Balancing Chemical Equations

Parts of a Chemical Equation

Molecules vs Compounds

• Molecules are two or more atoms bonded together…but compounds must have two different elements

– Ex- O2, NaCl, etc.

REMEMBER: – Not all molecules are compounds, but all compounds

are molecules…explain

Chemical reactions continued

–Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken• What do we call those “outermost parts”???• Valence electrons

Symbols represent elementsFormulas represent the molecules

Chemical equations represent the reaction

Law of conservation of matter“mass is neither created nor

destroyed in any ordinary chemical reaction. “

• Or more simply, the mass of substances produced (products) by a chemical reaction is always equal to the mass of the reacting substances (reactants).

• Ex’s– 1.00g carbon + 5.34g sulfur _____g carbon

disulfide?• 6.34g carbon disulphide

– 2.00g carbon + 10.68g sulfur _____g carbon disulfide? • 12.68g carbon disulphide

Law of conservation of matter

Balancing Equations

• To follow the law of conservation of mass, any equation must be BALANCED

• (Mass of reactants must equal the mass of the products)

BALANCING EQUATIONS

–When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but

you may not change the …• subscripts.

• Changing the subscripts changes the compound. –H2O is very different that H2O2

– (Water vs Hydrogen Peroxide!)

Subscripts tell you how many atoms

of each element you

have

Coefficients tell you how

many of each

molecule you have

Ex: 4 Al + 3 O2 2Al2O3

This equation means :“4 Al atoms + 3 O2 molecules

produces 2 molecules of Al2O3”

Steps to Balancing Chemical Equations

• 1. Write out the equation leaving room to add coefficients.

• Ex-

Na3PO4 + Fe2O3 Na2O + FePO4

Step 2: Record how many atoms of each element

2. Find the number of atoms for each element on each side. – YOU MUST COMBINE ALL of the same element together…– ex

Na3PO4 + Fe2O3 Na2O + FePO4

» Na- 3 Na-2» P-1 P-1» O- 7 Fe- 1» Fe-2 O-5

HINT- write them in the same order on both sides so that it is easier to compare them

STEP THREEDetermine where to place coefficients in front of

formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

2 Na3PO4 +___ Fe2O3 3 Na2O + 2 FePO4

» Na- 3 6 Na-2 6» P-1 2 P-1 2» Fe- 2 Fe- 1 2»O- 7 11 O-5 9 11

Step 4…4. Check your answer to see if:– The numbers of atoms on both sides of the

equation are now balanced.– The coefficients are in the lowest possible

whole number ratios. (reduced)

Some Helpful Hints for balancing equations:• Take one element at a time, working left

to right except for H and O. Save H for next to last, and O 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)

Example Illustration:

Let’s try a few

1. ___ C3H8 + ___ O2 ____ CO2 + ___ H2O

Ready for another?

2. _____B4H10 +___O2 __B2O3 + ____H2O

Balancing Equations when given the name (not the formula)

• Use “reacts with” and “produces” or “yields”

• Ex-• Hydrogen reacts with Oxygen to produce

dihydrogen monoxide.

• Be able to write the chemical equation from the word equation or vice versa!

Let’s try a few

1. ___ C3H8 + ___ O2 ____ CO2 + ___ H2O

Tricarbon octohydride reacts with oxygen to produce carbon dioxide

and dihydrogen monoxide

Ready for another?

2. _____B4H10 +___O2 __B2O3 + ____H2O

Tetraboron decahydride reacts with oxygen to produce diboron trioxide

and dihydrogen monoxide

Ready for another?

3. _____MgCl2 +___NaF __MgF2 + ____NaCl

Magnesium chloride reacts with sodium fluoride to produce

magnesium fluoride and sodium chloride