CHEM 1411 - General Chemistry I

Course Lecture Notes

Fall 2007

 

Instructor: Dr. Shawn Amorde

 

 

 

Chemical Equations

 

Text Chapters (2.1, 3.1-3.4)

Recommended Problems:  (2, 4a, 8, 10, 12, 14, 18, 20, 22, 24, 28, 32, 34, 38, 40, 46, 47)

 

I.                   Chemical Equations and Reaction Stoichiometry

A.    Chemical Formulas

B.     Chemical Equations

C.     Balancing Equations

D.    Limiting Reagent

E.     Percent Yields

 

 

Chemical Formulas

 

We just learned the chemical formula (molecular formula) is a the smallest unit in a substance which still possesses the properties of that substance.  All molecules has a strict ratio of elements which is maintained through out the substance.

There is a term for this relationship, stoichiometry, which is the quantitative relationship between elements in a molecule is composition stoichiometry.  We are now going to look at the quantitative relationship between reactants and products, or reaction stoichiometry.

 

Molecular formulas gives us the number of elements in the molecule, but not how they are bonded together.  The structural formula shows us the connectivity of the atom.

For example, C3H8, is propane.  The structural formula shows which atoms are bonding with which atoms by drawing the molecule like this, with each line showing a chemical bond.

 

 

 

 

 

 

 

 

 

Chemical Equations

 

Remember that a physical change in a substance does not change the chemical composition, or molecular formula of the substance.  For example, boiling water does not change the molecular formula of water.

 

However, when you ignite methane (light your stove burner), you are using up the methane and forming carbon dioxide instead, this is a chemical change.

 

First, let’s review the Law of Conservation of Matter

There is no observable change in quantity of matter in a chemical reaction or a physical change.

 

Second, let’s review the Law of Conservation of Energy

Energy can not be created or destroyed in a chemical reaction or physical change.  It can only be converted from one form to another.

 

Third, the Law of Conservation of Matter and Energy

            The combined amount of matter and energy in the universe is fixed.

 

 

So what?  What does this mean for us?

 

Chemical equations are used to describe a chemical reaction(or change).

Chemical equations follow the Laws of Conservation of Matter and Energy!!

 

Chemical equations show the molecular formulas of the substances reacting, called reactants, and the molecular formulas of the substances being formed, called products.

They are written from left to right with reactants always on the right and products always on the left.

For example,

 

 

 

 

Balancing Equations

 

Because chemical equations must not create or destroy matter……the number of each of the elements must remain the same!!

This means each side of the chemical equation must be “balanced” to equal the other side.

 

For example,

 

 

 

Note the coefficients are whole numbers that indicate how many of each molecule is needed to react.

The subscript numbers are representative of the actual molecular formula, these numbers do not change.

 

In balancing reactions, we only balance the number of molecules reacting and never change the molecules reacting.

 

These equations are based on experimental findings!!  We can not change our observations to fit our equations……it is always the other way around.

 

 Let’s look at the reaction of burning diethyl ether.

 

 

 

 

Let’s figure out the total atoms on either side.

 

 

 

 

The total atom count must be equal on each side of the equation.

So, we must change the number of molecules of reactants and products so our total atom count matches.

Book method….

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Let’s try another one, using a slightly different approach.

Fractional coefficient.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Now that our equation is balanced what does it tell us?

A balanced reaction equation tells us the number of moles for each molecule!!

 

Let’s look at our stove burner again,

 

  CH4 + 2O2 → CO2 + 2H2O

1 mol   2mol    1mol   2mol

 

This concept is extremely important to remember!!

We will use this time and again in solving problems for the rest of the course.

 

Let’s work some problems.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Limiting Reactants Concept

 

Unless your reaction has an infinite amount of reactants the reaction will end as soon as one of the reagents is used completely.  This reagent is called the “limiting reagent”.  When you light your stove, methane is being supplied and the reaction is proceding, but when you turn of the burner and the flow of methane, the reaction consumes the rest of the methane and then the burner turns off.  In this case, the methane is your limiting reagent.

 

Here is our reaction.

CH4 + 2O2 → CO2 + 2H2O

 

If we are given a finite amount of CH4 and O2, which will run out faster?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Percent Yield

 

You will always need to convert to moles for your percent yield!!

 

Percent yield = moles of product/ moles of reactant

 

The book shows something a little different here, but keep it simple……always use moles.

 

Theoretical yield simple means the yield if all of your reactant is converted to your product.  Always on paper, never in the lab!! 

Actual yield is the yield of product you actually make in the lab.