Mix an acid with a base and something surprisingly tidy happens. Whichever acid and whichever base you pick, the same tiny reaction is going on underneath: a proton meets a hydroxide ion and they make water. Everything else in the equation is just watching.
📘 What you need to know
acid + base → salt + water is the general pattern for neutralisation.
The reaction that actually happens is H+(aq) + OH−(aq) → H2O(l).
The other ions are spectator ions. They do not change, and together they make up the salt.
The salt is named after the acid: hydrochloric gives chlorides, nitric gives nitrates, sulfuric gives sulfates, ethanoic gives ethanoates.
Acids also react with metals (giving hydrogen), metal oxides, metal hydroxides, carbonates and hydrogencarbonates.
Only carbonates and hydrogencarbonates give off CO2. Only metals give off hydrogen.
The enthalpy of neutralisation for a strong acid with a strong base is close to −57 kJ mol−1, because it is always the same reaction.
The reaction hiding underneath
Take the standard example: hydrochloric acid and sodium hydroxide.
The equation you already know
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Both of these are strong, so in solution they are not really molecules at all — they are ions swimming about. Write them out as ions and something obvious appears.
The spectator ions are not useless — they are the salt. Evaporate the water off and Na+ and Cl− are left behind as solid sodium chloride.
This one picture explains why the enthalpy of neutralisation is the same number for HCl with NaOH, HNO3 with KOH, and every other strong-with-strong pair. It is literally the same reaction each time. Swap in a weak acid and the value drops, because some energy has to be spent breaking the weak acid apart first.
Naming the salt
The salt is built from two halves: the positive ion from the base and the negative ion from the acid. So the second word of the name tells you which acid was used.
Acid
Ion it leaves behind
Salts are called
Example
hydrochloric, HCl
Cl−
chlorides
NaCl, sodium chloride
nitric, HNO3
NO3−
nitrates
KNO3, potassium nitrate
sulfuric, H2SO4
SO42−
sulfates
MgSO4, magnesium sulfate
ethanoic, CH3COOH
CH3COO−
ethanoates
CH3COONa, sodium ethanoate
phosphoric, H3PO4
PO43−
phosphates
Na3PO4, sodium phosphate
any acid, with ammonia
NH4+ from the base
ammonium salts
NH4Cl, ammonium chloride
The five reactions acids do
“Base” is wider than “alkali”. An alkali is a soluble base, but oxides, hydroxides, carbonates and hydrogencarbonates all count as bases too, and they all give you a salt.
Only the bottom two rows fizz with carbon dioxide, which is how you test an unknown solid: add acid and look for a gas that turns limewater milky.
Watch the balancing numbers. They come from the charges. Zn2+ needs two Cl−, so you need 2HCl. Mg(OH)2 has two hydroxides, so it needs two protons and makes two waters. Get the formula of the salt right first and the rest follows.
🧩 Building any neutralisation equation
Identify the two ions in the salt: the metal (or ammonium) ion from the base, and the acid’s leftover ion.
Balance their charges to get the salt’s formula, e.g. Ca2+ with Cl− gives CaCl2.
Add the other products: water always, plus CO2 if you started from a carbonate, or H2 if you started from a metal.
Balance the whole equation and finish with state symbols. Solids that dissolve become (aq).
The energy released
The enthalpy of neutralisation is the enthalpy change when an acid and a base react to form one mole of water.
For any strong acid with any strong base it comes out at about −57 kJ mol−1, which should not surprise you now: the underlying reaction is H+ + OH− → H2O every time, and the spectator ions contribute nothing.
Use a weak acid and the value is smaller, perhaps −55 kJ mol−1. The reaction has to keep pulling the weak acid apart to release more protons, and that step absorbs energy, so less is left over to heat the solution.
Worked examples
WORKED EXAMPLE
Write the balanced equation, with state symbols, for nitric acid reacting with copper(II) carbonate.
Step 1: work out the salt
Nitric acid leaves NO3−, and copper here is Cu2+.
Charges balance as Cu(NO3)2Step 2: add the other products
A carbonate always gives water and carbon dioxide as well.
Step 3: balance and add states2HNO3(aq) + CuCO3(s) → Cu(NO3)2(aq) + H2O(l) + CO2(g)
Check: 2 N each side, 1 Cu, 2 H, and 9 O each side.
2HNO3(aq) + CuCO3(s) → Cu(NO3)2(aq) + H2O(l) + CO2(g)the 2 in front of HNO₃ comes straight from the 2+ charge on copper
WORKED EXAMPLE
25.0 cm3 of 0.200 mol dm−3 NaOH is exactly neutralised by 0.100 mol dm−3 H2SO4. Calculate the volume of acid used.
Step 1: moles of NaOHn = 0.200 × 0.0250 = 5.00 × 10−3 molStep 2: use the equation ratioH2SO4 + 2NaOH → Na2SO4 + 2H2O
One mole of acid neutralises two of alkali, so halve it.
n(H2SO4) = 2.50 × 10−3 molStep 3: convert moles to volumeV = 2.50 × 10−3 ÷ 0.100 = 0.0250 dm3V = 25.0 cm3the 2:1 ratio is the whole question — miss it and you get 50.0 cm³
WORKED EXAMPLE
Explain why the enthalpy of neutralisation of HCl with NaOH is almost identical to that of HNO3 with KOH.
Step 1: write both as ionic equationsBoth reduce to H+(aq) + OH−(aq) → H2O(l)Step 2: say what happened to the rest
Na+, K+, Cl− and NO3− are all spectators and stay aqueous throughout.
Step 3: link that to the energy
Since the same bonds are made in the same reaction, the same energy is released.
Both are really the same reaction, so both give about −57 kJ mol−1the mark is for naming the shared ionic equation, not for quoting the number
💡 Exam tip
Learn the ionic equation H+(aq) + OH−(aq) → H2O(l) with state symbols. It is asked for directly and often.
Name the salt from the acid, not the base. Sulfuric always gives a sulfate, whatever metal is involved.
Get the salt formula right first, then balance. Doing it the other way round leads to made-up formulas like NaCl2.
Only carbonates and hydrogencarbonates fizz with CO2. Metal oxides and hydroxides dissolve quietly.
The word “base” includes insoluble oxides and carbonates. “Alkali” means a base that dissolves in water.
For enthalpy questions, define it per one mole of water formed — not per mole of acid.
⚠ Common mix-up
Expecting hydrogen from every acid reaction. Only metals give H2. Carbonates give CO2, and oxides and hydroxides give neither.
Leaving spectator ions in the ionic equation. If a species is identical on both sides, cross it out.
Forgetting the 2 for sulfuric acid. It supplies two protons, so it needs twice as much of a 1+ alkali.
Thinking neutral always means the salt solution is pH 7. The salt of a weak acid is not, as you will see in salt hydrolysis.
Saying weak acids release less energy because they react less. They react just as fully — energy is used up splitting them, so the measured value is smaller.
Mixing up base and alkali. All alkalis are bases; only the soluble bases are alkalis.
Up next: pH Titration Curves. Instead of adding all the alkali at once, we will add it drop by drop and watch what the pH does. The shape of that graph tells you more than any single number could.
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