Unit 8 – Chemical Equations and Reactions• Notes p.1-2• These ones are in order!

Format of Chemical Equations:

2H2 + O2 2H2O

Reactants/Reagents

Format of Chemical Equations:

2H2 + O2 2H2O

Product(s)

Format of Chemical Equations:

2H2 + O2 2H2O

Yield(s)

Format of Chemical Equations:

2H2 + O2 2H2O

Subscript(s)

Format of Chemical Equations:

2H2 + O2 2H2O

Coefficients(s)

•Coefficients• Large numbers in front•Give how many molecules of each

substance are in the equation• Subscripts

• Small numbers after an element •Give how many of each atom are needed

in the formula

Format of Chemical Equations:

Now for some practice• These are not in the notes but are essential skills to have

What is the coefficient for …• 2Al + 3CuSO4 Al2(SO4)3 + 3Cu

• Aluminum: 2• Copper sulfate: 3 • Aluminum sulfate: 1 (when there is no number written, it is 1) • Copper: 3

What is the coefficient for …• 2H2O 2H2 + O2

•Water: 2• Hydrogen gas: 2• Oxygen gas: 1

How many ___ are there in ___•Mg(OH)2 + 2HCl MgCl2 + 2H2O

• How many H are in Mg(OH)2? • 2 (distribute the subscript through the parentheses)

• How many H are in HCl?• 2 (the coefficient means there are 2 H)

• How many H are in the reactants? • 4 total: 2 from HCl and 2 from Mg(OH)2

• How many H are in the products? • 4 (all from H2O)

•Law of Conservation of mass •Matter cannot be created nor destroyed, only

change form •Balancing Equations •Must be the same number of each type of atom

on the reactant side and the product side

Even though a reaction has taken place, the mass at the beginning and the mass at end are the same. No mass (or matter) is gained or lost in the reaction.

•Never change SUBSCRIPTS in balancing!!

•Only change COEFFICIENTS!!!

• To Balance an Equation:1. Total the number of each element for both sides 2. Check if each element is the same on both sides3. Change the total on the inventory list so that the number of atoms matches.4. Add a coefficient to the substance in the equation5. Adjust the inventory list to reflect any changes in other atoms6. Move on to each atom until the lists match for EVERY element

•Tricks:• Balance polyatomic ions as a group• If an element is present twice on one side, leave

it for last• If the list isn’t working because you have an odd

number on one side and an even number on the other, try doubling everything• Simplify: Check after balancing for any factor that

could divide through all the coefficients

EXAMPLE: WRITE A BALANCED EQUATION• Chlorine + sodium bromide bromine + sodium chloride Cl2 + NaBr Br2 + NaCl

Cl: 2 Cl: 1Na: 1 Na: 1Br: 1 Br: 2

• 1. Total the number of each element for both sides• Look at subscripts to tell how many are in the unbalanced equation• All coefficients in an unbalanced equation are 1

(You might need to brush up on writing chemical formulas if you are only given the name.)

EXAMPLE: WRITE A BALANCED EQUATION• Chlorine + sodium bromide bromine + sodium chloride Cl2 + NaBr Br2 + NaCl

Cl: 2 ≠ Cl: 1Na: 1 = Na: 1Br: 1 ≠ Br: 2

2. Check if each element is the same on both sides- Chlorine and bromine are unbalanced

EXAMPLE: WRITE A BALANCED EQUATION• Chlorine + sodium bromide bromine + sodium chloride Cl2 + NaBr Br2 + NaCl

Cl: 2 ≠ Cl: 1 2Na: 1 = Na: 1Br: 1 ≠ Br: 2

3. Change the total on the inventory list so that the number of atoms matches.- I like to start with the first element on my inventory list. It doesn’t really matter which one you start with.

EXAMPLE: WRITE A BALANCED EQUATION• Chlorine + sodium bromide bromine + sodium chloride Cl2 + NaBr Br2 + 2 NaCl

Cl: 2 ≠ Cl: 1 2Na: 1 = Na: 1Br: 1 ≠ Br: 2

4. Add a coefficient to the substance in the equation- 1 x coefficient = 2, therefore the coefficient is 2

EXAMPLE: WRITE A BALANCED EQUATION• Chlorine + sodium bromide bromine + sodium chloride Cl2 + NaBr Br2 + 2 NaCl

Cl: 2 = Cl: 1 2Na: 1 ≠ Na: 1 2Br: 1 ≠ Br: 2

5. Adjust the inventory list to reflect any changes in other atoms- The new coefficient applies to both Na and Cl. According to the

equation, there are now 2 Na in the products.

EXAMPLE: WRITE A BALANCED EQUATION• Chlorine + sodium bromide bromine + sodium chloride Cl2 + 2 NaBr Br2 + 2 NaCl

Cl: 2 = Cl: 1 2Na: 1 2 = Na: 1 2Br: 1 2 = Br: 2

6. Move on to each atom until the lists match for EVERY element- Na is now unbalanced. The new coefficient in front of NaBr should be 2. That means there are now also 2 Br. The equation is balanced!

EXAMPLE: WRITE A BALANCED EQUATION

aluminum sulfate+ calcium chloride aluminum chloride +calcium sulfate Al2(SO4)3 + CaCl2 AlCl3 + CaSO4

Al: 2 Al: 1SO4: 3 SO4: 1Ca: 1 Ca: 1Cl: 2 Cl: 3

• 1. Total the number of each element for both sides• Look at subscripts to tell how many are in the unbalanced equation• All coefficients in an unbalanced equation are 1

Trick: Because SO4 is present as a polyatomic ion on both sides, it (sulfate) can be treated as one unit instead as S and O separately.

EXAMPLE: WRITE A BALANCED EQUATION

aluminum sulfate+ calcium chloride aluminum chloride +calcium sulfate Al2(SO4)3 + CaCl2 AlCl3 + CaSO4

Al: 2 ≠ Al: 1SO4: 3 ≠ SO4: 1Ca: 1 = Ca: 1Cl: 2 ≠ Cl: 3

2. Check if each element is the same on both sides- Aluminum, sulfate, and chlorine are unbalanced

EXAMPLE: WRITE A BALANCED EQUATION

aluminum sulfate+ calcium chloride aluminum chloride +calcium sulfate Al2(SO4)3 + CaCl2 AlCl3 + CaSO4

Al: 2 = Al: 1 2SO4: 3 ≠ SO4: 1Ca: 1 = Ca: 1Cl: 2 ≠ Cl: 3

3. Change the total on the inventory list so that the number of atoms matches.- I like to start with the first element on my inventory list. It doesn’t really matter which one you start with – but do one at a time!

EXAMPLE: WRITE A BALANCED EQUATION

aluminum sulfate+ calcium chloride aluminum chloride +calcium sulfate Al2(SO4)3 + CaCl2 2 AlCl3 + CaSO4

Al: 2 = Al: 1 2SO4: 3 ≠ SO4: 1Ca: 1 = Ca: 1Cl: 2 ≠ Cl: 3

4. Add a coefficient to the substance in the equation- 1 x coefficient = 2, therefore the new coefficient is 2. Now Al is balanced.

EXAMPLE: WRITE A BALANCED EQUATION

aluminum sulfate+ calcium chloride aluminum chloride +calcium sulfate Al2(SO4)3 + CaCl2 2 AlCl3 + CaSO4

Al: 2 = Al: 1 2SO4: 3 ≠ SO4: 1Ca: 1 = Ca: 1Cl: 2 ≠ Cl: 3 6

5. Adjust the inventory list to reflect any changes in other atoms- The new coefficient applies to both Al and Cl. According to the equation, there

are now 6 Cl in the products (2 from coefficient x 3 from subscript).

EXAMPLE: WRITE A BALANCED EQUATION

aluminum sulfate+ calcium chloride aluminum chloride +calcium sulfate Al2(SO4)3 + 3 CaCl2 2 AlCl3 + CaSO4

Al: 2 = Al: 1 2SO4: 3 ≠ SO4: 1Ca: 1 = Ca: 1Cl: 2 6 = Cl: 3 6

6. Move on to each atom until the lists match for EVERY element- Cl in the reactants is now unbalanced. There need to be 6. 2 x coefficient = 6, so the coefficient for CaCl2 needs to be 3. Now Cl is balanced.

EXAMPLE: WRITE A BALANCED EQUATION

aluminum sulfate+ calcium chloride aluminum chloride +calcium sulfate Al2(SO4)3 + 3 CaCl2 2 AlCl3 + CaSO4

Al: 2 = Al: 1 2SO4: 3 ≠ SO4: 1Ca: 1 3 = Ca: 1 3Cl: 2 6 = Cl: 3 6

6. Move on to each atom until the lists match for EVERY element- Changing the coefficient of CaCl2 means there are now 3 Ca. To balance calcium we need to change the number in the products.

EXAMPLE: WRITE A BALANCED EQUATION

aluminum sulfate+ calcium chloride aluminum chloride +calcium sulfate Al2(SO4)3 + 3 CaCl2 2 AlCl3 + 3 CaSO4

Al: 2 = Al: 1 2SO4: 3 = SO4: 1 3Ca: 1 3 = Ca: 1 3Cl: 2 6 = Cl: 3 6

6. Move on to each atom until the lists match for EVERY element- Calcium now balances! The new coefficient means there are three sulfates. Now sulfate balances!

Practice Problems___ C3H8 + ___ O2 ___ CO2 + ___ H2O

___ NaBrO3 ___ NaBr + ___ O2

___ NH4NO3 ___ N2O + ___ H2O

___ H2 + ___ CO + ___ O2 ___ H2CO3

___ La2O3 + ___ H2O ___ La(OH)3

___ NaI + ___ Pb(NO3)2 ___ NaNO3 + ___PbI2

1 5 3 4

2 2 3

1 1 1 1

1 3 2

2 1 2 1

1 1 2

Balance oxygen LAST!

Balance oxygen LAST! Make sure to account for both N in the reactants.

It was already balanced! Yay!

Balance oxygen LAST!

Treat the nitrate ion as one unit.

HINTS