At this point we know how to determine the number of electrons, protons and neutrons in an isotope of an element. We also know that the number below the symbol of the element in the periodic table is called the relative, weighted average atomic mass of the element. The number has units of atomic mass units or Daltons.

We also know the symbols and formulas for the first twenty elements and some additional 'common' elements.

What about when atoms combine to form compounds? First of all we can write formulas of compounds. For example, water is a compound because it is composed of two diferent elements, hydrogen and oxygen. The formula for water is H₂O. The formula of another compound everyone is familiar with is sodium chloride, NaCl. How do we make these two compounds?

To make these two compounds we need to do a chemical reaction. We can do a chemical reaction by mixing substances.

How do we tell if a chemical reaction occurs? The best evidence is a difference in the physical and chemical properties of the reactants and the products. Characteristics to watch for when mixing two or more substances to know whether a chemical reaction occurs is;

• heat and light (flames)

• explosion

• color change

• evolution of a gas

• formation of a solid (precipitate)

Lets look at two reactions; the reaction between hydrogen (H₂) and oxygen (O₂) and the reaction between sodium (Na) and chlorine (Cl₂). To describe these reactions chemists use a symbolic representation, called a chemical equation. The reaction between hydrogen (H₂) and oxygen (O₂) can be described using the equation,

H₂(g) + O₂(g) --heat--> H₂O(g)

In the reaction between hydrogen and oxygen a balloon was filled with a 50/50 mixture of the two gases. Notice that the mixture appears to be stable. But if we bring a lighted match close to the balloon and touch it...blam!! The mixture reacts, rapidly and explosively! The product of the reaction is good old water-H₂O.

The reaction between sodium (Na) and chlorine (Cl₂) can be described using the equation,

Na(s) + Cl₂(g) --heat--> NaCl(s)

In the reaction between sodium and chlorine gas, we heated a sample of sodium in a deflagrating spoon and then placed the sodium into a flask containing chlorine gas. It took a moment for the reaction to get started but once it did, we could see a bring light, and a white cloud of sodium chloride was formed. A cloud is simply a fine dispersion of solid particles in a gas. The fine dispersion of solid is the solid chloride, and the gas is air.

Both of these two reactions were very violent, explosive, loud, and intensely bright. Yet the products are just sodium chloride and water. Stuff we depend on to live and we all eat everyday. That is amazing chemistry, hazardous elements combined together to form life giving compounds!

The form of a chemical equation involves writing the formulas of the reactants (the substances that are mixed together) on the left, using '+' when more than one substance is involved and the formula(s) of the product(s) on the right. The reactants and the products are separated by an arrow '--->'. Sometimes additional information about the reaction is placed above or below the arrow which separates the reactants and products. Such information include;

• heat (using a delta symbol)

• temperature (at which the reaction is run at)

• pressure

• time (length of time the reaction is allowed to proceed)

Chemists alway try to be sure the chemical equation is balanced.

There are some simple rules for balancing equations:

• Things not to do when balancing an equation:

  1. Change a subscripted number

  2. Add or remove an element arbitrarily

  3. Add or remove a compound arbitrarily

• Things to do when balancing an equation:

  1. Change only the coefficients preceding an element or compound.

  2. Try to balance non-oxygen and non-hydrogen elements first.

  3. Balance substances in their elemental form last.

So to balance the first equation between hydrogen and oxygen,

H₂(g) + O₂(g) --heat--> H₂O(g)

we want to be sure equal numbers of hydrogen and oxygen atoms are on both sides. Notice on the products (right side) there is only one oxygen atom, but on the reactants (left side) there are two oxygen atoms. To balance the equation we use a coefficient preceeding the water.

H₂(g) + O₂(g) --heat--> 2H₂O(g)

When we change the coefficient for water we change the number of hydrogen atoms. So the products now have four hydrogen atoms. To get four hydrogen atoms on the reactants side we change the coefficient from one to two.

2H₂(g) + O₂(g) --heat--> 2H₂O(g)

To balance the reaction between sodium (Na) and chlorine (Cl₂)

Na(s) + Cl₂(g) --heat--> NaCl(s)

we change the coefficient preceeding NaCl to a two and also preceeding Na.

2Na(s) + Cl₂(g) --heat--> 2NaCl(s)

You should practice writing and balancing chemical equations by doing Question 27 in the textbook on page 81.

Write a chemical equation which describes how to prepare each of the following compounds from their elements; CaO, MgCl₂, Al₂O₃, and SO₂.

The two reactions we observed in class (shown above) are examples of formation or synthesis reactions. We mixed two elements and produced a new compound.

Another interesting reaction is a combustion reaction. One that we are all familiar with is the combustion of methane (natural gas). Combustion reactions are characterized by a compound reacting with oxygen in the atmosphere. Anytime we use a gas stove to heat or cook we are depending on the combustion of methane to produce heat. How do we write an equation to describe the combustion of methane? First we'll begin with the reactants....methane and oxygen.

CH₄(g) + O₂(g) --heat-->

What are the products? In this case, we are combusing a hydrocarbon, the products are always carbon dioxide and water.

CH₄(g) + O₂(g) --heat--> CO₂(g) + H₂O(g)

Now comes the challenge...can you balance this equation? (we'll do this in class)

Can you write the balanced combustion equation for propane? The chemical formula for propane is C₃H₈(g).