Chapter 6 Notes

You are expected to know the chemical formula and the standard state phase(25 degrees C and 1 atmosphere of pressure) for all of the elements in the periodic table.

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. 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;

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;

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, SO42-, and sulfite, SO32- 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)

Let's look at another reaction. How about the reaction between sodium and chlorine. The equation is;

Na(s) + Cl2(g) ---->

What is the product of this reaction? (Since this the product is a binary ionic compund I expect you to know the correct formula!) If you are thinking sodium chloride, NaCl, you are CORRECT! So the reaction becomes;

Na(s) + Cl2(g) ----> NaCl(s)

We'll watch the video of the reaction. In the video of this reaction between sodium solid and chlorine we needed to add a few drops of water to initiate the reaction. What is the reaction between sodium and water?

Na(s) + H2O(l) ---->

Can you predict the products? Remember we did the reaction between potassium and water in class earlier in the semester and the reaction between sodium and water is identical. So the products are hydrogen gas and sodium hydroxide.

Na(s) + H2O(l) ----> NaOH(s) + H2(g)

In our discussion of chemical reactions and chemical equations we will organize reactions into certain types. We have discussed two types so far; formation and single replacement 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 three formation reactions so far.

Single replacement reactions are characterized by an element reacting with a compound forming a new compound and another element. The term replacement comes from the observation that the reactant element 'replaces' an element in the reactant compound. The reaction between sodium and water is an example of a single replacement reaction. We'll cover some additional examples of single replacement reactions later.

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 procede 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;

Fe3O4(s) + H2(g) ----> Fe(s) + H2O(l)

Since hydrogen and iron are elements we will balance them last, so only oxygen remains. How many oxygens on the left? I count 4. On the right? I count 1. To balance the oxygen atoms we place the coefficient 4 before water.

Fe3O4(s) + H2(g) ----> Fe(s) + 4H2O(l)

Now we can balance the iron and the hydrogen. There are 3 iron atoms on the right and 1 on the left. We need a coefficient of 3 for the iron on the right. Using the same logic for hydrogen we need a coefficient of 4 for hydrogen.

Fe3O4(s) + 4H2(g) ----> 3Fe(s) + 4H2O(l)

Ready for another?

Try

C4H10(g) + O2(g) ----> CO2(g) + H2O(l)

Since oxygen is an element we will balance it last. Another useful rule is to balance non-oxygen and non-hydrogen atoms first. In this case that means carbon. In the unbalanced equation there are 4 carbon atoms in the reactants and 1 in the products.

C4H10(g) + O2(g) ----> 4CO2(g) + H2O(l)

Now we can balance hydrogen. There are 10 hydrogen in the reactants and 2 in the products. So a coefficient of 5 is needed for water.

C4H10(g) + O2(g) ----> 4CO2(g) + 5H2O(l)

So only oxygen remains. There are 13 oxygen atom in the products and two in the reactants. We need 13 oxygen atom in the reactants, but how do we get 13 oxygen atoms on the reactants side? Thirteen oxygen atoms is the same as 6 and 1/2 O2 molecules.

C4H10(g) + 6 1/2O2(g) ----> 4CO2(g) + 5H2O(l)

or

C4H10(g) + 13/2O2(g) ----> 4CO2(g) + 5H2O(l)

We prefer to have whole number coefficients when balancing chemical equations. To adjust to whole numbers we will multiply all the coefficients by 2.

2 x (C4H10(g) + 6 1/2O2(g) ----> 4CO2(g) + 5H2O(l))

or

2C4H10(g) + 13O2(g) ----> 8CO2(g) + 10H2O(l)

Notice all the atoms are now balanced. There are 8 C's, 20 H's and 26 O's on both sides of the equation.

 

 

 

 

 

 

Na(s) + H2O(l) ----> NaOH(s) + H2(g)