KEY2CHEM

Descriptions of Change

Chemists describe chemical and physical changes in a variety of ways. Chemical equations convey how reactants are converted into products during a change and represent how a macroscopic observation relates to processes occurring on the microscopic (atomic) level.

Example 1.

Which is a balanced chemical equation for the formation of $$\require{mhchem}\ce{AB3}$$ from $$\require{mhchem}\ce{A2}$$ and $$\require{mhchem}\ce{B2}$$ that corresponds to the diagram drawn below?

A. $$\require{mhchem}\ce{2A + 2B->2AB3}$$

B. $$\require{mhchem}\ce{A2 + B2->2AB3}$$

C. $$\require{mhchem}\ce{2 A2 + 6 B2->4AB3}$$

Solution

C. $$\require{mhchem}\ce{2 A2 + 6B2->4AB3}$$

The diagram shows $$2$$ molecules of $$\require{mhchem}\ce{A2}$$ reacting with $$6$$ molecules of $$\require{mhchem}\ce{B2}$$. This forms a total of $$4$$ $$\require{mhchem}\ce{AB3}$$. Note: the simplest balanced equation for this process (with smallest whole number coefficients) is $$\require{mhchem}\ce{A2 + 3 B2 -> 2 AB3}$$.

Example 2.

Which diagram best describes the melting of solid ice to liquid water?

A.

B.

C.

Solution

A.

In a solid, the particles are close together and fixed in position. In a liquid, the particles are farther apart and have more freedom of motion. This is a physical change, as there is no change in composition (still $$\require{mhchem}\ce{H2O}$$).

Example 3.

Sodium metal reacts with chlorine gas to form solid sodium chloride. Write a balanced chemical equation to describe this change.

A. $$\require{mhchem}\ce{Na(s) + Cl(g) -> NaCl(s)}$$

B. $$\require{mhchem}\ce{2 Na(s) + Cl2(g) -> 2NaCl(s)}$$

C. $$\require{mhchem}\ce{S(s) + Cl2(g) -> SCl2(s)}$$

B. $$\require{mhchem}\ce{2Na(s) + Cl2(g) -> 2NaCl(s)}$$

Sodium is atomic symbol $$\require{mhchem}\ce{Na}$$ and chlorine gas exists as a diatomic molecule ($$\require{mhchem}\ce{Cl2}$$).