Chemical Reactions

How atoms and molecules react together chemically

Atoms and Molecules, Elements and Compounds

There are about a hundred different types of atoms in the Universe. Substances made up of a single type of atom are called Elements.

Some elements are made up of single atoms:

Helium He
Carbon C
Sodium Na
Sulphur S
Iron Fe

He, Fe, and Na are the Chemical Symbols of the elements.

Some elements are made up of groups of atoms:

Oxygen O2
Ozone O3
Nitrogen N2
Chlorine Cl2
Hydrogen H2

These groups of atoms are called molecules.

Molecules can also be made up of combinations of different types of atoms. These substances are called compounds:

Common Salt NaCl
Water H2O
Methane CH4
Ammonia NH3
Sulphuric Acid H2SO4

O2, CH4 and NH3 are the Chemical Formulas of Oxygen, Methane and Ammonia respectively. CH4 means that a single molecule of methane contains one atom of Carbon and four atoms of Hydrogen. This chemical formula could have been written C1H4 but the 1 is never written. Similarly, a molecule of Ammonia (NH3) contains one atom of Nitrogen and three atoms of Hydrogen.

Chemical Reactions

Chemical reactions occur when different atoms and molecules combine together and spit apart.

For example, if Carbon (C) is burnt in Oxygen (O2) to form Carbon Dioxide, a Chemical Reaction occurs. This reaction can be written:

C + O2 --> CO2

This is called a Chemical Equation. The substances on the left hand side of the equation are called the Reactants. The substances on the right hand side are called the Products.

There is one very important rule with chemical equations:

The number of individual atoms on each side of the equation must be the same.

On the left had side, there is an atom of Carbon and a molecule of Oxygen (containing two atoms). On the right hand side there is a molecule of carbon dioxide (containing one atom of carbon and two atoms of Oxygen). The number of atoms on the left hand side is equal to the number of atoms on the right hand side. All that has changed is the arrangement of the atoms. In a chemical reaction atoms are re-arranged; no atoms are destroyed or created.

In the next example, Hydrogen gas is mixed with Oxygen gas. If the mixture is sparked, it explodes to form water. This chemical reaction can be expressed as:

H2 + O2 --> H2O

On the left had side, there is a molecule of Hydrogen (containing two atoms) and a molecule of Oxygen (also containing two atoms). On the right hand side there is a molecule of water (containing two atoms of Hydrogen and one atom of Oxygen). The left hand side has one extra atom of Oxygen. This is not allowed by the Law of Conservation of Matter. Both sides must contain the same number of atoms.

To make the equation conform, we must balance the equation.

It is not possible to change the chemical formulas of the reactants or products. Water will always be H2O. Balancing the equation is achieved by changing the number of molecules involved. The balanced form of the above equation is:

2H2 + O2 --> 2H2O

Now, on the left had side, there are two molecules of Hydrogen (each containing two atoms making four atoms) and a molecule of Oxygen (containing two atoms). On the right hand side there are two molecule of water (each containing two atoms of Hydrogen and one atom of Oxygen making a total of four atoms of Hydrogen and two of Oxygen). The equation is now balanced.

In summary, when Hydrogen reacts with Oxygen, two molecules of Hydrogen react with one molecule of Oxygen to give two molecules of water.

This reaction gives out a lot of heat when it goes from left to right. It is said to be Exothermic (from two Greek words meaning out and heat). Because of the Law of the Conservation of Energy, if the reaction was made to go from right to left, energy would have to be added. Reactions that require energy are called Endothermic.

The following reaction is between Sulphuric Acid (H2SO4) and Sodium Hydroxide (NaOH) to give Sodium Sulphate (Na2SO4) and water (H20):

H2SO4 + 2NaOH --> Na2SO4 + 2H2O

This equation is balanced. Counting the individual atoms on both sides gives four atoms of Hydrogen, two of Sodium, one of Sulphur and six Oxygen. This is achieved because one molecule of Sulphuric Acid reacts with two molecules of Sodium Hydroxide.

The final reaction is between Nitrogen (N2) and Hydrogen (H2) to give Ammonia (NH3):

N2 + 3H2 --> 2NH3

The equation is balanced since there are 2 Nitrogen atoms and 6 Hydrogen atoms on both sides.

This reaction is different to the previous ones. When Hydrogen and Nitrogen are mixed together under room temperature and pressure, very little happens. When the temperature and pressure are raised, a partial reaction occurs.

The reaction goes in both directions. While the Nitrogen and Hydrogen are combining to form Ammonia, Ammonia splits to form Hydrogen and Nitrogen. A mixture of all three substances results. This type of reaction is called an Equilibrium and is represented by arrows going in both directions.

N2 + 3H2 <--> 2NH3

It is possible to push the reaction in one direction by adding a Catalyst. A catalyst is a substance that helps a reaction without being used up.

If Ammonia is removed from the equilibrium mixture, the reaction will move to produce more Ammonia so that equilibrium is attained.

Relative Atomic and Molecular Mass

Chemical equations tell us how many atoms or molecules react with each other. What is more interesting is how much of a substance is needed to react properly with another.

To answer these types of questions we must use a quantity called Relative Atomic Mass (RAM). In simple terms, this tells us how heavy different atoms are. In actual fact, it tells us their relative masses.

The table below lists Relative Atomic Masses for selected elements.

Hydrogen (H) 1.01
Carbon (C) 12.01
Oxygen (O) 16.00
Nitrogen (N) 14.01
Sodium (Na) 22.99
Sulphur (S) 32.06
Chlorine (Cl) 35.45

This table tells us that a Carbon atom is about 12 times heavier than a Hydrogen atom while an Oxygen atom is 16 times as heavy.

Molecules have a Relative Molecular Mass. This is the sum of the relative atomic masses of its atoms. An Oxygen molecule (made up of two atoms of Oxygen, each with a relative atomic mass of 16) has a relative molecular mass of 32.

Using the table we can put figures to the chemical equations.

As an example, how many grams of Oxygen (O) will be used if 12g of Carbon (C) are burnt to form Carbon Dioxide (CO2)?

Remember the chemical reaction is written:

C + O2 --> CO2

From the table above we can see that Carbon has a relative atomic mass of 12 while the Oxygen molecule has a relative molecular mass of 32. Since a single atom of Carbon reacts with a single molecule of Oxygen, 12g of Carbon will react with 32g of Oxygen.

The relative molecular mass of Carbon Dioxide is 44 (C = 12, O2 = 32; 12 + 32 = 44).

If the question had been how much Oxygen does 1g of Carbon require to burn completely to Carbon Dioxide, the figures can be divided by 12:

12g of C combine with 32g of O2 to give 44g of CO2

Dividing by all figures by 12 gives:

1g of C combines with 2.67g of O2 to give 3.67g of CO2

Note that 2.67 is 32 ÷ 12 and 3.67 is 44 ÷ 12. Also, the reactants always weigh as much as the products. In a chemical reaction matter cannot be created or destroyed.

For the formation of water from Hydrogen and Oxygen we have

2H2 + O2 --> 2H2O

H2 has a relative molecular mass of 2 (1 + 1). 2H2 have a relative molecular mass of 4. O2 has a relative molecular mass of 32. H2O has a relative molecular mass of 18 (1 + 1 + 16). Two molecules of H2O will have a relative molecular mass of 36. Therefore:

4g of H2 combine with 32g of O2 to give 36g of H2O

Any chemical reaction (as long as the equation is balanced) can be analysed in this way.

© KryssTal 2001

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