Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 42

A chemical equation provides a shorthand method for describing the process taking place in a chemical reaction. The chemical formulae for the reactants are written on the left‐hand side of the equation and the formulae for the products on the right‐hand side. The number of each type of atom on the left‐hand side of the equation must be the same as the number of each type of atom on the right‐hand side. This involves balancing the equation by writing the number of moles of each substance in front of the formula for the substance. These numbers are called stoichiometric coefficients.

The mole is the term used in chemistry to describe a specific amount of a material. It will be explained fully in Chapter 3.

To balance a chemical equation, first write the reactants and products separated by a reaction arrow. For example, in the reaction of hydrogen gas with oxygen gas to form water, the reactants hydrogen and oxygen should be written on the left‐hand side and the product water on the right‐hand side of the reaction arrow. Note that both hydrogen and oxygen exist as molecules whose formulae are H₂ and O₂, respectively. The formula for water is H₂O.

The numbers of atoms of hydrogen are balanced on both sides, but the equation shows 2 atoms of oxygen on the left and only 1 atom on the right. To balance the equation, we therefore need to increase the number of oxygen atoms on the right to 2 by including 2 moles (or units) of H₂O, as shown in red:

However, we now have 4 atoms of hydrogen on the right and only 2 on the left. In order to fully balance the equation, we must increase the number of atoms of hydrogen on the left to 4, as shown:

The equation is now balanced. The total number of atoms on the left of the equation is equal to the number on the right.

As another example, consider the reaction of calcium hydroxide, Ca(OH)₂, with nitric acid, HNO₃. The products in this reaction are calcium nitrate, Ca(NO₃)₂, and water.

Calcium has a charge of +2 and so requires two hydroxide ions of charge −1 for the compound to remain neutral. Calcium also requires two nitrate ions, NO₃⁻, to form a neutral formula unit of calcium nitrate, Ca(NO₃)₂.

Again, write the formulae for the reactants on the left‐hand side of the equation and the products on the right‐hand side, as shown:

In this case, it is simpler to treat the nitrate ion as a unit with formula NO₃⁻ rather than separately balancing nitrogen and oxygen atoms on both sides of the equation. It can be seen that there is one nitrate ion on the left‐hand side but there are two on the right. Therefore we need to add 2 moles of nitric acid to the left‐hand side:

This has balanced the nitrate ions but not the hydrogen and oxygen atoms. There are 4 hydrogen atoms on the left‐hand side but only 2 on the right. There are also 2 oxygen atoms (apart from in the NO₃⁻ ion) on the left‐hand side but only 1 on the right. We therefore need to increase the number of water molecules to 2:

The equation is now balanced.

If a formula unit is written with a number in front of it in a chemical equation, for example, 2Ca(NO₃)₂, this means that there are 2 units or lots of Ca(NO₃)₂ taking part in the reaction. The scientific term for this quantity is a mole. You will meet the mole and its definition in Chapter 3.

The word equation for this reaction is spoken as:

1 mole of calcium hydroxide reacts with 2 moles of nitric acid to give 1 mole of calcium nitrate and 2 moles of water.