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CHEMISTRY FORM 2


1. STRUCTURE OF THE ATOM, AND THE PERIODIC TABLE
2. CHEMICAL FAMILIES AND PATTERNS IN PROPERTIES
3. CHEMICAL BONDING AND STRUCTURE
4. SALTS
5. EFFECT OF AN ELECTRIC CURRENT ON SUBSTANCES
6. CARBON AND SOME OF ITS COMPOUNDS
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Salts: Reacting a Metal with an Acid

4.0 Salts


4.4.1 Reacting a Metal with an Acid


The choice of metal and acid depends on the salt we want to make. Which salt do you want: iron (II) sulphate (used in iron tablets), or zinc sulphate (medicine for zinc deficiency in humans)? Suppose we pick zinc sulphate.


Questions 4.4.1(a)


  1. Which metal should we use to prepare zinc sulphate?

  2. Which acid is required to make this salt?

  3. The salt produced is soluble; so it will be in solution. Why are we certain of this, without testing?

  4. Write a balanced chemical equation for the reaction.

  5. How can we proceed to obtain crystals of zinc sulphate from the solution?

  6. Why is it not advisable to prepare sodium sulphate (or potassium sulphate) in a similar manner although they also produce salts with acids?

  7. Magnesium also reacts with acids to produce salts. But the salt is not pure. Suggest a reason why the salt is not pure. Hint: What else does magnesium produce?

  8. Besides carefully handling apparatus and chemicals, explain any safety precaution we should take during the reaction.


Answers to Questions 4.4.1(a)


In this method, we need a metal and an acid that make up the salt we want. For example, if we want iron (II) chloride, we use iron metal and hydrochloric acid. For aluminium sulphate, we need aluminium metal and sulphuric acid, and so on.


This method is suitable for metals that do not react with cold water but react with dilute acids. They are zinc, aluminium, lead, and iron which are above copper in the reactivity series.


Caution: Care must be taken to avoid bare flame during the reaction because hydrogen gas is produced. This burns explosively and can cause accident.


Observe the video on preparation of zinc sulphate crystals below

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(courtesy Youtube-C11 - Salts (Salts Preparation using Method 1) by Nick Ng)

Questions 4.4.1(b)

Describe step-by-step how you would prepare the salt, zinc sulphate.


Answers to Questions 4.4.1(b)


The acid is warmed so that the reaction occurs faster. Warming is stopped before the metal is added to avoid presence of a flame during the reaction. A small excess of the metal is an indication that all the acid is used up; so it will not contaminate the salt made.


A glass rod is used to stir the reacting mixture because it (glass) is unreactive. Soluble salts should not be washed after filtration because they would re-dissolve in water. Figure 4.4.1 shows zinc sulphate crystals produced by this method.


Zinc sulphate crystals

Figure 4.4.1(a) Zinc sulphate crystals


Questions 4.4.1(c)

Describe how you would prepare iron (II) sulphate from iron metal dust and a suitable dilute acid. Include an equation for the reaction.


Answers to Questions 4.4.1(c)


Have you noticed that preparation of salts from metals and acids follows the same procedure? It makes work easier. Figure 4.4.1(b) shows the appearance of iron (II) sulphate.


Iron (II) sulphate, high school chemistry

Figure 4.4.1(b) Iron (II) sulphate


Due to reaction with oxygen of the air (oxidation), iron (II) sulphate slowly turns brown as it gets oxidized to iron (III) sulphate.