2.0 g, 3.0 g and 4.0 g give volumes of 120 cm³, 240 cm³, 360 cm³, 480 cm³, 600 cm³ and 600 cm³ respectively.Describe the trend in the results for masses of calcium carbonate from 0.5 g to 2.0 g.2 marks
- A student in Singapore wants to find out how much 0.100 mol dm⁻³ hydrochloric acid is needed to neutralise 25.0 cm³ of sodium hydroxide solution. She uses a pipette, a burette, a conical flask and phenolphthalein indicator, which is pink in alkali and colourless in acid. She repeats the titration several times. Her titres are 24.8 cm³, 24.3 cm³, 24.4 cm³ and 24.5 cm³.Describe how the student should carry out the first titration, from filling the burette to finding the titre.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).