Chapter 3a

Formulas, Equations and Moles

Lecture Notes

Dr. Sammia Shahid

Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken

Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.

Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction

Introduction

The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction is:

C + O2 CO2

It contains the same information as the English sentence but has quantitative meaning as well.

Because of the principle of theBecause of the principle of the

conservation of matterconservation of matter

an equation must be an equation must be

balancedbalanced

It must have the same numberIt must have the same number

of atoms of the same kind onof atoms of the same kind on

both sides.both sides. Lavoisier, 1788Lavoisier, 1788

Chemical Equations

Balancing Chemical Equations

• A balanced chemical equation represents the conversion of the reactants to products such that the number of atoms of each element is conserved.

reactants products

limestone quicklime + gas

Calcium carbonate calcium oxide + carbon dioxide

CaCO3(s) CaO(s) + CO2(g)

Balancing Chemical Equations

CaCO3(s) CaO(s) + CO2(g)

The letters in parentheses following each substance are

called State Symbols

(g) → gas (l) → liquid (s) → solid (aq) → aqueous

Balancing Chemical Equations

A balanced equation MUST have the same number of

atoms of each element on both sides of the equation.

H2 + O2 → H2O Not Balanced

H2 + ½O2 → H2O Balanced

2H2 + O2 → 2H2O Balanced

Balancing Chemical Equations

The numbers multiplying chemical formulas in a chemical

equation are called: Stoichiometric Coefficients (S.C.)

2H2 + O2 → 2H2O Balanced

Here 2, 1, and 2 are stoichiometric coefficients.

Balancing Chemical Equations

A balanced chemical equation shows that the law of conservation of mass is adhered to.

In a balanced chemical equation, the numbers and kinds of atoms on both sides of the reaction arrow are identical.

2NaCl(s)2Na(s) + Cl2(g)

right side:

2 Na2 Cl

left side:

2 Na2 Cl

Balancing Chemical Equations

2. Find suitable coefficients—the numbers placed before formulas to indicate how many formula units of each substance are required to balance the equation.

2H2O(l)2H2(g) + O2(g)

1. Write the unbalanced equation using the correct chemical formula for each reactant and product.

DO NOT TRY TO BALANCE IT YET!

H2O(l)H2(g) + O2(g)

DO NOT CHANGE THE FORMULAS!

Balancing Chemical Equations

3. Reduce the coefficients to their smallest whole-number values, if necessary, by dividing them all by a common denominator.

2H2O(l)2H2(g) + O2(g)

4H2O(l)4H2(g) + 2O2(g)

divide all by 2

Balancing Chemical Equations

4. Check your answer by making sure that the numbers and kinds of atoms are the same on both sides of the equation.

2H2O(l)2H2(g) + O2(g)

right side:

4 H2 O

left side:

4 H2 O

Some of the Helpful Hints for balancing equations:

• Take one element at a time, working left to right except for H and O. Save O for next to last, and H until last.

• IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

• (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

• MINOH: try Metals first, then Ions, then Nonmetals, and Oxygen, then Hydrogen last

Steps to Balancing Equations

Balancing Chemical Equations

Do not change subscripts when you balance a chemical equation. You are only allowed to change the coefficients.

H2O(l)H2(g) + O2(g) unbalanced

2H2O(l)2H2(g) + O2(g)

Chemical equation changed!

H2O2(l)H2(g) + O2(g)

Balanced properly

Balancing Chemical Equations

Example 1: B2H6 + O2 → B2O3 + H2O

Balance O last

B is already balanced

Start by changing S.C. of H2O:

B2H6 + O2 → B2O3 + 3H2O

Balancing Chemical Equations

Example 1: B2H6 + O2 → B2O3 + 3H2O

B and H are balanced

Balance O by changing S.C. of O2

B2H6 + 3O2 → B2O3 + 3H2O

BALANCED!

Balancing Chemical Equations

Example 2: MnO2 + KOH + O2 → K2MnO4 + H2O

Balance O last

Mn is already balanced

Change S.C. of KOH to balance K

MnO2 + 2KOH + O2 → K2MnO4 + H2O

Balancing Chemical Equations

Example 2: MnO2 + 2KOH + O2 → K2MnO4 + H2O

Mn, K, and H are balanced (H was balanced by chance)

Balance O

MnO2 + 2KOH + ½O2 → K2MnO4 + H2O

or

2MnO2 + 4KOH + O2 → 2K2MnO4 + 2H2O

Balancing Chemical Equations

• Balance the following equations:

–C6H12O6 → C2H6O + CO2

–Fe + O2 → Fe2O3

–NH3 + Cl2 → N2H4 + NH4Cl

Balancing Chemical Equations

• Balance the following equations:

–C6H12O6 → C2H6O + CO2 Unbalanced

–C6H12O6 → 2C2H6O + 2CO2 Balanced

Balancing Chemical Equations

• Balance the following equations:

–Fe + O2 → Fe2O3 Unbalanced

–4Fe + 3O2 → 2Fe2O3 (balance O first)

Balancing Chemical Equations

• Balance the following equations:

–NH3 + Cl2 → N2H4 + NH4Cl Unbalanced

–N:H is 1:3 on left, must get 1:3 on right!

–4NH3 + Cl2 → N2H4 + 2NH4Cl Balanced

The student will be able to:

describe and apply mass relationships in chemical reactions;

define and write chemical formulas;

write and balance chemical reactions and chemical equations.

Learning Outcomes

Day Time

Tuesday

09:00 am - 01:00 pm 03:00 pm - 04:00 pm

Wednesday09:00 am - 11:00 am

Thursday

09:00 am - 01:00 pm 03:00 pm - 04:00 pm

Friday02:00 pm - 03:00 pm

COUNSELING HOURS