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GCSE Chemistry - Acids

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Acids - Introduction

Acids are a group of compounds that exhibit very specific properties. They all have at least one hydrogen atom as part of the molecule. It is this hydrogen atom that takes a major role in its reactions.

The strength of an acid is given by its pH value. This stands for per hydrogen a Scandinavian term meaning strength of hydrogen. The more hydrogen ions that an acid gives out the lower the pH value so acids have pH values of -1 to 7.

Acids react with metals, such as magnesium and zinc, with carbonate compounds and with bases, such as oxides and hydroxides.

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Acids - Reaction with Metals

Experimental sheet for the reaction of metals with dilute hydrochloric acid.

The reaction of metals with acids is much more vigorous than the corresponding reaction with water. Just as with the reaction with water, hydrogen gas is released. A salt, i.e. a combination of two ions, is also formed; the particular salt formed depends on the acid used in the reaction.

The three common laboratory acids are hydrochloric acid (HCl), nitric acid (HNO3) and sulphuric acid (H2SO4). The salts formed by these acids are chloride, nitrate and sulphate respectively. During the reaction the hydrogen atoms are replaced by metal ions to form the salt.

General word equations -

metal + hydrochloric acid → metal chloride + hydrogen gas

metal + nitric acid → metal nitrate + hydrogen gas

metal + sulphuric acid → metal sulphate + hydrogen gas

Exemplar formulae equations -

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Zn(s) + 2HNO3(aq) → Zn(NO3)2(aq) + H2(g)

2Na(s) + H2SO4(aq) → Na2SO4(aq) + H2(g)

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Acids - Reaction with Carbonates

The reaction of carbonates with acids follows a very similar pattern to their reaction with metals. The same salt is produced as with the reaction with a metal; however, instead of hydrogen gas being evolved though water and carbon dioxide gas are formed.

The carbon dioxide gas can be tested for by bubbling the gas produced through limewater, a dilute solution of calcium hydroxide. If the limewater turns milky, i.e. produces a white precipitate, then carbon dioxide is produced.

General word equations -

metal carbonate + hydrochloric acid → metal chloride + water + carbon dioxide

metal carbonate + nitric acid → metal nitrate + water + carbon dioxide

metal carbonate + sulphuric acid → metal sulphate + water + carbon dioxide

Exemplar formulae equations -

MgCO3(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) + CO2(g)

ZnCO3(s) + 2HNO3(aq) → Zn(NO3)2(aq) + H2O(l) + CO2(g)

Na2CO3(s) + H2SO4(aq) → Na2SO4(aq) + H2O(l) + CO2(g)

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Acids - Reaction with Bases

Bases are compounds that react with acids to produce a salt and water only. Both oxide and hydroxide compounds can be classified as bases, though not all possible oxides and hydroxides will react with acids. For example iron(III) oxide (rust) does not react with acids.

No gases are produced in these reactions and so the only way to see the progress of any reaction is to notice any solid dissolving into the acid.

General word equations -

metal oxide + hydrochloric acid → metal chloride + water

metal oxide + nitric acid → metal nitrate + water

metal oxide + sulphuric acid → metal sulphate + water

metal hydroxide + hydrochloric acid → metal chloride + water

metal hydroxide + nitric acid → metal nitrate + water

metal hydroxide + sulphuric acid → metal sulphate + water

Exemplar formulae equations -

MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)

2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)

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Acids - Reaction Rates

Experimental sheet for measuring the rate of reaction between HCl and CaCO3.

The speed at which a chemical reaction occurs is known as its rate. Some chemical reactions occur very slowly, such as the chemical weathering of rocks and the rusting of iron. Some reactions occur very quickly, such as the reaction of metals with acids and chemical explosives, such as TNT.

The rate of a reaction can be followed by measuring either the rate of formation of one of the products or the rate of consumption of one of the reactants.

Kinetic theory -

The arrangement of particles in solids, liquids and gases has a direct affect on how fast a reaction will occur.

solid liquid gas

In the diagrams above it can be seen that in solid the particles are extremely close to one another and movement is extremely restricted, only molecular vibration is permitted. This leads to very slow reactions but good thermal conductivity.

In a liquid the particles have more freedom, however there are still attractions between particles keeping them close to one another.

In a gas the particles are spread apart with very little, if any, attraction between particles. Movement is unrestricted and so reactions involving gases generally occur very quickly.

Collision theory -

One of the most important parts of a chemical reaction is when the reactant molecules collide with each other. Without this initial collision a chemical reaction will not occur. It must be noted that as well as a collision the molecules must have enough energy for the reaction to actually occur, so not all collisions will result in a positive reaction and the formation of a product.

Any change in reaction conditions that affects this efficiency of the collision between molecules will directly affect the rate. For example, if the concentration of reactants, i.e. the number of molecules in a given volume, is increased there will be more collisions and therefore the rate will increase. With a reaction between gases an increase in pressure will force more molecules into a smaller volume, therefore the number of collisions and the rate will increase.

With a solid reactant the amount of its surface that is available is a very important factor. If the solid is finely powdered its surface area will be very high and therefore more molecules can collide with each other, and the rate of reaction will increase.

The final major factor that affects the rate of reaction is the temperature used. The higher the temperature a reaction is conducted at the more energy the molecules will have and the faster the molecules will move. Therefore, there will be more frequent collisions and more effective collisions and the rate will increase.

Rate graphs -

By finding the concentration of a reactant or a product at particular times during a reaction graphs of the data can be plotted. They should take the form of either,

for the loss of a reactant for the formation of a product

The rate of a reaction is simply the absolute value of the gradient of these lines.

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

updated : Sunday, 15th July, 2012

mail to: chemistryrules

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