1. 12 Weeks to TAKS Week 12

2. Objective 4: 8c: Law of Conservation of Mass
The student is expected to investigate and identify the law of conservation of mass.

3. Chemical Equations: An expression in which symbols and formulas are used to represent a chemical reaction.
sodium metal + chlorine gas  table salt
(sodium chloride)

4. The meaning of chemical equations
A mathematical equation: x+2x=3x
A chemical equation identifies the starting and finishing chemical as reactants and products: reactants products
Example: Formation of water
2H2 + O2  2H20
A chemical equation is balanced when it reflects the conservation of mass and charge.

5. Law of Conservation of Mass
Mass is neither created nor destroyed during chemical or physical reactions.
Total mass of reactants =
Total mass of products
Antoine Lavoisier

6. The Law of conservation of mass states that matter cannot be created or destroyed in any chemical reaction
The bonds between atoms in the reactants are rearranged to form new compounds, but none of the atoms disappear, and no new atoms are formed.
So: Chemical equations must be balanced, meaning the numbers and kinds of atoms must be the same on both sides of the reaction arrow.
The numbers placed in front of formulas to balance equations are called coefficients, and they multiply all the atoms in the chemical formula.

7. Balancing Chemical Equations
The following five steps can be used as a guide to balance chemical equations.
Balance this chemical reaction.
Sulfuric Acid reacts with sodium hydroxide to yield sodium sulfate and water
Step 1: Write an unbalanced equation, using correct formulas for all reactants and products.
H2SO4 + NaOH  Na2SO4 + H2O

8. Step 2: Inventory all atoms found in the equation
H2SO4 + NaOH  Na2SO4 + H2O
2 H + 1 H = 3H Na
1 S S
4 O + 1 O = 5 O O + 1 O= 5 O
1 Na H

9. Step 3: Compare the number of each atom on each side of the equation
Step 3: Compare the number of each atom on each side of the equation. Add coefficients to balance the number of atoms. Remember that adding a coefficient affects all elements in the compound.
H2SO4 + NaOH  Na2SO H2O
2 H + 1 H = 3H Na
1 S S
4 O + 1 O = 5 O O + 1 O= 5 O
1 Na H 2 2 4H
Equal
Equal 6O
6 O 4H
2 Na

10. Step 4: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are same.
H2SO NaOH  Na2SO H2O
2 H + 2 H = 4H Na
1 S S
4 O + 2 O = 6 O O + 2 O= 6 O
2 Na H

11. Step 5: Make sure the coefficients are reduced to their lowest whole-number value (ok here).
H2SO4 + 2 NaOH Na2SO H2O
1 : : :

12. Your Turn! Balance the following equations. KClO3 → KCl + O2
P4 + O2 → P2O5
Al2O3 → Al + O2
Al2(SO4)3 + Ca(OH)2 → Al(OH)3 + CaSO4
Ca(OH)2 + H3PO4 → Ca3(PO4)2 + H2O

13. Answers Balance the following equations. 2 KClO3 → 2 KCl + 3O2
P4 + 5O2 → 2 P2O5
2 Al2O3 →4 Al + 3O2
Al2(SO4)3 +3 Ca(OH)2→ 2Al(OH)3 + 3CaSO4
3Ca(OH)2 + 2H3PO4 → Ca3(PO4)2 + 6H2O

14. What do coefficients mean?
1. They indicate the number of particles of atoms, molecules, and formula units found in the reaction
2. They are used to determine the amount of reactants and products.
2 H2 + O2  2 H2O
Indicates that 2 molecules of hydrogen react with 1 molecule of oxygen to produce 2 molecules of water.

15. Before and After Reaction
Particles always react in the same ratio and they are always conserved
5 molecules + 15 molecules  10 molecules
of N of H of NH3
N H  2 NH3

16. Water from a Camel fat
Camels store the fat, tristearin (C57H110O6), in the hump. As well as being a source of energy, the fat is a source of water, because when it is used the reaction takes place. Given the following information, what mass of water can be made from 1000 g of fat?
2 C57H110O6(s) O2(g)  114 CO2(g) H2O(l)
1000 g C57H110O g O2  2817g CO2 + ? H2O

17. = 3930 g Mass of reactants = Mass of products = 3930 g – 2817g 1000 g
Remember that mass is conserved. Therefore the mass of reactants = mass of products
1000 g C57H110O g O2  2817g CO2 + ? H2O
Mass of reactants =
Mass of products
= 3930 g
– 2817g
1000 g g
= 1113 g H2O
= 3930 g

18. Your Turn! Water in Space
In the space shuttle, the CO that the crew exhales is removed from the air by a reaction within canisters of lithium hydroxide. On average, each astronaut exhales about 880 g of CO2 daily. What mass of water will be produced when this amount of CO2 reacts with 956 g of LiOH according to the following equation?
CO2(g) LiOH(s)  Li2CO3(aq) + H2O(l)
880 gCO g LiOH  1476 g Li2CO3 + ? H2O

19. Remember that mass is conserved
Remember that mass is conserved. Therefore the mass of reactants = mass of products
880 gCO g LiOH  1476 g Li2CO3 + ? H2O
Mass of reactants = Mass of products
= 1836 g g
880 g
+ 956 g
= 1836 g
= 360 g H2O

20. This demonstration is the combustion of diethyl ether in air. If 33
This demonstration is the combustion of diethyl ether in air. If 33.8 g of diethyl ether is added to the balloon, how many grams of carbon dioxide are given off?
C2H5OC2H O  4 CO H2O
33.8 g C2H5OC2H g O2 ? g CO g H2O
= 80.3 g CO2

21. If this experiment was done in your classroom, why would it be difficult to prove the law of conservation of mass?
C2H5OC2H5 (l)+ 6 O2 (g)  4 CO2 (g) + 5 H2O (g)
CO2 and H2O are gases and they would move throughout the room.

22. To produce 12 molecules of water, the flask must have how many molecules of ammonia (NH3)?
                                    
4 NH3 + 3 O2 2 N2+ 6 H2O
8 NH3 + 6 O2  4 N H2O (the ratio will still be 4:3:2:6)
8 molecules of NH3

23. Methane gas is burned in excess oxygen to produce carbon dioxide & water. If 25.0 grams of methane is burned in 100. g of oxygen (O2) and 68.8 g CO2 are produced, how many grams of water is produced?
CH4 (g) + 2 O2 (g)  2 H2O (l) + CO2 (g)
25.0 g g = 68.8 g + ?
56.2 g H2O

24. Phosphorus reacts with oxygen to produce diphosphorus pentoxide according to the equation:
P4 (s) + 5 O2 (g)  2 P2O5
How many particles of phosphorus must be present to produce 30 molecules of P2O5?
15 P O2  30 P2O5 (ratio is still 1: 5: 2)
15 particles of P4