Chemistry, Third edition

WRITING AND BALANCING EQUATIONS 27

Example 2.6

The equation

AgNO 3 (aq)NaCl(aq)AgCl(s)NaNO 3 (aq)

shows that silver nitrate and sodium chloride solutions react together to form solid

silver chloride and sodium nitrate solution. It is important to include the state

symbols in this equation, because solid silver nitrate and solid sodium chloride do

not react– they must first be dissolved in water or no reaction takes place.

BOX 2.6

John Dalton

John Dalton (1766–1844) was the son of

a poor Quaker couple. He spent most of

his life in Manchester teaching and

researching. Although the Greeks, as

early as 400 BC, believed that matter

consisted of small particles which they

called atoms, the theory was revived

and elaborated on by Dalton. In 1808 he

presented his atomic theoryin which he

stated:

●Elements are made up of particles

called atoms, which are indestructible.

●The atoms of an element are all the

same, but different from those of other

elements. (We now know this is not

quite true because of the existence of

isotopes (Chapter 3).)

●‘Compound atoms’ (molecules as we

know them) are formed by small, whole-

number combinations of atoms.

●A chemical change involves a new

arrangement of atoms.

The theory, with a few important

modifications, still holds true today. To

help him in his work, Dalton used balls

with different colours for atoms. For

example he used one black ball (carbon)

and one white ball (oxygen) to represent

CO, or carbon monoxide. Although

Dalton proposed symbols for the elements

(see Fig. 2.6), the symbols he proposed

were found to be difficult to use.

It was left to Berzelius of Sweden to

propose that each element be given a

letter symbol and that this should

represent an atomof the element. He

proposed that all elements be given a

symbol corresponding to the first letter

of their names. Any repetitions were

avoided by giving some symbols a

second letter from the name. This is the

system we use today.

Fig. 2.6Early symbols for the elements

as proposed by Dalton.

Equations using state symbols

Write balanced equations (including state symbols) for the following reactions:

(i) Solid calcium carbonate decomposes on heating into solid calcium oxide and carbon dioxide

gas.

(ii) Solutions of lead(II) nitrate and sodium iodide react together to form solid lead(II) iodide and

sodium nitrate solution.

(iii)When chlorine gas is passed over heated aluminium, solid aluminium chloride is formed.

(iv)North Sea gas chiefly consists of a compound called methane(CH 4 ). Methane burns in

oxygen gas to form carbon dioxide and steam.

Exercise 2L