12 to TAKS topic 12

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12 to TAKStopic 12
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• Objective 4 8c Law of Conservation of Mass
• 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