Conservation of Mass and Reactions

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Conservation of Mass and Reactions
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• 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.

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

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• The meaning of chemical equations
• A mathematical equation x2x3x

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.
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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
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• 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.
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• 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

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Step 2 Inventory all atoms found in the equation
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
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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
2
4H
6O
6 O
4H
2 Na
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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
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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
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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

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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

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

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

Mass of reactants
Mass of products
3930 g
2817g
1000 g
2930 g
1113 g H2O
3930 g
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Water in Space
Your Turn!
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
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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
1836 g
- 1476 g
880 g
956 g
1836 g
360 g H2O
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• 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
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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.

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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 N2 12 H2O (the ratio will
still be 4326)
8 molecules of NH3
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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 100.0g 68.8 g ?
56.2 g H2O
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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 P4 75 O2 ? 30 P2O5 (ratio is still 1 5 2)
15 particles of P4