Additional Science-Chemistry Chapter 7

GCSE AQA Additional Science-Chemistry Chapter 7

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  • Created by: Mel
  • Created on: 24-05-12 10:32

Acids and Alkalis

When we dissolve a substance in water we make an aqueous solution.

  • Pure water is neutral and has a pH value of 7.
  • Acids are substances that produce hydrogen ions, H+ (aq), when they are added to water. This makes the solution acidic and it has a pH value of less than 7.
  • Bases react with acids and neutralize them. Alkalis are bases that dissolve in water to make the solution alkaline. They produce hydroxide ions, OH- (aq), in the solution. Alkaline solutions have a pH value greater than 7.
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Acids and Alkalis

Acids include chemicals like citric acid, sulfuric acid and ethanoic acid. All acids taste very sour, although many acids are far too dangerous to put in the mouth. Ethanoic acid (vinegar) and citric acid (the sour taste in citric fruit, fizzy drinks and squashes) are acids which we regularly eat.

One acid that is used in the laboratory is hydrochloric acid which is formed when the gas hydrogen chloride (HCl) dissolves in water:

HCl (g) (+ water) ---> H+ (aq) + Cl- (aq)

Bases are the opposite of acids in the way they react. Because alkalis are bases which dissolve in water they are the bases which we use most commonly. E.g. Sodium hydroxide solution is formed when we dissolve solid sodium hydroxide in water:

NaOH (s) (+ water) ---> Na+ (aq) + OH- (aq)

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Universal Indicator

The pH scale has values from 0 to 14. Solutions that are very acidic have low pH values between 0 and 2, and solutions that are very alkaline have high pH values of 12 to 14.

(http://www.btinternet.com/~mr.larsen/images/pHScale.gif)

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Making Salts from Metals or Bases

Salts are formed when acids react with metals or bases. Acids will react with metals that are above hydrogen in the reactivity series. These metals react with acids to form hydrogen gas and a salt.

ACID + METAL --> SALT + HYDROGEN

Bases are metal oxides or metal hydroxides. They react with acids to form a salt and water.

ACID + BASE --> SALT + WATER

A metal, or a base that is insoluble in water, is added a little at a time to the acid until all of the acid has reacted. The mixture is then filtered to remove the excess solid, leaving a solution of the salt. The solid salt is made when water is evaporated from the solution so that it crystallizes.

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Making Salts from Metals or Bases (cont.)

The salt made depends on the metal or the base that is used in the reaction and the acid. So bases that contain sodium ions will always make sodium salts, while those that contain potassium ions will always make potassium salts.

Bases are metal oxides or metal hydroxides. They react with acids to form a salt and water. The oxide of a transition metal, such as iron(III) oxide, is an example of a base that we can use to make a salt in this way:

Acid + Base --> Salt + Water

6HCl (aq) + Fe2O3 (s) --> 2FeCl3 (aq) + 3H2O (l)

Hydrochloric Acid + Solid Iron(III) oxide --> Iron (III) Chloride + Water Solution

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Making Salts from Solutions

Soluble salts can be made by reacting an acid and an alkali.

ACID + ALKALI --> SALT + WATER

Reaction between acid and alkali can be summarized by showing ions that react:

H+ (aq) + OH- (aq) --> H2O (l)

(from acid) (from alkali)

However, there is no visible change when the solutions react so we need an indicator to show when the reaction is complete.

Alternatively, the volumes of acid and alkali needed to produce the salt are found and these volumes of fresh solutions are mixed. The pure salt is obtained by crystallization.

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Forming Salts

A metal, or a base that is insoluble in water, is added a little at a time to the acid until all of the acid has reacted. The mixture is then filtered to remove the excess solid, leaving a solution of the salt. The solid salt is made when water is evaporated from the solution so that it crystallizes.

Ammonia solution is an alkali that does not contain a metal. It forms ammonium salts, such as ammonium nitrate, which are used as fertilisers.

Insoluble salts can be made by mixing solutions of soluble salts that contain the ions needed. E.g., we can make lead iodide by mixing solutions of lead nitrate and potassium iodide. The lead iodide forms a precipitate that can be filtered and dried.

Some pollutants can be removed from water as precipitates by adding ions that react with them to form insoluble salts.

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