Making salts, Study Guides, Projects, Research of Chemistry

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Making salts

In this topic we are going to draw on aspects of last year’s course – what do we need to recap?

• Reactions of acids
• How to write formulae for compounds using valency The methods for making different salts depend on the solubility of the salt in water. We need to be able predict if a salt will be soluble: Solubility Rules for ionic compounds in water
  • sodium, potassium and ammonium salts are soluble
  • all nitrates are soluble
  • chlorides are soluble except silver and lead(II) chlorides
  • sulphates are soluble except barium, calcium and lead(II)
  • calcium hydroxide is slightly soluble
  • predict that other ionic compounds are insoluble

Separate Sciences

Solubility Curves

These show how the solubility of a substance varies with temperature. 10

Solubility [g/100g water] here the solution is not saturated – more solute can be added here the solution is saturated and some solute remains undissolved

Separate Sciences

Using Solubilty Curves i) What is the lowest temperature at which we can dissolve 20g of the solute in 100g of water? 60°C ii) How much solute will dissolve in 100g of water at 40°C? 15g iii) How much solute will dissolve in 50g of water at 60°C?

20g in 100g water, so 10g in 50g water

iv) What is the lowest temperature required to dissolve 30g of solute in 200g of water?

Same as 15g in 100g water so 40°C

Separate Sciences

Investigating solubility practically

Method:

1. Add known mass of solute and known volume of water in a boiling tube.
2. Calculate solubility (g of solute per 100g of water) when this forms a saturated solution.
3. Heat mixture in hot water bath until all the solute dissolves.
4. Cool in an ice bath until crystals just start to appear – record temperature.
5. Add known volume of water to the boiling tube.
6. Repeat steps 2 – 5 until sufficient points have been obtained to plot solubility curve. Use this method to plot a solubility curve for ammonium chloride, starting with 2.6g of solute and 4cm 3 (4g) of water, and adding 1cm 3 (1g) each time.

Methods for making salts

A salt is made when the hydrogen ions in an acid are replaced by metal ions (or ammonium ions). The name of the salt begins with the name of the metal, followed by a name reflecting the acid it was made from:

e.g sodium chloride

magnesium sulphate

potassium nitrate

from hydrochloric acid from sulphuric acid from nitric acid

Method for making soluble salts Choose a solid reactant that will react with an acid to make the salt we want. This is likely to be:

• a metal e.g. Mg, Zn, Fe

Reaction: metal + acid à salt + hydrogen

• a metal oxide e.g. CuO

Reaction: metal + acid à salt + water

• a metal carbonate e.g. CuCO 3 , ZnCO 3

Reaction: metal + acid à salt + water + CO

2 How will we know when the reaction is complete? How can remove unreacted reactants? How can remove any products we don’t want?

Stepwise Instructions

1. Add solid reactant to acid until no more will react and some solid is left. Warming may be needed if the reaction is slow (e.g. with metal oxides).
2. Filter to remove unreacted solid reactant.
3. Boil the salt solution in an evaporating dish to remove water until the solution becomes saturated; heat until crystals start to form e.g. on the end of a glass rod after dipping in the solution.
4. Allow the salt solution to cool so that crystals form.
5. Filter to obtain the crystals and discard the remaining solution.
6. Dry the crystals (warm oven), or leave in warm place for the remaining water to evaporate. Use this method to prepare the salt copper sulphate from a metal oxide. A full write-up of this practical is required.

Making a salt with acid + metal oxide General equation: METAL OXIDE + ACID à SALT + WATER e.g. CuO (s)

+ H

2

SO

4(aq) à CuSO 4(aq)

+ H

2

O

(l) Works for: All metal oxides. Useful for making salts of unreactive metals e.g. copper or lead. Observations: The solid metal oxide will disappear as it reacts Note: The acid may require warming to speed up the reaction

Making a salt with acid + metal carbonate General equation: METAL CARBONATE + ACID à SALT + WATER + CARBON DIOXIDE e.g. ZnCO 3(s)

+ H

2

SO

4(aq) à ZnSO 4(aq)

+ H

2

O

(l)

+ CO

2(g) Works for: All metal carbonates. Useful for making salts of unreactive metals e.g. copper Observations: Solid metal carbonates will disappear as they react There will be bubbling/fizzing as a gas is produced

Separate Sciences

METAL HYDROXIDE^ +^ ACID^ à^ SALT^ **+ WATER e.g. NaOH (aq)

• HCl (aq)** à NaCl (aq)

+ H

2

O

(l) Works for: All alkalis. Useful for salts of reactive metals such as sodium or potassium and to make ammonium salts. Ammonia solution contains ammonium hydroxide: NH 3(g)

+ H

2

O

(l) à NH 4

OH

(aq) Observations: None! You are adding two colourless solutions together to form a colourless solution and water. Use this method to prepare the salt ammonium chloride. A full write-up of this practical is required.

Separate Sciences

Method for making insoluble salts

Why not just use the same method we have learnt for making soluble salts? Which step in that method will make it fail when the product is an insoluble salt? When we need to make an insoluble salt, we can’t filter to remove unreacted solid reactants, so we choose for both of our reactants to be solutions (liquids). Making an insoluble solid by reacting solutions of two soluble substances together is called a precipitation reaction. The precipitate of our insoluble salt can be separated from the remaining solutions after the reaction by filtration.

Separate Sciences

barium nitrate + sodium sulphate

à barium sulphate + sodium nitrate

precipitate

• collect by

filtering

remaining solution

• pour away Explain how you would make insoluble silver bromide by this method.

Separate Sciences

Stepwise Instructions

1. Mix two solutions in a to form a precipitate of the insoluble salt.
2. Remove the precipitate from the remaining solution by filtering.
3. Wash the solid precipite with distilled water to remove traces of the soluble reactants and products.
4. Dry the precipitate (warm oven) to remove the remaining water. Use this method to prepare the salt lead sulphate. A full write-up of this practical is required.