The reactions of acids pop up throughout A-Level Chemistry. You met them all at GCSE.
Yet, I bet if I asked you to write an equation for one of them right now, half of you wouldn’t be able to do it.
In fact, probably more than half.
The problem is, you don’t really get taught these reactions at A-level because they’re kind of assumed knowledge. But because you’re not taught them, they won’t be fresh in your mind so you’re going to struggle. This will cost you easy marks.
This tutorial will help you revise the reactions of acids that you’re expected to know when you start A-level Chemistry. You’ll learn more about acids in year 2 of A-level but these are the very basics.
The reactions of acids you’re expected to know
1.The reaction of an acid with a metal
2. The reaction of an acid with an alkali
3. The reaction of an acid with a metal oxide
4. The reaction of an acid with a carbonate
5. The reaction of an acid with ammonia
The reaction of acids with metals
IMPORTANT!!! This is the only reaction in this handout that is a redox reaction. This could come up as a multiple choice question, so if you’re given the choice of several reactions of acids and asked which is a redox reaction it will be the one with the met.
Acid + metal → salt + hydrogen
Example: Sulfuric acid reacts with Potassium to make Potassium Sulfate and Hydrogen
H2SO4 + 2K → K2SO4 + H2
The reaction of acids with alkalis (e.g. KOH)
Acid + alkali → salt + hydrogen
Example: Hydrochloric acid reacts with Sodium Hydroxide to make Sodium Chloride and water.
HCl + NaOH → NaCl + H2O
The reaction of acids with metal oxides
Acid + metal oxide → salt + water
Example: Nitric Acid reacts with Magnesium Oxide to make Magnesium Nitrate and water.
2HNO3 + MgO → Mg(NO3 )2 + H2O
The reaction of acids with metal carbonates
Acid + metal carbonate → salt + carbon dioxide + water
Example: Ethanoic acid reacts with Sodium carbonate to make Magnesium ethanoate, carbon dioxide and water
CH3COOH + Na2CO3 → CH3COONa + CO2 + H2O
The reaction of acids with ammonia
Acid + ammonia→ ammonium salt
Example: Hydrochloric acid reacts with Ammonia to make Ammonium Chloride.
HCl + NH3 → + NH4Cl
How to identify the salt
The salt is made from the conjugate base of the acid (the bit left over when it loses a proton) and the metal ion from the other reactant. For example, in H2SO4 the conjugate base is SO42- and in CH3COOH the conjugate base is CH3COO–.
Make sure you don’t make this mistake
Using the wrong formula for the salt is the most common mistake, for example NaSO4 instead of Na2SO4 This usually happens when you don’t consider the oxidation state of the metal and just assume that one metal ion will bond with one conjugate base ion. Of course, this isn’t always the case.
Using Sodium Sulfate as our example: Sodium is in group 1 of the periodic table so forms 1+ ions. Sulfate is the SO42- ion (you’re supposed to know this) so the formula of Sodium Sulfate is Na2SO4.
This is super important as if your formula is incorrect your equation will be balanced incorrectly. Then, any calculations you do based on the equation will be based on the wrong mole ratio so you won’t be able to get the right answers. So remember to always consider the oxidation state when working out formulae.
To understand what salt you get when acids react with Ammonia you’ll need to understand a bit more about how the reaction works.
Acids donate a proton to Ammonia to make the positively charged Ammonium ion:
H+ + NH3 → NH4+
The ammonium ion then forms a salt with the conjugate base from the acid. For example if the acid was Hydrochloric acid..
HCl + NH3 → NH4Cl