Neutralisation of Acids and Salt Production (AQA GCSE Chemistry: Combined Science)
Revision Note
Author
StewartExpertise
Chemistry Lead
Acid - Base Reactions
- When an acid reacts with a base, a neutralisation reaction occurs
- Bases have pH values above 7
- Many bases are insoluble in water but the ones that do dissolve in water are called alkalis
- They thus form an alkaline solution
- Examples of alkalis are soluble metal hydroxides such as NaOH and Ca(OH)2
- In all acid-base neutralisation reactions, salt and water are produced:
acid + base ⟶ salt + water
- If the base is a metal carbonate, carbon dioxide is also produced:
acid + base ⟶ salt + water + carbon dioxide
- The identity of the salt produced depends on the acid used and the positive ions in the base
- Hydrochloric acid produces chlorides, sulfuric acid produces sulfate salts and nitric acid produces nitrates
Reactions of Acids with Metal Oxides and Metal Hydroxides
- Metal oxides and metal hydroxides act as bases
- When they react with acid, a neutralisation reaction occurs
- In all acid-base neutralisation reactions, salt and water are produced
- The following are some specific examples of reactions between acids and metal oxides:
2HCl + CuO ⟶ CuCl2 + H2O
H2SO4 + Na2O⟶ Na2SO4 + H2O
2HNO3 + MgO ⟶ Mg(NO3)2 + H2O
- The following are some specific examples of reactions between acids and metal hydroxides:
2HCl + 2LiOH ⟶ 2LiCl + H2O
H2SO4 + 2NaOH ⟶ Na2SO4 + 2H2O
HNO3 + KOH ⟶ KNO3 + H2O
Reactions of Acids with Metal Carbonates
- Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water
- These reactions are easily distinguishable from acid – metal oxide/hydroxide reactions due to the presence of effervescence caused by the carbon dioxide gas
- The following are some specific examples of reactions between acids and metal carbonates:
2HCl + Na2CO3 ⟶ 2NaCl + H2O + CO2
H2SO4 + CaCO3⟶ CaSO4 + H2O + CO2
2HNO3 + MgCO3 → Mg(NO3)2 + H2O + CO2
Exam Tip
If in an acid-base reaction there is effervescence produced then the base must be a metal carbonate which produces carbon dioxide gas.
Predicting the Salts
Salts
- A salt is a compound that is formed when the hydrogen ion in an acid is replaced by a metal ion
- For example if we replace the hydrogen ion in HCl with a potassium ion, then the salt potassium chloride is formed, KCl
- Salts are an important branch of chemistry due to the varied and important uses of this class of compounds
- These uses include fertilisers, batteries, cleaning products, healthcare products and fungicides
Naming salts
- The name of a salt has two parts
- The first part comes from the metal, metal oxide or metal carbonate used in the reaction
- The second part comes from the acid
- The name of the salt can be determined by looking at the reactants
- For example hydrochloric acid always produces salts that end in chloride and contain the chloride ion, Cl–
- Other examples:
- Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride
- Zinc oxide reacts with sulfuric acid to produce zinc sulfate
- A list of the common ions and their formulae is shown below:
Naming Salts Table
- Salts have no overall charge since the sum of the charges on the ions is equal to zero
- From the table of the common ions it is clear the charges on each the Group I elements is always 1+, Group II is 2+, Group VI is 2- and Group VII is 1-
- If you know the ions present in a salt you can identify the formula from balancing the charges
Worked example
What is the formula of magnesium phosphate?
Answer:
- Step 1: Write out the formulae of each ion, including their charges
Mg2+ and PO43-
- Step 2: Balance the charges by multiplying them so that the charges are equal but opposite and they cancel out:
(Mg2+) x 3 = 6+ and (PO43-) x 2 = 6-; so (6+) + (6-) = 0
- Step 3: The multiplying number for each ion tells you how many ions there must be present in the formula so use these to construct the formula:
The formula is Mg3(PO4)2
Note: Use brackets around the ion if there is more than one ion and the ion contains more than one element
Exam Tip
You should know by heart the names of the common ions, their charges and the formulae of compounds they often appear in.
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