Balancing More Equations and Types of Chemical Reactions

Wednesday, October 9th

Homework Review

We spent the first part of class today going over the two worksheets that were assigned for homework last night.  The key for these worksheets is posted here.  If you were absent on Tuesday, the blank copies are in the Unit 3 Handouts folder.  We were only supposed to do the odd number problems for the Balancing Chemical Equations worksheet, but the answers to the even problems are also posted if you want extra practice.

Number 9 on the Balancing Chemical Equations worksheet is a good problem to look at because it uses a decimal to balance the equation (which we learned how to do yesterday):

__ C8H18 + __ O2          >         __ CO2 + __ H2O

On the left side, we start off with 8 carbons, 18 hydrogens, and 2 oxygens.

On the right, we have 1 carbon, 2 hydrogens, and 3 oxygens.

First we balance the carbon atoms:

__ C8H18 + __ O2          >        8CO2 + __ H2O

This gives us 8 carbons, 18 hydrogens, and 2 oxygens on the left, and 8 carbons, 2 hydrogens, and 17 (8*2+1) oxygens on the right. To balance the hydrogen, you multiply the H2O by 9.

__ C8H18 + __ O2          >        8CO2 + 9H2O

We now have 8, 18, and 2 on the left, and 8 carbons, 18 hydrogens, and 25 (8*2+9) oxygens on the right. Now we can balance out the oxygens by multiplying the oxygen gas on the left side by the decimal 12.5.

__ C8H18 + 12.5O2          >        8CO2 + 9H2O

The equation is now balanced, but we have to multiply the whole thing by 2 to get all whole numbers, which gives you the answer

2C8H18 + 25O2          >        16CO2 + 18H2O

*tip: when given a reaction with carbon, hydrogen, and oxygen, balance them in the order CHO!

Pop Quiz

After we checked our homework, Mrs. Friedmann announced that we had a pop quiz, which caused anxiety attacks for many of us. However, it was only one problem for a single point and we were allowed to use our notes. The problem we had to answer read:

Aluminum nitrate reacts with magnesium oxide to make magnesium nitrate and aluminum oxide.

Put into equation form (don't forget to balance the charges!), this would look like:

__ Al(No3)3 + __ MgO          >         __ Mg(No3)2 + __ Al2O3

On the left, we have 1 aluminum atom, 3 nitrates (if you don't split it up), 1 oxygen and 1 magnesium. On the right, we have 2 aluminums, 2 nitrates, 3 oxygens, and 1 magnesium. First, I balanced the aluminum:

2Al(No3)3 + __ MgO          >         __ Mg(No3)2 + __ Al2O3

There are now 2 aluminums, 6 nitrates, 1 oxygen, 1 magnesium on the left, and 2 aluminums, 2 nitrates, 3 oxygens, and 1 magnesium on the right.  I now balanced the nitrates like so:

2Al(No3)3 + __ MgO          >         3Mg(No3)2 + __ Al2O3

You now have 2 aluminums, 6 nitrates, 1 oxygen, and 1 magnesium on the left, and 2 aluminums, 6 nitrates, 3 oxygens and 3 magnesiums on the right. What is now left to balance are the magnesium and oxygen atoms, which we can conveniently balance by multiplying MgO by 3.

2Al(No3)3 + 3MgO          >         3Mg(No3)2 + __ Al2O3

The equation is now balanced. The answer is:

2Al(No3)3 + 3MgO          >         3Mg(No3)2 + Al2O3

Notes

We spent the rest of class taking notes on the five common types of chemical reactions. Here is an overview.

1. "Put It Together" Combination Reaction

Two elements combine to make a compound. Opposite of a decomposition reaction.

A + B > AB

Ex: 2Mg + O2 > 2MgO

2. "Take It Apart" Decomposition Reaction

A more complex compound breaks up into simple elements. Opposite of a combination reaction.

AB > A + B

Ex: 2H2O2 > 2H2O + O2

3. "Trade One Place" Single Replacement Reaction

An element replaces one of the ions in an ionic compound ONLY if element is more active than the ion. We received an Activity Series Chart to use for this (posted on moodle) that shows which elements are more or less active than others. Elements cannot replace anything above them. If this happens, the reaction does not occur. For finding out which ions to switch with the element, think "like replaces like."

If the element is a metal it replaces the metal (positive) ion:

A(<metal) + B(<metal)C > B + AC (A replaces B)

If the element is a nonmetal it replaces the nonmetal (negative) ion:

A(<nonmetal) + BC(<nonmetal) > C + BA (A replaces C)

Metal Ex: 2Al + 3CuSO4 > 3Cu + Al2(SO4)

*notice how in the product, there are 2 atoms of Al and 3 of SO4, instead of just Al replacing Cu. Don't forget to balance out the charges!

Nonmetal Ex: F2 + 2NaCl > Cl2 + 2NaF

4. "Trade Two Places" Double Replacement Reaction

Two ionic compounds trade positive (front) ions. Remember "like replaces like." (You do not need to use the Activity Series Chart for these.)

AB + CD > CB + AD

5. "Burn!" Combustion Reaction (aka CHO Reaction)

A compound made of H and C (known as a hydrocarbon) reacts with O2 to make CO2 and H2O. You always get carbon dioxide and water with combustion reactions. Combustion reactions are the burning of anything organic.

CxHy + O2 > CO2 + H2O

Homework

We have two sheets due tomorrow. These can be found on moodle under the name Classifying Reactions HW Sheets in the Unit 3 Handouts.