Chemical Reactions

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Chemical Changes

When water boils, the steam that rises into the air is still composed of water, but it is now in the form of a gas. It can change back to liquid again on a cold mirror or window. This is a change of state. But when you burn wood on a fire, it won't change back to wood... so what's going on?

Sometimes substances change in ways that are difficult, if not impossible to reverse. These are chemical changes, and they involve the formation of new substances.

For example, the black bits on burnt toast are carbon left behind after other substances in the bread have been driven off by heat. The black bits won't change back into bread as the toast cools.

The change is called a chemical reaction and the new substance(s) has different physical and chemical properties from the old substance(s).

In the chemical reaction shown, baking soda is mixed into vinegar and a frothy new substance is made. On testing, the new substance turns out to be the gas carbon dioxide. So when the white powder is mixed into a brown liquid a chemical reaction occurs resultling in a clear gas.

A chemical reaction involves combining or breaking down substances to produce new chemicals. The chemicals you start out with are called the reactants (e.g. the vinegar and baking soda. The chemicals you finish up with are called the products (e.g. the carbon dioxide gas).

A chemical reaction involves changing reactants into products.

Elements react differently, depending on their position in the periodic table. Some elements explode in air, some go green, while others do not change at all over many years.

Elements in group 1 are very reactive metals, while those in group 17 are very reactive non-metals. This is because the elements in both of these groups have only to lose or gain one electron to have full, stable valence electron shells. Elements with half-filled valence shells, like carbon, are less reactive, while elements in group 18 with full shells of electrons are unreactive.

Reaction Evidence

It is not always easy to decide whether a change is a chemical reaction. We can observe reaction evidence to help decide if a chemical change has occurred.

If you observe two or more of the following pieces of evidence, then the change is likely to be a chemical reaction.

Chemical Properties

The chemical properties of a substance describe how a particular substance reacts and changes when it is with other substances.

Common chemical properties that chemists consider for a substance are shown in the following table, with an example for each.

Chemists are interested in the strength of the reaction (reactivity) and the products formed. Chemical properties can be compared to guage the reactivity of substances. For example, magnesium metal reacts more quickly and vigorously than iron metal does, so magnesium is more reactive than iron.

Recording Reactions as Chemical Equations

During chemical reactions, all the substances that we start with are reactants. The new substances that formed during the reaction are products. Scientists use word and symbol equations to describe and summarise what happens in a reaction.

The reactants, or substances that take part in the reaction, are written on the left.
An arrow sign '→' to show that a chemical reaction occurs. The arrow summarises the statement: 'The reactant substances on the left have reacted together to form the product substances on the right.'
The products, or substances produced in the reaction, are written on the right.
A plus sign '+' to take the place of 'and'. The plus sign shows which substances are mixed together as reactants, and which are present as products.

In a word equation, the names for all the substances are written out in full. In a chemical equation, chemical sumbols (from the periodic table) or formulae of each substance are used instead of their names.

For example, inside the space shuttle's main engine, hydrogen and oxygen gases react in a controlled explosion called combustion. The product is very hot steam. (A different reaction occurs inside the booster rockets.)

The rapidly expanding hot gas is forced out of the back of the rocket, propelling the shuttle upwards.

Example: Coal

Coal is made of carbon. When a piece of coal burns in oxygen gas, carbon dioxide is formed. The reactants are carbon (C) and oxygen (O₂). The product is carbon dioxide (CO₂), so:

Word equation:

carbon + oxygen → carbon dioxide

Chemical equation:

C + O2 → CO2

3D model of carbon dioxide gas (CO₂). To change the size, scroll up or down. Grey = carbon atom; red = oxygen atom.

Example: Iron

Iron metal (Fe) reacts with sulfur (S), to form iron sulfide (FeS), so:

Word equation:

iron + sulfur → iron sulfide

Chemical equation:

Fe + S → FeS

3D model of iron sulfide (FeS). To change the size, scroll up or down. Orange = iron atom; yellow = sulfur atom.

Describing Chemical Changes

Magnesium ribbon was heated in a flame till it ignited, then it was plunged into a gas jar of oxygen. There are several ways of describing a chemical reaction, each designed to give different information.

Here are four ways of describing the reaction:

1) Visual Description

Simple labelled drawings can show must aspects of a chemical reaction.

2) Written Description

The burning silvery-grey magnesium ribbon flared up when placed in oxygen gas to give an intense white light with dense white smoke. Some of the smoke formed a white powder afterwards. A white solid was left on the tips of the tongs.

3) Word Equation

Summarising our observations:

Magnesium heated in oxygen gas ignited producing a dense white smoke and much heat and light.

We know that the white smoke is magnesium oxide, so the basic word equation can be written as:

magnesium + oxygen → magnesium oxide

Although heat and light are forms of energy rather than substances, they need to be included as 'products' of the reaction.

magnesium + oxygen → magnesium oxide + heat + light

4) Chemical Equation

If you know the formulas involved, you can write out a chemical equation using the chemical symbols. Mg is the formula for magnesium, O is for oxygen gas, and MgO for magnesium oxide.

Mg + O2 → MgO + heat + light

Notice that the umber of atoms on each side is not balanced. This is a more complex skill.

Compounds are Made from Chemical Reactions

Chemical changes - from a marble statue slowly dissolving in acid rain, through magnesium bruning in air, to awesome power of a rocket motor - involve elements reacting. The atoms gain and lose electrons, and combine together into compounds. In compounds, elements are bonded together.

The compounds have completely different properties to the elements that made them up.

The elements in a compound combine in fixed amounts. This makes it possible to write a formula for a molecule - an individual particle - of a compound. For example:

Water (H₂O) is an unreactive liquid made of:

two parts hydrogen (explosive gas) and
one part oxygen (reactive gas)

Carbon dioxide (CO₂) is an unreactive gas made of:

one part carbon (black powder solid) and
two parts oxygen (reactive gas)

All types of food contain the element carbon - a black powder - but the carbon doesn't show because it is in compounds with other elements. When food is overheated, some of these compounds break down, and carbon is left. The food looks black, we say it is burnt.

Sugar has the formula C₁₂H₂₂O₁₁. Each molecule of sugar has:

12 atoms of carbon
22 atoms of hydrogen
11 atoms of oxygen

Sugar is called a carbohydrate because, in effect, it is a combination of carbon atoms and water (two hydrogen atoms for each oxygen atom). Two gases and a black powder (carbon) have combined to make a sweet, white, crystalline solid compound!