Chemical Properties of Acids and Bases
Senior 2 · Chemistry
Chemical Properties of Acids and Bases
Lesson 2 of 4
25 min
Leave an iron nail in lemon juice and it slowly bubbles and dissolves. Pour vinegar on a rusty knife and the rust fades. Wipe an aluminium pan with a strong alkaline cleaner and it fizzes. Each of these is a reaction of an acid or a base with a metal or a metal oxide. The patterns are simple enough to predict the products every time.
Metals above hydrogen in the reactivity series react with dilute hydrochloric acid and dilute sulfuric acid, fizzing as hydrogen is given off and forming a salt:
The more reactive the metal, the faster the reaction: magnesium > zinc > iron. Copper, silver and gold are below hydrogen and do not react with dilute acids. Sodium and potassium react explosively and must never be added to acids. Lead hardly reacts, because a layer of insoluble lead(II) chloride or lead(II) sulfate quickly coats it. Nitric acid is unusual: it attacks metals but rarely gives hydrogen, so it is not used to prepare hydrogen.
Test for hydrogen: a burning splint held at the mouth of the tube goes out with a squeaky pop.
Ionic equation. The chloride or sulfate ions are spectator ions. What really happens is: Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g).
Metal oxides are bases. They neutralise acids to form a salt and water, with no gas:
Ionic equation: CuO(s) + 2H+(aq) → Cu2+(aq) + H2O(l).
For every neutralisation of an acid by an alkali the ionic equation is the same: H+(aq) + OH−(aq) → H2O(l). The reaction gives out heat (it is exothermic).
What volume of hydrogen, measured at STP, is produced when 0.24 g of magnesium reacts with excess dilute hydrochloric acid? (Mg = 24; molar gas volume at STP = 22.4 dm³ = 22 400 cm³)
"Excess" acid means there is more than enough acid, so the magnesium is the reactant that runs out and fixes the amount of product.
Tip
Three word equations cover most exam questions
Acid + metal → salt + hydrogen. Acid + metal oxide → salt + water. Acid + alkali → salt + water. The salt's name is the metal plus the acid's "family name": hydrochloric acid gives chlorides, sulfuric acid gives sulfates and nitric acid gives nitrates. If an iron salt forms from the metal and dilute acid, it is iron(II).
Write a balanced equation for zinc reacting with dilute hydrochloric acid, and name the salt.
Solution: Zn + 2HCl → ZnCl2 + H2. The salt is zinc chloride.
A student adds copper turnings to dilute sulfuric acid and sees no bubbles. Explain.
Solution: Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid, and no reaction occurs.
Name the products when magnesium oxide reacts with dilute nitric acid, and write the equation.
Solution: Magnesium nitrate and water: MgO + 2HNO3 → Mg(NO3)2 + H2O.
Write the ionic equation for the neutralisation of any acid by any alkali.
Solution: H+(aq) + OH−(aq) → H2O(l).
What mass of zinc sulfate forms when 6.5 g of zinc reacts with excess dilute sulfuric acid? (Zn = 65, S = 32, O = 16)
Solution: Moles of Zn = 6.5 ÷ 65 = 0.10 mol. Zn + H2SO4 → ZnSO4 + H2 is 1 : 1, so 0.10 mol of ZnSO4 forms. M(ZnSO4) = 65 + 32 + 64 = 161 g/mol, so the mass = 0.10 × 161 = 16.1 g.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
