Explain why the energy needed to separate different elements within a compound

is an indication of the strength of the bonding within the compound. BOND MODEL ATTRACTIVE ENERGY EXAMPLE

FORCE (kJ/mole) Ionic bond Covalent bond Electrostatic attraction between positive and negative ions Electrostatic attraction between bonding electrons and both nuclei 400– 4000 150– 1100 NaCl HCl + + + + + – – – – + +

Figure 4.7 Chemical bonds and bond energies

76 MODULE 1: The chemical Earth

Simple covalent molecular substances consist of molecules in which the atoms are held together by covalent bonds. The bond energy indicates the strength of the forces holding the atoms together in the molecule. For example, to decompose

an HCl molecule into separate H and Cl atoms requires 431 kJ mol–1 of energy.

These large bond energies explain the observation that chemical change is usually accompanied by the absorption or release of considerable quantities of energy.

Covalent network substances consist of a three-dimensional arrangement of atoms

joined by covalent bonds. For example, in silicon dioxide (Figure 3.9) every silicon atom is covalently bonded to four oxygen atoms and each oxygen atom is covalently

bonded to two silicon atoms. Each Si–O bond has a bond energy of 163 kJ mol–1,

which makes the disruption of the entire lattice very difficult indeed. For this reason silicon dioxide has a very high melting point.

Unit 3.5 describes ionic bonding as the electrostatic attraction of oppositely charged ions within a three-dimensional lattice of positive and negative ions (Figure 3.10). In sodium chloride the lattice consists of Na+ and Cl– ions. The separation of the Na+ and Cl– ions requires a large amount of energy, called the lattice energy, which is a measure of the strength of the ionic bond. The ionic lattice energies for several compounds are shown in Table 4.3.

Table 4.3 Ionic lattice energies for several ionic compounds

Ionic compound _{Ionic lattice energy (kJ mol}–1₎

NaCl 788 KCl 718 MgCl₂ 2523 CaCl₂ 2255 MgO 3800 CaO 3419

For NaCl the energy required to separate the Na+ and Cl– ions is 788 kJ mol–1, which indicates the considerable strength of ionic bonds. Similarly, the quantity

of energy needed to separate Mg2+ and O2– ions in magnesium oxide is 3800 kJ

mol–1. The range of ionic lattice energies is approximately 400–4000 kJ mol–1. When compounds are decomposed into their constituent elements, large quantities of energy are usually required. This is because the decomposition reaction

involves breaking strong covalent or ionic bonds within these substances. On the other hand, changes of state such as liquid water to steam require much smaller quantities of energy. This process only involves breaking weak intermolecular forces between molecules. These intermolecular forces, which will be described

in more detail in Chapter 5, range up to about 40 kJ mol–1 in strength. They are

*Key

points

CHAPTER 4: Chemical change 77

• Chemical changes are those in which new substances with different compositions and properties are formed.

• Chemical changes are often accompanied by the absorption or release of large quantities of energy, generally in the form of heat, light or electricity.

• Chemical changes involve changes in the nature and arrangement of atomic particles so that different combinations of atoms are produced.

• Physical changes are those in which substances undergo changes in physical properties such as volume or density, or changes in state such as changes from solid to liquid or from liquid to gas.

• Physical changes occur without a change in the composition of the particular substance involved.

• Physical changes are often accompanied by the absorption or release of small quantities of energy.

• When water boils, it changes state from liquid to gas; however, the two states

both consist of water (H₂O) molecules.

• When water is electrolysed, it decomposes into two new substances, hydrogen gas (H₂) and oxygen gas (O₂).

• Elements cannot be decomposed by chemical change into simpler substances. • Compounds can be chemically decomposed into elements or simpler compounds

using heat, light or electricity.

• Chemical reactions are represented by chemical equations.

• Chemical equations show the relationships between the numbers of particles of reactants and products in a chemical reaction.

• In balanced chemical equations:

– the numbers of atoms of each element on both sides of the equation are equal – the sums of the electrical charges on each side of the equation are equal – the symbols (g), (l), (s) and (aq) are used to represent gas, liquid, solid and

aqueous solution respectively.

• In balanced chemical equations, the coefficients in front of the formulas are altered so that the number of atoms of each element are equal on both sides of the equation.

• The law of conservation of mass states that in chemical reactions there is no gain or loss in mass.

• The extraction of metals involves separating metals from their ores and involves chemical change.

• Chemical synthesis is the process of forming compounds from elements or other compounds.

• Chemical synthesis produces many of the products, foods and medicines we use today.

78 MODULE 1: The chemical Earth

• Covalent bonds and ionic bonds are strong chemical bonds.

• Large quantities of energy are needed to separate the different elements within a compound—this energy is an indication of the strength of the chemical bonds in compounds.

*ApplicAtion And inVEStiGAtion

1 Classify each of the following as a physical or chemical change: