Friday, Aug. 30th: “A” DayTuesday, Sept. 3rd: “B” Day

AgendaGo over quiz/review worksheetSection 8.2: “Balancing Chemical Equations”Homework:

1. Sec. 8.2 review, pg. 274: #1-62. Chemical Formulas/Equations Worksheet

a. Pg. 329: #15 – 24/ Pg. 330: # 1- 203. “Writing Chemical Formulas” worksheet4. Concept Review: “Balancing Chemical

Equations”

Reactions Conserve MassA basic law of science is the conservation of

mass.Law of conservation of mass: in ordinary

chemical or physical changes, mass is neither created nor destroyed.

If the products of a chemical reaction are all collected, their total mass will be the same as the total mass of the reactants.

Reactions Rearrange AtomsThe products and reactants of a chemical

reaction are made up of the same number and kinds of atoms.

The atoms are just rearranged and connected differently.

? Na + ? H2O ? NaOH + ? H2

To show that a reaction satisfies the law of conservation of mass, the equation must be BALANCED!

Balancing EquationsTo balance an equation, the number of atoms

of each element needs to be the same on each side of the arrow.

But, the formulas cannot be changed.CO cannot be changed to CO2 to balance the

oxygen atoms – these are two different molecules with different properties.

Place numbers, called coefficients, in FRONT of the formulas…

CoefficientsCoefficient: a small, whole number that appears as a

factor in front of a formula in a chemical reaction.A coefficient multiplies the number of atoms of each

element in the formula.

For example:H2O: 1 water molecule contains 2

hydrogen atoms and 1 oxygen atom

2 H2O: 2 water molecules contain 4 hydrogen atoms and 2 oxygen

atoms

Diatomic Elements

For this section, you will need to remember that the following elements are diatomic and exist as molecules:

H2, O2, N2, F2, Cl2, Br2, I2

**Write these at the top of your notes**

How to Balance Chemical EquationsSkills Toolkit (pg. 268)

1. Identify reactants and products.a. Write an unbalanced equation.

2. Count atoms.a. Balance the atoms that appear in only 1 reactant and

1 product first. b. Balance O2 and H2 last.

c. Treat polyatomic ions as a unit if they appear on both sides of arrow.

3. Insert coefficients to balance 1 element at a time.4. Check to make sure you did it right!

Balancing an EquationSample Problem A, pg. 269

Balance the equation for the reaction of iron(III) oxide with hydrogen to form iron and water.

1. Identify reactants and products.Iron(III) oxide + hydrogen iron and water

? H2 Fe H2O

We need to figure out the formulas for our reactants and products.

Balancing an EquationSample Problem A, pg. 269 cont.

What is the formula for iron(III) oxide?Iron(III) means that the charge on the iron cation is 3+

Oxide is oxygen, whose ion charge is always 2-

Fe3+ O2-

Fe2O3

Fe2O3 + H2 Fe + H2O (unbalanced)

Write an unbalanced equation:Fe2O3 + H2 Fe + H2O

Sample Problem A, cont.Fe2O3 + H2 Fe + H2O

2. Count atoms. Balance Fe (Iron) 1st because it appears in only 1 reactant and 1 product.

Fe2O3 + H2 2 Fe + H2O

Balance O2 (Oxygen) next:

Fe2O3 + H2 2 Fe + 3 H2O

Lastly, balance H2 (Hydrogen):

Fe2O3 + 3 H2 2 Fe + 3 H2O

Is everything balanced? YES

Additional ExampleBalance the following equation:

C3H8 + O2 CO2 + H2OBalance C (carbon) first:

C3H8 + O2 3 CO2 + H2ONext, balance H (hydrogen):

C3H8 + O2 3 CO2 + 4 H2OLastly, balance O (oxygen):

C3H8 + 5 O2 3 CO2 + 4 H2O

10 oxygen atoms = 6 + 4 oxygen atoms

Never Change Subscripts to Balance an Equation

Unbalanced Equation: H2 + O2 H2O

Incorrect: H2 + O2 H2O2

H2O H2O2

H2O is water; H2O2 is hydrogen peroxideThey are NOT the same thing!

Instead, add coefficients to balance…

Correct: 2 H2 + O2 2 H2O

Sample Problem BPg. 271

The reaction of ammonia with oxygen produces nitrogen monoxide and water vapor. Write a balanced equation for this reaction.

1.Identify reactants and products. Reactants: NH3 and O2

Products: NO and H2OWrite unbalanced equation:

NH3 + O2 NO + H2O

Sample Problem B, Cont.NH3 + O2 NO + H2O

2. Count atoms.Balance N (nitrogen) first:

It’s already balanced – yeah!Balance H (hydrogen) next:

2 NH3 + O2 NO + 3 H2ORecheck the N, it is now unbalanced, fix it:

2 NH3 + O2 2 NO + 3 H2OLastly, try to balance the O (oxygen):

2 NH3 + O2 2 NO + 3 H2O 2 oxygen 5 oxygen atoms

Sample Problem B, Cont.2 NH3 + 2.5 O2 2 NO + 3 H2O

5 oxygen 5 oxygen atomsIn order to get the oxygen atoms to be equal, on

both sides, the O2 coefficient needs to be 2.5But, coefficients need to be small, whole

numbers so this is incorrect.To solve, simply double ALL of the coefficients:

4 NH3 + 5 O2 4 NO + 6 H2O

Is everything balanced? YES

Sample Problem C, Pg. 273Aluminum reacts with arsenic acid, HAsO3, to

form H2 and aluminum arsenate. Write a balanced equation for this reaction.

1. Identify reactants and products. Reactants: Al and HAsO3

Products: H2 and Al(AsO3)3

Write unbalanced equation:Al + HAsO3 H2 + Al(AsO3)3

Sample Problem C, Cont.Al + HAsO3 H2 + Al(AsO3)3

2.Count atoms. First, balance the Al

It’s already balanced – yeah!Next, balance the arsenate polyatomic ion:

Al + 3 HAsO3 H2 + Al(AsO3)3

Next, balance the H (hydrogen):Al + 6 H AsO3 3 H2 + Al(AsO3)3

This messes up the AsO3 ion, fix it:

Al + 6 H AsO3 3 H2 + 2 Al(AsO3)3

Sample Problem C, Cont.Al + 6 H AsO3 3 H2 + 2 Al(AsO3)3

Are we balanced yet? NO!

Now the Al (aluminum) is unbalanced, fix it:2 Al + 6 H AsO3 3 H2 + 2 Al(AsO3)3

How about now?Yes, we’ve finally balanced the

equation!

Practice Makes PerfectThe best way to get better at balancing

equations is to practice, a lot!You don’t have to balance polyatomic ions as

a unit if you don’t want to. You can balance the individual elements if you find that easier.

Sometimes, balancing equations is tricky and you need to use trial and error…

Homework1. Sec. 8.2 review, pg. 274: #1-62. Chemical Formulas/Equations WS

a. Pg. 329: #15 – 24b. Pg. 330: # 1- 20

3. “Writing Chemical Formulas” worksheet4. Concept Review: “Balancing Chemical Equations”

Next time: Quiz over section 8.2: “Balancing

Chemical Equations”High School Open House:

Wednesday, Sept. 4th: 6:30 – 8:00 pm