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Acids, Bases and Salts - The characteristic properties of acids and bases

Grade 12IGCSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Acids are substances that act as proton (H+H^+) donors in aqueous solutions (Brønsted-Lowry theory).

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Bases are substances that act as proton (H+H^+) acceptors. Alkalis are a sub-set of bases that are soluble in water and release hydroxide ions (OH−OH^-).

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Acids have a pH<7pH < 7, turn blue litmus paper red, and have a sour taste.

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Bases have a pH>7pH > 7, turn red litmus paper blue, and often have a slippery or soapy feel.

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The characteristic reaction of an acid with a metal produces a salt and hydrogen gas: Acid+Metal→Salt+H2(g)Acid + Metal \rightarrow Salt + H_2(g).

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The reaction of an acid with a metal carbonate produces a salt, water, and carbon dioxide: Acid+Carbonate→Salt+H2O(l)+CO2(g)Acid + Carbonate \rightarrow Salt + H_2O(l) + CO_2(g).

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Neutralization is the reaction between an acid and a base to produce a salt and water: H+(aq)+OH−(aq)→H2O(l)H^+(aq) + OH^-(aq) \rightarrow H_2O(l).

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Strong acids (e.g., HClHCl, H2SO4H_2SO_4) completely dissociate in aqueous solution, whereas weak acids (e.g., CH3COOHCH_3COOH) only partially dissociate.

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Indicators such as Phenolphthalein (colorless in acid, pink in alkali) and Methyl Orange (red in acid, yellow in alkali) are used to distinguish between acidic and alkaline solutions.

📐Formulae

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

[H+]=10−pH[H^+] = 10^{-pH}

Kw=[H+][OH−]=1.0×10−14 at 25∘CK_w = [H^+][OH^-] = 1.0 \times 10^{-14} \text{ at } 25^\circ C

Acid+Base→Salt+H2OAcid + Base \rightarrow Salt + H_2O

2HCl(aq)+Mg(s)→MgCl2(aq)+H2(g)2HCl(aq) + Mg(s) \rightarrow MgCl_2(aq) + H_2(g)

💡Examples

Problem 1:

Calculate the pHpH of a 0.05 mol/dm30.05\text{ mol/dm}^3 solution of sulfuric acid (H2SO4H_2SO_4), assuming complete dissociation.

Solution:

Since H2SO4H_2SO_4 is a diprotic acid, [H+]=2×0.05=0.1 mol/dm3[H^+] = 2 \times 0.05 = 0.1\text{ mol/dm}^3. Therefore, pH=−log⁡10(0.1)=1pH = -\log_{10}(0.1) = 1.

Explanation:

Sulfuric acid releases two protons per molecule. The pHpH is the negative logarithm of the total hydrogen ion concentration.

Problem 2:

Write the balanced chemical equation and the net ionic equation for the reaction between hydrochloric acid (HClHCl) and sodium hydroxide (NaOHNaOH).

Solution:

Chemical Equation: HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l). Net Ionic Equation: H+(aq)+OH−(aq)→H2O(l)H^+(aq) + OH^-(aq) \rightarrow H_2O(l).

Explanation:

In a neutralization reaction between a strong acid and a strong base, the spectator ions (Na+Na^+ and Cl−Cl^-) are removed to show the formation of water.

Problem 3:

Identify the observation when dilute nitric acid (HNO3HNO_3) is added to solid copper(II) carbonate (CuCO3CuCO_3).

Solution:

CuCO3(s)+2HNO3(aq)→Cu(NO3)2(aq)+H2O(l)+CO2(g)CuCO_3(s) + 2HNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + H_2O(l) + CO_2(g).

Explanation:

Effervescence (fizzing) is observed due to the release of carbon dioxide gas, and the green solid dissolves to form a blue solution of copper(II) nitrate.

The characteristic properties of acids and bases Revision - Grade 12 Chemistry IGCSE