Page 24 - Focus SPM KSSM F4 2020 - Chemistry

Chemistry Form 4 Chapter 3 The Mole Concept, Chemical Formula and Equation

3.4 Chemical Equations

1. A chemical equation is a precise description of a chemical reaction.
2. A chemical equation can be written in words or chemical formulae. Using chemical formulae is easier,
faster and accurate.

Starting substances are called New substances formed are
reactants and are written 2H (g) + O (g) → 2H O(l) called products and are
on the left-hand side of an 2 2 2 written on the right-hand side
equation. of an equation.
Figure 3.16 An example of chemical equation
3. According to the law of conservation of mass, a matter can neither be created nor destroyed. So, the
numbers of atoms before and after a chemical reaction are the same. Therefore, a chemical equation
must be balanced.

Chapter
3 Example: Hydrogen + Oxygen → Water Chemical equation
before being
balanced Balancing of
Products chemical
Reactants equation
Chemical equation
after being VIDEO
balanced
2H 2 (g) + O 2 (g) → 2H 2 O(l)

Figure 3.17 Chemical equation must be balanced

4. Table 3.12 shows the steps in constructing a chemical equation.

Table 3.12 Constructing chemical equation
Reaction: Iron filings react with copper(III) chloride solution to form iron(III) oxide and copper metal.
Step Example and explanation
① Identify the reactants, products and their Reactants : Iron, Fe dan copper(II) chloride, CuCl
2
formulae. Products: Iron(III) chloride, FeCl dan copper, Cu
3
② Write the main part of the equation. Fe + CuCl → FeCl + Cu
2 3
③ Check the number of atoms of each Left-hand side: Atom Fe = 1 Right-hand side: Atom Fe = 1
element on both sides of the equation. Atom Cu = 1 Atom Cu = 1
Atom Cl = 2 Atom Cl = 3
The numbers of atoms are not balanced.
④ Balance the equation by adjusting the Balancing the number of Cl atoms: Fe + 3CuCl → 2FeCl + Cu
3
2
coefficients in front of the formulae. Balancing the number of Fe atoms: 2Fe + 3CuCl → 2FeCl + Cu
3
2
Balancing the number of Cu atoms: 2Fe + 3CuCl → 2FeCl + 3Cu
2 3
⑤ Check whether equation is now 2Fe + 3CuCl → 2FeCl + 3Cu
3
2
balanced. Left-hand side: Atom Fe = 2 Right-hand side: Atom Fe = 2
Atom Cu = 3 Atom Cu = 3
Atom Cl = 6 Atom Cl = 6
The numbers of atoms are balanced.
⑥ Write the state symbol of each 2Fe(s) + 3CuCl (aq) → 2FeCl (aq) + 3Cu(s)
substance. 2 3

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