law of conservation of mass 1
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Chemical Reactions
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Lesson 1. The Law of Conservation of Mass
In the late eighteenth century, Antoine Lavoisier, a French chemist, recognized the importance of accurate
measurements. He extensively studied and explained the nature of
combustion. He found out that combustion involved
reaction with oxygen.
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Law of Conservation of Mass
Total mass of reactants
= Total mass of
products
Antoine Lavoisier
Mass is neither created nor
destroyed during chemical or
physical reactions.
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Reactants are the starting material in a chemical reaction.
Products are the substance formed as a result of a chemical
reaction.
In a chemical equation, reactants are found on the left side and the products are on the right side. A chemical reaction can therefore be summarized as:
Reactants → Products
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CHEMICAL REACTION
sodium metal + chlorine gas ---- table salt
(sodium chloride)
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The Law of conservation of mass states that matter cannot be created or destroyed in any
chemical reaction The atoms in the reactants are
rearranged to form new compounds, but none of the atoms disappear, and no new atoms are
formed.H
H
HH O O
Reactants Products
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The same number of each kind of atom must be on the left side of the arrow as are on
the right side when an equation is balanced.
Al Al
Al Al
O O
O OO O
Al AlOO O
Al AlOO O
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Oxygen, as a product, must be multiplied by 2. A coefficient is placed in front of the compounds that contains O.
Oxygen atoms are =When a coefficient is added you must
multiply all atoms in the compound by this
number. Redo the inventory
After redoing the inventory, add coefficients to balanced other atoms. With each additional coefficient the inventory must be updated.
Oxygen atoms are≠
Hydogen atoms are =
2 H atoms 2 H atoms
2 O atoms 1 O atom
Hydogen atoms are ≠Oxygen atoms are =
First: Inventory the atoms on both sides of the yield arrow
Balancing an equation: H2 + O2 H2O
HH
HO O O
H
Second: Determine which atoms are not equal and add coefficients to balance these atoms.
2
4 H atoms
2 O atoms
2
4 H atoms
2 O atoms
Hydogen atoms are =
This is now a balanced equation
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So: Chemical equations are balanced, when the numbers and kinds of atoms on each
side of the reaction arrow are equal.
HH
HH O O
Reactants Products
HH
HH
OO
4 Hydrogen 4 Hydrogen
2 Oxygen 2 Oxygen
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Because atoms are conserved, the mass of the reactants in the chemical reaction is equal to the mass of the products.
H2 + F2 2 HF
2g + 38g = 2( 20g) 40g = 40g
40 g 40 g
Reactants Products
H H F F H F H F
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The Law of Conservation of Mass allows the calculation of the reactants
or products.
If 27.0 g of mercuric oxide (the red solid compound) is heated and
completely decomposes to give the elements oxygen and mercury. 2.0 g of oxygen are produced. How
many grams of mercury are produced?
mercuric oxide mercury + oxygenHgO Hg + O2
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Reactants Products
27.0 g HgO
? g Hg
2.0 g O2
27 g HgO = 2 g O2 + ? Hg =25 g Hg
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Which of the following chemical reactions indicate that mass in conserved?
A. Mg + Cl2 2 MgClB. 2 Ca + O2 2 CaOC. Zn + S 2 ZnSD. C + O2 2 CO
The correct answer is B
CaCa
OO
OOCa
Ca
Because atoms must be equal on each side of the equation:
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Mg + HCl → MgCl2 + H2
When the above equation is balanced, the coefficient for magnesium is —
A 0B 1C 2D 4
2 Mg + 2 HCl → MgCl2 + H2
The correct answer is C
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Suppose 100 g of iron metal rusts. We weigh the rust and find that the rust has a mass of 143 g. What mass of oxygen reacted with the iron?
A. 243 g B. 57 g C. 100 gD 43 g
43 g Oxygen Choice D
Iron + Oxygen Rust 100 g + ?g 143g What mass of oxygen is needed?