1. Combustion Reactions

2. What are combustion reactions?
A combustion reaction occurs when a compound that contains carbon and hydrogen is burned in Oxygen gas (O2). It always produces carbon dioxide and water vapor.
Example:
CH4 + 2 O2  CO H2O
Many times the compound also contains oxygen
2C3H4O3 + 5 O2  6 CO H2O
Notice the products remain the same.

3. Using a combustion reaction to find empirical formula.
Vitamin C is a compound made up of carbon, hydrogen and oxygen atoms. When we combust 1.00g of vitamin C we produce 1.50g of carbon dioxide and 0.408g of water vapor. Find the empirical formula of vitamin C.
#1 We know that it is a combustion reaction so:
CxHyOz + O2  CO2 + H2O
#2 We notice that all the carbon atoms from vitamin C became part of carbon dioxide so;
1.50g CO2 x 1 mole CO2 / 44 g = moles of CO2
Since there is only 1 atom of carbon in CO2 we have moles of C

4. #3 We have 0.408 g of H2O so all the hydrogen atoms from vitamin C became part of the water vapor.
0.408 g H2O x 1 mole H2O / 18 grams = moles of H2O
Notice thee are TWO atoms of hydrogen in H2O so;
We have 2x = moles of Hydrogen
#4 We now go back to Vitamin C, CxHyOz
We know we have:
moles of C x 12g/1 mole of C= .409g of C
moles of H x 1g / 1mole of H = .0454g of H
Oxygen = ( )= .546 g of O

5. #5 We now know the grams of each of the elements that make up vitamin C
C = .409g x 1mole C/ 12g = moles
H = g x 1mole H/ 1g = moles
O = g x 1mole O/ 16 g = moles
Divide all moles by smallest answer
C= .0341/.0341 =1
H= .0454/.0341 = 1.33
O = .0341/ .0341= 1
Multiply all answers by 3 and we get
C3 H4 O3

6. Homework 1.Balance the following reactions
A) CaO + HNO3  Ca(NO3)2 + H2O
B) NH3 + H2SO4  (NH4)2SO4
2.Given the following reaction, find the grams of H2O are produced
2C6H O2  12 CO H2O
You start with 125 grams of C6H6 and 875 g of O2
3. Find the empirical formula of a compound that is made up of C,H and O. When you combust 2.00g of the compound in excess oxygen gas you produced 3.83g of CO2 and 2.35g of H2O