chemical reactions
1. Signs of a chemical reaction
There are several signs that indicate that a chemical reaction may have taken place:
- Gas is produced
- A permanent colour change occurred
- A solid is formed
- Energy as heat, light, electricity and/or sound is being produced
- Heat is being consumed (causing the temperature to decrease)
- An odour develops
2. Chemical equations
Every second of the day chemica reactions take place, both inside you and around you. In a chemical reactions, one or more substances (the reactants) react and change into one or more new substances (the products).
Over the course of centuries, chemists have developed shorthand ways to describe what happens in a chemical reaction, known as chemical equations.
Over the course of centuries, chemists have developed shorthand ways to describe what happens in a chemical reaction, known as chemical equations.
word equation
There are a variety of ways to write chemical equations. One of them is word equation. In this method, the reactants are written in the left and the products on the right. An arrow pointing to the right separates them. The arrow can be read yields, gives or produces.
reactants --> products
For example, to describe the rusting of iron, you could say: " Iron reacts with oxygen to produce iron(III) oxide (rust)". A quicker way to describe this process is to identify the reactants and the product by means of a word equation:
iron + oxygen --> iron(III) oxide
Note that no capital letter is used to write the name of chemicals. Another example is the word equation used to describe the reaction of methane with oxygen (called combustion) to produce water and carbon dioxide:
methane + oxygen --> water + carbon dioxide
reactants --> products
For example, to describe the rusting of iron, you could say: " Iron reacts with oxygen to produce iron(III) oxide (rust)". A quicker way to describe this process is to identify the reactants and the product by means of a word equation:
iron + oxygen --> iron(III) oxide
Note that no capital letter is used to write the name of chemicals. Another example is the word equation used to describe the reaction of methane with oxygen (called combustion) to produce water and carbon dioxide:
methane + oxygen --> water + carbon dioxide
Rules to write a word equation:
- write the names of the reactants to the left of the arrow separated by '+' signs. If there is only one reactant, then no '+' sign is needed.
- write the name of the products to the right of the arrow, also separated by '+' signs. If there is only one product, as in the reaction above, then no '+' sign is needed.
formula equation
Word equations are adequate to describe a chemical reaction but cumbersome. It is easier to use the chemical formula for the reactants and products instead, to write a chemical equation. A chemical equation is a representation of a chemical reaction. The formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right).
Here is as an example the chemical equation for rusting:
Fe + O2 --> Fe2O3
Equations that just show the formulas of the reactants and products are called formula equations (or skeleton equations in some textbooks). IT is very common to add more information on the physical state of the reactants and products by adding a symbol after each formula. Use (s) for a solid, (l) for a liquid, (g) for a gas.
Here is as an example the chemical equation for rusting:
Fe + O2 --> Fe2O3
Equations that just show the formulas of the reactants and products are called formula equations (or skeleton equations in some textbooks). IT is very common to add more information on the physical state of the reactants and products by adding a symbol after each formula. Use (s) for a solid, (l) for a liquid, (g) for a gas.
For example the the chemical equation for rusting is more accurately described as:
Fe(s) + O2(g) --> Fe2O3(s)
indicating that elemental iron and iron oxide occur as solid and oxygen as a gas, in normal conditions.
Fe(s) + O2(g) --> Fe2O3(s)
indicating that elemental iron and iron oxide occur as solid and oxygen as a gas, in normal conditions.
However, formula equations do not indicate the relative amount of the reactants and products. Balanced reactions are used to provide information on the relative amount of reactants and products, which is necessary to make calculations and predictions on the amount of reactants needed for a reaction or the amount of products that can be expected. The steps to balance chemical equation will be learned in Year 9.