Chemical Equations

The content that follows is the substance of lecture 11. In this lecture we cover Chemical Equations, Balancing and the determination of the Net Ionic Equation.

A Chemical Equation:

2H₂(g) + O₂(g) → 2H₂O(g)

This equation reads: 2 moles of Hydrogen gas plus one mole of oxygen gas reacts to yield 2 moles of water. As we discussed previously, all of the elements in the molecules themselves are in mole amounts. In other words, in the water molecule there are 2 moles of hydrogen and 1 mole of oxygen. The same is true for chemical equations, all of the amounts shown are in moles.

Another aspect of chemical equations that we have discussed previously is the Law of Conservation of Mass. This means that you cannot lose or gain any moles of an element when you perform a reaction. In order to show this conservation, you always have to make sure the equation is in "balance". The number of moles of each element must be the same on both the reactant side (left side of the equation) and the product side (right side of the equation. The arrow in the center indicates the direction in which the reaction is running.

Balancing Chemical Equations:

The process of balancing an equation is not difficult so long as you remember a few guidelines:

1) The compounds are made to balance by multiplying the entire compound by a coeffcient placed in front of the compound.

2) You NEVER change the subscripts within a compound to create balance. This actually changes the identity of the compound so NOT GOOD!

3) Always start with the non-hydrogen/oxygen compounds first when balancing as there are normally less of them.

4) Balance polyatomic ions as whole units rather than trying to balance the elements inside them individually.

5) Leave the balance of elements that are alone in the reaction until last if you can since they can be multiplied by any number without affecting any other elements in the reaction.

Let's do an example that uses the guidelines above:

Balance the following reaction:

 Al    +    H₂SO₄   →    Al₂(SO₄)₃     +    H₂ 

Based on the guidelines above we should start this balancing with Aluminum. There are 2 moles of Aluminum on the product side but only 1 on the reactant side, so we add a coefficient of 2 in front of the Al on the reactant side:

 Al    +    H₂SO₄   →    Al₂(SO₄)₃     +    H₂ 

Next according to the guidelines we should look at the polyatomic ion, sulfate. There are 3 SO₄^2- ions on the right but only 1 on the left so we need to input a coefficient of 3 in front of the sulfuric acid molecule:

2Al    +   3H₂SO₄   →    Al₂(SO₄)₃     +    H₂ 

Finally we can addess the hydrogen in the equation. Since it is alone it will be easy to balance. There are 3(2) =6 moles of hydrogen on the left and currently only two on the right so we need to place a coefficient of 3 on the right in front of the hydrogen:

2Al    +   3H₂SO₄   →    Al₂(SO₄)₃     +   3H₂ 

Now we should be balanced:

Reactant Moles: 2 Al, 6 H, 3 S, 12 O        Product Moles: 2 Al, 6 H, 3 S, 12 O

All equal so we are balanced!

Now you try:

 

What can we do with a balanced equation?

A chemical reaction equation is essentially a table of conversion factors that we can use to predict amounts of products that can be made, reactants needed to make a specific amount of product or exact amounts of reactants needed to completely consume another reactant (eg. acid and base neutralization).

While the use of the stoichiometry (fancy words for mole relationships in a reaction equation) may seem difficult, it really isn't. Let's use an example that you all can understand first:

1 slice bologna + 2 slices of bread → 1 Sandwich

If I asked you how many sandwiches you could make with 12 slices of bread, you would immediately say 6, right? How did you know this? Well, obviously based on the equation, you make 1 sandwich for every 2 slices of bread you have.

12 slices bread x 1 sandwich/2 slices bread = 6 sandwiches

If I asked you how many slices of bologna and bread you need to create 10 sandwiches, you would immediately say 10 slices of bolgna and 20 slices of bread, right? Again the relationship given in the equation tells you how much is needed.

10 sandwiches x 1 slice bologna/1 sandwich = 10 slices bologna

and

10 sandwiches x 2 slices bread/1 sandwich = 20 slices bread

Finally if I asked you how many sandwiches you could make with 5 slices of bologna and 8 slices of bread? Which of the two sandwich makings would run out first and how much of the excess makings would be left over? Well, the answers are 4 sandwiches could be made and 1 slice of bologna is left over.

5 slices of bologna x 1 sandwich/1 slice bologna = 5 sandwiches

8 slices bread x 1 sandwich/2 slices bread = 4 sandwiches

8 slices bread x 1 slice bologna/2 slices bread = 4 slices of bologna used

5 slices bologna - 4 slices used = 1 slice left over

So what do bologna sandwiches have to do with chemistry? The same types of calculations you did in your head above can be done using a balanced reaction equation:

2H₂(g) + O₂(g) → 2H₂O(g)

How many moles of water can you make with 6 moles of hydrogen and 5 moles of oxygen? Which will run out first? Which will be left over? How much will be left over?

6 mol H₂ x 2 mol H₂O/2 mol H₂ = 6 mol 2H₂O

5 mol O₂ x 2 mol H₂O/1 mol O₂ = 10 mol 2H₂O

6 mol H₂ x 1 mol O₂/2 mol H₂ = 3 mol O₂ used

5 mol O₂ - 3 mol O₂ used = 2 mol O₂left over

You should note that the calculation process is identical to the one we completed for the sandwiches. So long as you take whatever you are given in a question (grams, volume and molarity, etc.) and convert it to moles, you can then use the balanced chemical reaction to make predictions for both product and reactant amounts.

We will come back to this in a few lectures but for now just try to get used to the concept of how you can use a balanced chemical reaction.

Net Ionic Equations

Now that you know how to balance equations, let's take a brief moment to talk about why a reaction happens. When two chemicals are placed together in a reaction, there has to be a driving force to make them come together to produce a product. Generally, the driving force is the production of a molecule or compound that will not fall back apart in solution.

Electrolytes are defined as compounds that dissociate into separate ions when placed in aqueous solution. They are called electrolytes because the ions will support the transfer of electricity in the solution.

Ionic compounds, acids and bases are electrolytes.

When placed into solution these compounds separate into ions and if a reaction is to take place they "swap partners". This process is called Metathesis.

The reaction will take place if one of the following is formed: A insoluble compound (precipitate), a gas or a covalent compound (eg. water).

The Net Ionic Equation demonstrate the part of a reaction that is the driving force for a reaction.

For example the molecular reaction for the reaction of an acid and a base:

HCl(aq) + NaOH(aq) → NaCl(aq)+ H₂O(l)

To see the ions for the reaction we create the Total Ionic Equation by separating everything into ions:

H⁺(aq)+ Cl^-(aq)+ Na⁺(aq) + OH^-(aq) → Na⁺(aq) + Cl^-(aq) + H₂O(l)

You can now cancel anything that is in the same amounts on both sides to produce the Net Ionic Equation. This will remove both the Na and Cl ions from the reaction, leaving the driving force of the reaction; the production of water:

H⁺(aq) + OH^-(aq)→ H₂O(l)

Another example of the process for determining the Net Ionic Equation would be a reaction involving the production of a solid:

Molecular Equation: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)

Total Ionic Equation: Pb²⁺(aq) + 2NO₃^-(aq) +2 K⁺(aq) + 2I^-(aq) → 2K⁺(aq) + 2NO₃^-(aq) +PbI₂(s)

Net Ionic Equation: Pb²⁺(aq) + 2I^-(aq) → PbI₂(s)

Now you Try: