A chemical equation is a symbolic representation of all of the substances involved in a chemical reaction. We use the chemical formulas of substance to represent each chemical specie involved in the reaction. We also use the notation (g), (l), (s), or (aq) following the chemical formula to identify the phase of the substances in the equation.

As an example of a chemical reaction we watched the reaction between iron, Fe(s), and sulfur, S₈(s), on video. There are some additional reactions on our Web site that you can review. We noted the physical properties of both iron and sulfur before the reaction and the physical property of the product of the reaction. The magnetic property of iron, present when iron was in its elemental form, was absent in the product. The yellow color of the sulfur, present in the elemental form as a reactant was absent in the product. Yet both iron and sulfur are in the product. Since the properties changed the product is a new substance.

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. This can be obvious as in the case of the reaction between iron and sulfur. Other characteristics to watch for when mixing two or more substances;

• heat and light (flames)

• explosion

• color change

• evolution of a gas

• formation of a solid (precipitate)

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)

Well, how about writing the equation for the reaction between iron and sulfur? To do that we have to know the formula for both elements and their phases;

Fe(s) + S₈(s) --heat-->

(Note: I can not use a delta symbol on the WEB easily). Now we have to know what the products are. This is a little more difficult but in this case is it easy. We are reacting a metal with a nonmetal so we know the product is an ionic compound. Since there are only two elements reacting the product has to be binary. So we need the formula for a binary ionic compound. You know how to do that. Metals form cations and the nonmetal forms an anion and we need to use the principle of electroneutrality to balance the charges. Our only other problem is that iron is a transition metal and as such it can have several different charges. For our purposes we limit the possibilities to 2+ or 3+. If we use 2+ for iron and we know sulfur is always 2-, the formula of the product would have to be, FeS(s). We know it is a solid because we saw on the video it was solid, but also ALL ionic compounds are solids!

So the reaction is,

Fe(s) + S₈(s) --heat--> FeS(s)

Another reaction (side reaction) which occurred as the above reaction occurred was between sulfur and oxygen in the air. This side reaction was evident because of the bluish flame that appeared as the sulfur melted. At one point in the video as we looked the crucible, containing the reaction, from the side the blue flame was erupting from the crucible. The reaction was surpressed when the hand placed the crucible lid onto the crucible. But what was the reaction?

S₈(s) + O₂(g) --heat-->

This is a harder question because we are combining two nonmetals. The product of this reaction we know will be a binary covalent compound. But we do not know what the formula of the product might be because we have not discussed any rules for determining formulas of covalent compounds. So what do we do? Well, unfortunately, we have to know some formulas of binary compounds containing sulfur and oxygen. You know sulfate, SO₄^2-, and sulfite, SO₃^2- are either of these likely? No! And the reason is the charged nature of those species. We will not form a charged substance in this particular case. What else you ask? How about SO? Is that possible? Well, know it is not. If we try to find such a compound in the Merck Index or the CRC we will not. So how about SO₂? In this case you would be right on! That is the correct formula. Sulfur dioxide is an evil smelling, colorless, gas. So the reaction is;

S₈(s) + O₂(g) --heat--> SO₂(g)

These two reactions we have discussed are called formation reactions. (Our textbook also refers to these reactions as synthesis or combination reactions.)

Formation reactions are characterized by the fact that the reactant are elements in their standard state and the product is a compound containing the reactant elements combined in some way. We have seen two formation reactions so far.

• Fe(s) + S₈(s) --heat--> FeS(s)

• S₈(s) + O₂(g) --heat--> SO₂(g)

Formation reactions involving metals and nonmetals are the more straightforward. Writing the formula of the product is guided by the knowledge that the metal becomes positively charged and the nonmetal becomes negatively charged. We use the principal of electroneutrality (balance the charges) to write the formula of the product. When the elements are both nonmetals predicting the products is a little more difficult. Besides the one example of this type above, others we have discussed include;

• Na(s) + Cl₂(g) ----> NaCl(s)

• N₂(s) + O₂(g) --heat--> 2NO₂(g)

• C(s) + O₂(g) ----> CO₂(g)

• P₄(s) + 5O₂(g) --heat--> P₄O₁₀(s)

Several of these formation reaction we have discussed are not balanced. So let's take a moment and talk about balancing chemical equations.

How about balancing these reactions? Balancing equations is the process of equating the elements on both sides of the reaction arrow. A balanced equation has equal numbers of each element on both sides of the equation. Balancing consists of introducing coefficients which proceed the formula in the equation. Subscripts are NEVER changed when balancing equations.

For many reactions a method of trail and error is used. By practicing you're skills will sharpen. As you balance more and more equations certain 'rules' begin to emerge which can be useful. One rule which is very useful is to balance elemental forms last when balancing equations. This rule derives from the fact that changing the coefficient preceding an element only effects that element. Changing a coefficient preceding a compound changes all the elements in the compound and may effect another substance in the equation.

So shall we balance a few of the equations we've covered so far?

Let's begin with;

Fe(s) + S₈(s) --heat--> FeS(s)

In this equation if we begin with the element iron, we see there one Fe atom on the left and one Fe atom on the right. So iron is balanced. Sulfur has eight atoms on the left and only one on the right. Sulfur is not balanced. To balance the sulfur atoms we need to replace the coefficient preceeding FeS with the number 8.

Fe(s) + S₈(s) --heat--> 8FeS(s)

Now the sulfur atoms are balanced on both sides, but by adding the coefficient we've unbalanced the iron atoms. To fix that we need to place a coefficient of 8 before the elemetn Fe on the left side of the equation.

8Fe(s) + S₈(s) --heat--> 8FeS(s)

Now the equation is balanced.

We've used the reaction between iron and sulfur and between sulfur and oxygen as examples of formation reactions. There was another reaction which occurred when the iron and sulfur mixture was heated. That reaction occurred when we lit the Bunsen burner to heat the mixture of iron and sulfur. The reaction that was used to heat the mixture was a combustion reaction between methane (CH₄) and oxygen (O₂).

CH₄(g) + O₂(g) --heat-->

Methane is an example of a hydrocarbon. It is not the only example of a hydrocarbon. But more about that shortly. The products of this combustion reaction are carbon dioxide gas and water vapor (liquid).

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

Other hydrocarbons you are familiar with include;

You should know the name and formula for the first ten hydrocarbons (memorize).

Methane is a colorless, odorless gas which is used as a fuel in most gas stoves to heat/cook food. Gas companies add a compound which has an odor to help detect gas leaks should they arise. The compound I mentioned in class that was added to natural gas was CH₃SH. This is an error, the compound that is added to natural gas is call t-butyl mercaptan and has the formula C₄H₉SH. So lets write the combustion reaction of this compound.

C₄H₉SH(g) + O₂(g) --heat--> CO₂(g) + H₂O(g) + SO₂(g)

Hey we get more practice balancing equations!

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