Acids, Bases and Salts PDF - Chemistry Revision Notes

```markdown # 9 Acids, bases and salts ## Acids Acids in their pure anhydrous form are composed of covalent molecules. However, acids dissolve in water to form solutions which contain ions. These solutions are described as **acidic.** Acids may be defined in three ways: 1. An acid is a substance which contains hydrogen which can be replaced directly or indirectly by a metal to form a salt, e.g. $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$ The zinc replaces the hydrogen to form the salt, zinc chloride. 2. An acid is a substance which forms hydroxonium ions when dissolved in water. When an acid dissolves in water it initially forms hydrogen ions. Each hydrogen ion then becomes associated with a water molecule to form a hydroxonium ion, e.g. $HCl(g) + water \rightarrow H^+(aq) + Cl^-(aq)$ $H^+(aq) + H_2O(l) \rightarrow H_3O^+(aq)$ hydroxonium ion Overall reaction: $HCl(g) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)$ For simplicity, it is usual to represent $H_3O^+(aq)$ as $H^+(aq)$. 3. An acid is a proton donor. When a hydrogen atom loses an electron to form an $H^+$ ion, it becomes a hydrogen nucleus which contains a single proton. Hydrogen ions are, therefore, protons and acids can donate these to other substances. For example, when an acid dissolves in water it donates its hydrogen ions (protons) to the water molecules, e.g. $HCl(g) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)$ ## GENERAL PROPERTIES OF AQUEOUS ACIDS The properties of acids in their pure form are quite different from those of their aqueous solutions. It is the $H^+(aq)$ ions that cause the acidic properties listed below and these are only formed in the presence of water. 1. Acids have a sour taste. 2. Acids are corrosive. 3. Acids turn litmus from blue to red (see Table 9.1, p. 40). 4. Acids conduct electricity, i.e. they are electrolytes. --- 5. Acids react with bases to form a salt and water only: base + acid $\rightarrow$ salt + water e.g. $Mg(OH)_2(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + 2H_2O(l)$ $CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l)$ Ionically:\ $OH^-(s \text{ or } aq) + H^+(aq) \rightarrow H_2O(l)$ or $O^{2-}(s) + 2H^+(aq) \rightarrow H_2O(l)$ This type of reaction is called a neutralisation reaction. 6. Acids react with carbonates or hydrogencarbonates to form a salt, water and carbon dioxide: carbonate or $+$ acid $\rightarrow$ salt + water + carbon dioxide hydrogencarbonate e.g. $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$ $NaHCO_3(aq) + HNO_3(aq) \rightarrow NaNO_3(aq) + H_2O(l) + CO_2(g)$ Ionically: $CO_3^{2-}(s \text{ or } aq) + 2H^+(aq) \rightarrow H_2O(l) + CO_2(g)$ or $HCO_3^-(aq) + H^+(aq) \rightarrow H_2O(l) + CO_2(g)$ 7. Acids, except nitric acid, react with the reactive metals (iron and above in the reactivity series - see Table 17.1, p. 90) to form a salt and hydrogen: reactive metal + acid $\rightarrow$ salt + hydrogen\ e.g. $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$ Ionically $Mg(s) + 2H^+(aq) \rightarrow Mg^{2+}(aq) + H_2(g)$ Since nitric acid is an oxidising agent it releases oxides of nitrogen, e.g. nitrogen dioxide, $NO_2$, and not hydrogen when reacted with metals. ## THE BASICITY (PROTICITY) OF ACIDS The basicity of an acid is the number of moles of $H^+$ ions produced per mole of acid. 1. Monobasic acids produce one mole of $H^+$ ions per mole of acid, e.g.\ $HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$ 1 mole 1 mole $HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)$ 1 mole 1 mole Monobasic acids form normal salts only (see p. 42). 2. Dibasic acids produce two moles of $H^+$ ions per mole of acid, e.g. $H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)$ 1 mole 2 moles Dibasic acids can form normal salts and acid salts (see p. 42). 3. Tribasic acids produce three moles of $H^+$ ions per mole of acid, $H_3PO_4(aq) \rightarrow 3H^+(aq) + PO_4^{3-}(aq)$ 1 mole 3 moles Tribasic acids can form normal salts and acid salts. --- ## ACID ANHYDRIDES An acid anhydride is an acidic oxide of a non-metal which reacts with water to form an acid, e.g. $CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)$ $SO_2(g) + H_2O(l) \rightleftharpoons H_2SO_3(aq)$ $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$ ## ACIDS IN DAILY LIFE 1. Antacids contain sodium hydrogencarbonate and usually citric or tartaric acid. The sodium hydrogencarbonate neutralises stomach acid, but has an unpleasant taste. When water is added, some of the sodium hydrogencarbonate reacts with the acid producing carbon dioxide. The carbon dioxide causes the antacid to fizz and be less unpleasant tasting. When taken as a drink, the remaining sodium hydrogencarbonate neutralises the stomach acid. 2. Baking powder is tartaric acid combined with sodium hydrogencarbonate. Heat causes the sodium hydrogencarbonate to decompose releasing carbon dioxide: $2NaHCO_3(s) \rightleftharpoons Na_2CO_3(s) + H_2O(l) + CO_2(g)$ The tartaric acid then reacts with the sodium carbonate in the presence of moisture forming more carbon dioxide. The carbon dioxide forms bubbles in the cake causing it to rise. 3. Fire extinguishers contain carbon dioxide under pressure. When pressure is reduced by opening the valve, the carbon dioxide is released. Carbon dioxide is non-flammable and, due to its high density, it smothers the fire. ## Bases A base is chemically opposite to an acid and may be defined in two ways: 1. A base is a substance which will react with an acid to form a salt and water only. 2. A base is a proton ($H^+$ ion) acceptor. For example, when a metal hydroxide reacts with an acid, the hydroxide ions accept hydrogen ions (protons) from the acid forming water: $OH^-(s \text{ or } aq) + H^+(aq) \rightarrow H_2O(l)$ (from base) (from acid) Bases include ammonia and most metal oxides and hydroxides, e.g. magnesium oxide, MgO; copper(II) hydroxide, $Cu(OH)_2$. ## Alkalis An alkali is a base which dissolves in, or reacts with, water to form a solution which contains $OH^-$ ions. The solution is described as alkaline, e.g. $NaOH(s) + water \rightarrow Na^+(aq) + OH^-(aq)$\ $Na_2O(s) + H_2O(l) \rightarrow 2Na^+(aq) + 2OH^-(aq)$\ $NH_3(g) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$ Most bases are insoluble, and are therefore not alkalis. --- ## GENERAL PROPERTIES OF AQUEOUS ALKALIS 1. Alkalis have a bitter taste. 2. Alkalis are soapy to touch. 3. Alkalis are corrosive. 4. Alkalis turn litmus from red to blue (see Table 9.1, below). 5. Alkalis conduct electricity, i.e. they are electrolytes. 6. Alkalis react with solutions containing metal ions (except potassium and sodium) to form insoluble precipitates, e.g. $CuSO_4(aq) + 2NaOH(aq) \rightarrow Cu(OH)_2(s) + Na_2SO_4(aq)$ Ionically: $M^{n+}(aq) + nOH^-(aq) \rightarrow M(OH)_n(s)$ 7. Alkalis react with zinc and aluminium forming a salt and hydrogen: zinc or\ $+$ alkali $\rightarrow$ salt + hydrogen\ aluminium e.g. $Zn(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2(g)$ sodium zincate 8. Alkalis react with ammonium salts forming a salt, water and ammonia: ammonium\ alkali $+$ $\rightarrow$ salt + water + ammonia\ salt e.g. $NaOH(aq) + NH_4Cl(aq) \rightarrow NaCl(aq) + H_2O(l) + NH_3(g)$ Ionically: $OH^-(aq) + NH_4^+(aq) \rightarrow H_2O(l) + NH_3(g)$ Insoluble bases react slightly in this way. 9. All bases react with acids forming a salt and water only (see p. 38). ## Recognising acids and alkalis Acids and alkalis are recognised using indicators. An indicator is a substance which has one colour when mixed with an acidic solution and a different colour when mixed with an alkaline solution. | Indicator | Colour in acidic solution | Colour in alkaline solution | | :-------------------- | :------------------------ | :-------------------------- | | Litmus | Red | Blue | | Phenolphthalein | Colourless | Pink | | Methyl orange | Pink/red | Yellow | | Screened methyl orange| Red | Green | | Bromothymol blue | Yellow | Blue | ## The strength of acids and alkalis 1. Strong acids and strong alkalis are fully ionised when they dissolve in water, i.e. their solutions contain a high concentration of $H^+$ ions or $OH^-$ ions, respectively. They are strong electrolytes (see p. 68), e.g. $H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)$ $NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)$ --- 2. Weak acids and weak alkalis are only partially ionised when they dissolve in water, i.e. their solutions contain a low concentration of $H^+$ ions or $OH^-$ ions, respectively. They are weak electrolytes, e.g. $CH_3COOH(aq) \rightleftharpoons CH_3COO^-(aq) + H^+(aq)$ $NH_3(g) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$ | Acids | Strong | Weak | Hemi | | :---------------- | :---------------- | :------------- | :--- | | | Hydrochloric acid HCl | Nitrous acid $HNO_2 $ | | | | Nitric acid $HNO_3$ | Sulphurous acid $H_2SO_3$ | | | | Sulphuric acid $H_2SO_4$ | Carbonic acid $H_2CO_3$ | | | | Phosphoric acid $H_3PO_4$ | Ethanoic acid $CH_3COOH$ | | | Alkalis | Potassium hydroxide KOH | Ammonia solution $NH_3(aq)$| | | | Sodium hydroxide NaOH | Calcium hydroxide $Ca(OH)_2$ | | ## THE pH SCALE The strength of an acid or alkali is measured on the pH scale using universal indicator. A visual representation of the pH scale is shown. The scale ranges from 0 to 14. The colors of the scale change to show increasing acidity and alkalinity: * pH < 7 the color changes from dark red through orange and yellow to green * pH = 7 the color is green * pH > 7 the color changes from Blue, light blue, dark blue to purple ## Amphoteric oxides and hydroxides An amphoteric substance is a substance which can react with both acids and strong alkalis. $strong alkali + amphoteric oxide \rightarrow salt + water$\ $or hydroxide$ $amphoteric oxide + acid \rightarrow salt + water$\ $or hydroxide$ Examples * Aluminium hydroxide reacts with the strong alkali, sodium hydroxide:\ $NaOH(aq) + Al(OH)_3(s) \rightarrow NaAlO_2(aq) + 2H_2O(l)$ sodium aluminate * Aluminium hydroxide reacts with hydrochloric acid:\ $Al(OH)_3(s) + 3HCl(aq) \rightarrow AlCl_3(aq) + 3H_2O(l)$ ```

Acids, Bases and Salts PDF - Chemistry Revision Notes

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