The Characteristic Properties of Acids and Bases Worked ExamplesIGCSE Chemistry 0620

(a) pH 7 is neutral: C. (b) Alkalis have pH above 7; a weakly alkaline solution is only a little above 7, so D (pH 10). E (pH 13) is strongly alkaline.

(c) The lower the pH, the higher the concentration of $\text{H}^+$ ions: A (pH 1). (d) pH 4 is orange.

(e) In the acid A: litmus red, thymolphthalein colourless, methyl orange red. In the alkali E: litmus blue, thymolphthalein blue, methyl orange yellow.

Use the general patterns: acid + metal → salt + hydrogen; acid + base → salt + water; acid + carbonate → salt + water + carbon dioxide. Hydrochloric acid makes chlorides.

(a) Zinc: zinc chloride + hydrogen. Copper(II) oxide: copper(II) chloride + water. Potassium carbonate: potassium chloride + water + carbon dioxide.

(b) Potassium chloride is KCl, so two are needed for the two $\text{K}^+$ ions in the carbonate:

(c) The potassium carbonate reaction — carbon dioxide turns limewater milky.

(a)(i) A base is an oxide or hydroxide of a metal. (ii) An alkali is a soluble base.

(b) Sodium hydroxide and potassium hydroxide dissolve in water: alkalis. Copper(II) oxide and magnesium oxide are insoluble: bases that are not alkalis.

(c) Hydroxide ions, $\text{OH}^-$.

(d) The damp red litmus paper turns blue, because ammonia is given off (an alkaline gas). Ammonium salts react with bases to release ammonia:

An acid is a proton donor and a base is a proton acceptor (a proton is $\text{H}^+$). Follow the hydrogen.

Reaction 1: $\text{HCO}_3^{\,-}$ accepts an $\text{H}^+$ (the extra H ends up in water) — it is acting as a base.

Reaction 2: $\text{HCO}_3^{\,-}$ loses its H to become $\text{CO}_3^{\,2-}$; it donates $\text{H}^+$ to $\text{OH}^-$ — it is acting as an acid. Answer: B.

C is the reverse. A and D assume a species always behaves the same way — but its role depends on what it reacts with.

A strong acid is completely dissociated in aqueous solution; a weak acid is partially dissociated.

Only E is completely dissociated, so only E is strong. F and G are both only partially dissociated, so both are weak — even though F dissociates more than G. Answer: A.

B wrongly treats the more-dissociated weak acid as strong. C confuses being an acid with being strong. D is backwards. At equal concentration G has the highest pH (fewest $\text{H}^+$ ions).

(a) Hydrochloric acid is strong — completely dissociated, so a single arrow:

Ethanoic acid is weak — partially dissociated, so a reversible arrow:

(b) $n(\text{HCl}) = 0.0500 \times 0.100 = 0.00500$ mol. From $\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$, 2 mol acid gives 1 mol hydrogen.

(c) X. Ethanoic acid has a lower concentration of $\text{H}^+$ ions, so collisions with the magnesium are less frequent and the curve is less steep. But the same number of moles of acid is present, and as $\text{H}^+$ ions are used up more acid molecules dissociate, so the same $60\text{ cm}^3$ of hydrogen is made eventually.

W is too fast; Y gives only half the hydrogen, which would need half the moles of acid.

(a) Bubbles / fizzing (effervescence); the green solid dissolves (disappears); a green solution forms.

(b) The limewater turns milky — the gas is carbon dioxide.

(c) Nickel(II) sulfate is $\text{NiSO}_4$; one carbonate reacts with one sulfuric acid:

(d) Any base of nickel: nickel(II) oxide or nickel(II) hydroxide (acid + base → salt + water). Nickel metal would also work (giving hydrogen).

(a) The pH passes through 7 in the middle of the steep drop, at $20.0\text{ cm}^3$.

(b) Only the hydrogen ions and hydroxide ions react; $\text{Na}^+$ and $\text{Cl}^-$ are spectator ions.

(c) At the start the solution is alkaline (pH 13): blue. After $30\text{ cm}^3$ the pH is below 2 (acidic): colourless.

(d) Moles of acid at the end-point, then use the 1 : 1 ratio in $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$.

(e) Universal indicator changes through many colours gradually, so it has no sharp end-point.

(a)(i) J — pH 1 and rapid reactions: a strong acid. (ii) K — pH 4 at the same concentration, slower reactions: a weak acid. (iii) M — pH 13 and it releases ammonia from an ammonium salt: an alkali. (iv) L — pH 7 and no reactions.

(b) Hydrogen — it 'pops' with a lighted splint.

(c) K is only partially dissociated, so at the same concentration it has a lower concentration of $\text{H}^+$ ions. There are fewer collisions per second between $\text{H}^+$ ions and the magnesium or carbonate.

(d) A weak acid such as ethanoic acid.

(a) Convert volumes to $\text{dm}^3$ and use moles = concentration × volume.

(b) $0.00320 \div 0.00160 = 2$, so the ratio is Q : NaOH = 1 : 2. Each $\text{OH}^-$ ion reacts with one $\text{H}^+$ ion, so each molecule of Q must release two $\text{H}^+$ ions (Q donates two protons).

(c) Hydrochloric and nitric acids release one $\text{H}^+$ per molecule; sulfuric acid, $\text{H}_2\text{SO}_4$, releases two.

(a) Calcium hydroxide has two $\text{OH}^-$, so two ammonium nitrate are needed.

(b) Only the ammonium and hydroxide ions change:

$\text{NH}_4^{\,+}$ loses $\text{H}^+$ — it is the proton donor (acid). $\text{OH}^-$ gains $\text{H}^+$ to form water — it is the proton acceptor (base).

(c) $M_r(\text{NH}_4\text{NO}_3) = 14 + 4 + 14 + 48 = 80$; $M_r(\text{NH}_3) = 17$. 1 mol ammonium nitrate gives 1 mol ammonia.

(d) The base reacts with the ammonium salt and nitrogen is lost as ammonia gas, so less nitrogen is left in the soil for the plants.

(a) Moles of carbon dioxide at r.t.p.:

$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$: 1 mol $\text{CaCO}_3$ gives 1 mol $\text{CO}_2$. $M_r(\text{CaCO}_3) = 100$.

Percentage purity = mass of pure substance ÷ mass of sample × 100.

(b) 2 mol HCl per mol $\text{CaCO}_3$:

(c) Calcium carbonate is a solid, so it stays as a formula; $\text{Cl}^-$ is a spectator ion.