Chemistry, Third edition

REACTIONS OF ACIDS 93

The reaction of organic bases such as propylamine C 3 H 7 NH 2 with acids also

produces salts:

C 3 H 7 NH 2 (l) + HCl(aq) C 3 H 7 NH 3 · Cl(aq)

(C 3 H 7 NH 3 · Cl is an ionic salt similar to NaCl, and is better represented as

C 3 H 7 NH 3 ,Cl. It dissociates in aqueous solution forming C 3 H 5 NH 3 (aq) and

Cl(aq) ions.)

Organic acids containing long carbon chains are known as ‘fatty acids’. The

neutralization of fatty acids by alkalis produces salts which are soapy to the feel. Fatty

acids are present on our skin, and this explains why alkalis feel soapy. Sodium

stearate (common soap) is produced by neutralising stearic acid with sodium

hydroxide. (For more about soap, see page 175.)

3. Reaction with metals

Solutions of acids react with some metals, making hydrogen gas and a salt. For

example, magnesium ribbon reacts with hydrochloric acid, producing magnesium

chloride and hydrogen gas. The hydrogen gas is produced so quickly that the solu-

tion fizzes:

Mg(s)2HCl(aq)MgCl 2 (aq)H 2 (g)

The ionic equation for this reaction is:

Mg(s)2H(aq)Mg^2 (aq)H 2 (g)

We may test for hydrogen on a smallscale by exploding a hydrogen–air mixture in a

flame (Fig. 6.6):

2H 2 (g) + O 2 (g)2H 2 O(l)

Other metals that react with H(aq) include calcium, zinc, aluminium and iron.

Potassium and sodium react with explosive violence. Copper, silver and gold do not

react.

Reaction of iron

with dilute sulfuric

acid

Write(i)a chemical

equation and (ii)an ionic

equation, for the reaction of

iron with dilute sulfuric acid

to produce iron(II) sulfate

and hydrogen gas.

Exercise 6K

Fig. 6.6Testing for hydrogen gas. An inverted test-tube is used to collect the hydrogen (which
rises because it is less dense than air). After about 20 seconds, the open end of the test-tube is
exposed to a flame. A ‘pop’ confirms the presence of hydrogen.