Balancing chemical equations

Main Ideas Chemical Reactions are represented by Chemical Equations. Chemical Equations are balanced to show the same number of atoms of each element on each side. The Law of Conservation of Mass says that atoms won’t be created or destroyed in a chemical reaction. That is why you have to balance chemical equations!

Chemical Reactions are Everywhere Cooking Respiration

CHEMICAL REACTIONS Zn + I2 Reactants Zn I2 Product

Parts of a Reaction Equation Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). A + sign separates molecules on the same side The arrow is read as “yields” Example C + O2  CO2 This reads “carbon plus oxygen react to yield carbon dioxide”

Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

Chemical Equations – Counting Atom Counting Atoms 3H2O coefficient subscript How do I know how many atoms are in a molecule? Easy!! There are rules to follow:

Chemical Equations – Counting Atoms Subscripts Multiply the atom directly in front of them (no subscript = 1) CH4 has 1 atom of C and 4 atoms of H C6H14 has 6 atoms of C and 14 atoms of H Multiply all atoms in preceding parenthesis Ca(OH)2 subscript multiplies both O and H. 1 Ca, 2 O and 2 H

WARNING! Don’t ever play around with subscripts (those little numbers that tell you how many atoms are in a molecule) e.g. C6H22O11

Chemical Equations – Counting Atoms Coefficients - Multiply all atoms in the compound they precede, including those in parenthesis. 2CH4 has 2 C and 8 H 2C6H12O6 has 12 C, 24 H and 12 O 3Ca(OH)2has 3 Ca, 6 O and 6 H Note: Coefficients do not carry past + signs or the reaction arrow ( ) in a chemical equation. They are for a single molecule or compound.

Chemical Equations – Counting Atoms MnO2 _________________ 2 MnO2 _________________ 4 MnO2 _________________ C2H6 _________________ 5 C2H6 _________________ Ca(HCO3)2 __________________ 2 Ca(HCO3)2 __________________ C2H2OH _______________ 3 C2H2OH _______________ Mrs. Johannesson

Chemical Equations – Counting Atoms MnO2 Mn = 1 O = 2 2 MnO2 Mn = 2 O= 4 (2x2) 4 MnO2 Mn = 4 O= 8 (4x2) C2H6 C = 2 H = 6 5 C2H6 C = 10 (5)x2 H = 30 (5x6) Ca(HCO3)2 Ca = 1 H =2 C = 2 O = 6 2 Ca(HCO3)2 Ca = 2 H =4 C = 4 O = 12 C2H2OH C = 2 H = 3 O = 1 3 C2H2OH C = 6 H = 9 O = 3 Mrs. Johannesson

Chemical Equations – Counting Atoms Mass is neither created nor destroyed in a chemical reaction total mass stays the same atoms can only rearrange 2 H2 + O2 2H2O 4 H 2 O 4 H 2 O Mrs. Johannesson

Balancing Equations Na + Cl2 NaCl Example Reactants Products Na Na Cl

Balancing Equations NaCl Na + Cl2 Example Reactants Products Total 3 1 Na 1 Cl 2 Cl 1 Total 3 Total 2

Balancing Equations 2NaCl Na + Cl2 Add coefficients only Example to balance Example 2NaCl Na + Cl2 Reactants Products Na 1 Na 1 2 Cl 2 Cl 1 2 Total 3 Total 2 4

WE SAY THAT THE EQUATION IS BALANCED!! Balancing Equations 2Na + Cl2 2NaCl Reactants Products Na 1 2 Na 1 2 Cl Cl 1 2 2 Total 3 4 Total 2 4 NOW BOTH SIDES HAVE EQUAL NUMBERS OF ATOMS WE SAY THAT THE EQUATION IS BALANCED!!

Chemical Equations – Balancing Practice Step 1: Determine the reactants and products. Step 2: Complete an atom inventory of both sides of the chemical equation. C3H8 + O2 → CO2 + H2O C = 3 C = 1 H = 8 H = 2 O = 2 O = 3 (2+1) Total left = 13 Total right = 6 Is this equation balanced?? NO.

Chemical Equations - Balancing Practice Step 3: Use coefficients to balance the equation. NEVER change the subscript in order to balance the equation!! C3H8 + O2 → 3CO2 + H2O C =3 C = 3 H = 8 H = 2 O = 2 O = 7 (6+1) Is this equation balanced?? No.

Chemical Equations - Balancing Practice C3H8 + 5O2 → 3CO2 + 4H2O C = 3 C = 3 H = 8 H = 8 O = 10 O = 10 (6 + 4) Total = 21 Total = 21 Is this equation balanced?? Yes. Hint: Whenever possible, save the “O” until last. The oxygen atoms will often take care of themselves.

Balance Equations in the simplest form Check to see that the coefficients used are the smallest integers that give the balanced equation. The balanced equation is: 4H2(g) + 2O2(g)  4H2O(l) or could be: 2H2(g) + O2(g)  2H2O(l) preferred

Chemical Equations – Balancing Practice Na + O2 → Na2O CuCl2 + H2S → CuS + HCl Al2O3 → Al + O2 4. NaCl + F2 → NaF + Cl2

Conservation of Mass Lab Mass Reactants = Mass of Products State Law of Conservation of Mass in your own words