Conservation of Mass and Conservation of Mass and ReactionsReactions

Objective 4

TEK 8 The student knows tat changes in matter affect everyday life.

(C ) The student is expected to investigate and identify the law of conservation of mass.

Chemical Equations: An expression in An expression in which symbols and formulas are used to which symbols and formulas are used to represent a chemical reaction.represent a chemical reaction.

sodium metal + chlorine gas sodium metal + chlorine gas table table saltsalt

((sodium chloride)sodium chloride)

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 2H20A chemical equation is

balanced when it reflects the conservation of mass and charge.

Law of Conservation of MassLaw of Conservation of Mass

Total mass of reactants =

Total mass of productsAntoine Lavoisier

Mass is neither created nor destroyed during chemical or physical reactions.

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.

Balancing Chemical EquationsThe 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

Step 2: Inventory all atoms found in the equation

H2SO4 + NaOH Na2SO4 + H2O

2 H + 1 H = 3H 2 Na1 S 1 S4 O + 1 O = 5 O 4 O + 1 O= 5 O 1 Na 2 H

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 Na2SO4 + H2O

2 H + 1 H = 3H 2 Na 1 S 1 S 4 O + 1 O = 5 O 4 O + 1 O=

5 O 1 Na 2 H

2 24H6O

4H6 O

2 Na

Equal Equa

l

Step 4: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are same.

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

2 H + 2 H = 4H 2 Na 1 S 1 S 4 O + 2 O = 6 O 4 O + 2 O= 6 O 2 Na 4 H

Step 5: Make sure the coefficients are reduced to their lowest whole-number value (ok here).

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

1 : 2: 1 : 2

Balance the following equations.

1. KClO3 → KCl + O2

2. P4 + O2 → P2O5

3. Al2O3 → Al + O2

4. Al2(SO4)3 + Ca(OH)2 → Al(OH)3 + CaSO4

5. Ca(OH)2 + H3PO4 → Ca3(PO4)2 + H2O

AnswersAnswers Balance the following equations.Balance the following equations.

1.1. 22 KClO3 → 22 KCl + 33O2

2. P4 + 55O2 → 22 P2O5

3.3. 22 Al2O3 →44 Al + 33O2

4. Al2(SO4)3 +33 Ca(OH)2→ 22Al(OH)3 + 33CaSO4

5.5. 33Ca(OH)2 + 22H3PO4 → Ca3(PO4)2 + 66H2O

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.

• Before and After Reaction Before and After Reaction Particles always react in the same ratio and they are

always conserved

• 5 molecules + 15 molecules 10 molecules of N2 of H2 of NH3

N2 + 3 H2 2 NH3

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?

Water from a Camel fat

2 C57H110O6(s) + 163 O2(g) 114 CO2(g) + 110 H2O(l)1000 g C57H110O6 + 2930 g O2 2817g CO2 + ? H2O

1000 g C57H110O6 + 2930 g O2 2817g CO2 + ? H2O

Mass of reactants =

Remember that mass is conserved. Therefore the mass of reactants = mass of products

Mass of products

1000 g + 2930 g

= 3930 g

= 3930 g

– 2817g

= 1113 g H2O

Water in Space

In the space shuttle, the CO2 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) + 2 LiOH(s) Li2CO3(aq) + H2O(l) 880 gCO2 + 956 g LiOH 1476 g Li2CO3 + ? H2O

Remember that mass is conserved. Therefore the mass of reactants = mass of products

880 gCO2 + 956 g LiOH 1476 g Li2CO3 + ? H2O Mass of reactants = Mass of products

880 g + 956 g

= 1836 g

= 1836 g - 1476 g

= 360 g H2O

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?

C2H5OC2H5 + 6 O2 4 CO2 + 5 H2O

33.8 g C2H5OC2H5 + 87.7 g O2 ? g CO2 + 41.2 g H2O = 80.3 g CO2

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.

                                    

4 NH3 + 3 O2 2 N2+ 6 H2O

To produce 12 molecules of water, the flask must have how many molecules of ammonia (NH3)?

8 molecules of NH3

8 NH3 + 6 O2 4 N2 + 12 H2O (the ratio will still be 4:3:2:6)

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)

56.2 g H2O

25.0 g + 100.0g = 68.8 g + ?

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 particles of P4

15 P4 + 75 O2 30 P2O5 (ratio is still 1: 5: 2)