Compounds and Chemical Equations

From elements to compounds

An element contains only one type of atom. During a chemical reaction, atoms from different elements can be rearranged and chemically joined to produce a compound. The product is a new substance, so its properties can be very different from those of the elements from which it formed.

For example, magnesium is a metal and oxygen is a gas, but they react to form solid magnesium oxide:

$\text{magnesium} + \text{oxygen} \rightarrow \text{magnesium oxide}$

Atoms are rearranged during a reaction; they are not created or destroyed. Bonds are broken and formed, so energy is transferred between the reacting substances and the surroundings. Chemical reactions often involve a detectable energy change, such as a temperature change or the production of light. Burning magnesium releases energy as intense light and heat, but not every reaction has an energy change that is easy to detect.

Fixed proportions and chemical formulae

A compound contains two or more different elements chemically combined in fixed proportions. Magnesium oxide has the formula $\mathrm{MgO}$, showing a simplest ratio of one magnesium ion to one oxide ion. Water is always $\mathrm{H_2O}$, with two hydrogen atoms for every oxygen atom.

A formula uses element symbols and small lower numbers called subscripts. In $\mathrm{CO_2}$, the subscript 2 shows that there are two oxygen atoms for each carbon atom. If no subscript is shown, the number is understood to be 1.

Formulae can also be used to name compounds:

• $\mathrm{NaCl}$ is sodium chloride;
• $\mathrm{MgO}$ is magnesium oxide;
• $\mathrm{CO_2}$ is carbon dioxide.

In a simple compound made from a metal and a non-metal, the metal is usually named first and the non-metal's name ends in -ide. The correct formula must be retained because changing a subscript changes the identity or fixed composition of the substance.

Because the elements in a compound are chemically combined, physical separation methods cannot separate them. Breaking a compound down into its elements requires one or more chemical reactions.

Word equations describe the substances

A word equation places the reactants on the left and the products on the right:

$\text{reactants} \rightarrow \text{products}$

For magnesium burning in oxygen:

$\text{magnesium} + \text{oxygen} \rightarrow \text{magnesium oxide}$

The plus sign means “reacts with”, while the arrow means “forms” or “produces”. A word equation identifies the substances but does not show their formulae or the relative numbers of particles involved.

Symbol equations conserve atoms

Replacing the names with the correct formulae first gives:

$\mathrm{Mg + O_2 \rightarrow MgO}$

This is not balanced: there are two oxygen atoms on the left but only one on the right. Place whole-number coefficients before the formulae:

$\mathrm{2Mg + O_2 \rightarrow 2MgO}$

There are now two magnesium atoms and two oxygen atoms on each side. This represents conservation of atoms and, therefore, conservation of mass in a closed system.

A coefficient multiplies the entire formula. Thus $2\mathrm{MgO}$ represents two magnesium oxide formula units, containing two Mg atoms and two O atoms altogether. Changing $\mathrm{MgO}$ to $\mathrm{MgO_2}$ would not balance the original reaction: it would invent a different formula.

Use this sequence when balancing:

1. write the correct formula for every reactant and product;
2. count the atoms of each element on both sides;
3. place coefficients before formulae until every count matches;
4. reduce the coefficients to the smallest possible whole-number ratio;
5. recount every element.

Where the physical states are known, they can be shown using $(s)$ for solid, $(l)$ for liquid, $(g)$ for gas and $(aq)$ for a substance dissolved in water:

$\mathrm{2Mg(s) + O_2(g) \rightarrow 2MgO(s)}$

Higher Tier: ionic equations show the particles that react

Dissolved ionic compounds contain separate, mobile ions. A complete ionic equation shows those dissolved ions individually. Ions that appear unchanged on both sides are spectator ions; cancelling them produces the ionic equation for the actual chemical change.

For example, mixing aqueous calcium chloride and silver nitrate forms a silver chloride precipitate:

$\mathrm{CaCl_2(aq) + 2AgNO_3(aq) \rightarrow Ca(NO_3)_2(aq) + 2AgCl(s)}$

When the aqueous ionic substances are split into ions, $\mathrm{Ca^{2+}}$ and $\mathrm{NO_3^-}$ occur unchanged on both sides. They are spectators. Cancelling them and simplifying the coefficients gives:

$\mathrm{Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)}$

The equation conserves atoms and charge: the total charge is zero on each side. State symbols matter because aqueous ions are separated, whereas solid silver chloride remains together.

Higher Tier: half equations show electron transfer

A half equation represents either electron gain or electron loss. During the electrolysis of molten lead bromide, lead ions gain electrons at the negative electrode:

$\mathrm{Pb^{2+} + 2e^- \rightarrow Pb}$

The atoms and total charge balance: the left-hand charge is $+2-2=0$, matching the neutral lead atom. Gaining electrons is reduction.

Bromide ions lose electrons at the positive electrode:

$\mathrm{2Br^- \rightarrow Br_2 + 2e^-}$

The two bromine atoms balance and both sides have an overall charge of $-2$. Losing electrons is oxidation. The electrons are written on opposite sides in the two half equations; when suitable half equations are combined, equal numbers of electrons cancel.

Use each representation for a different purpose: an overall symbol equation shows the reacting substances, an ionic equation shows only the particles that change, and a half equation isolates electron gain or loss.