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When chemicals react, they break apart and form new chemicals
Chemical Reactions
Physical: no new substances are formed• Melting, boiling, breaking, cutting,
ripping, dissolving
Chemical:
• totally new substances are formed
Types of Changes
• Bubbling – gas formed
• Turns cloudy – precipitate formed
• Color change
• Smell
• Temperature change• Exothermic – heat given off => the reaction
gets hot• Endothermic – heat taken in => the reaction
gets cold
Evidence of a chemical change
Chemical Equation
2H2 + O2 → 2H2O
Before After
Reactants
Products
Coefficients: # of molecules/compounds
Subscripts: # of atoms/ions in the formula
Example: 2Li3N = Li3N + Li3N
6 Li atoms and 2 N atoms
Example:Be(NO3)2 = Be(NO3)
(NO3)
1 Be atom, 2 N atoms, 6 O atoms
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.
HH
HH O O
Reactants Products
A chemical equation identifies the starting and ending chemicals as reactants and products:
reactants products
Formation of water
2H2 + O2 2H20
A chemical equation is balanced when it reflects the conservation of mass.
Graphic: http://www.chemistry.ohio-state.edu/betha/nealChemBal/
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
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
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
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
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
Reactants Products
27.0 g HgO
? g Hg
2.0 g O2
27 g HgO = 2 g O2 + ? Hg =25 g Hg
Which of the following chemical reactions indicate that mass in conserved?A. Mg + Cl2 2 MgCl
B. 2 Ca + O2 2 CaO
C. Zn + S 2 ZnS
D. C + O2 2 CO
The correct answer is B
CaCa
OO
OOCa
Ca
Because atoms must be equal on each side of the equation:
Mg + HCl → MgCl2 + H2
When the above equation is balanced, the coefficient for magnesium is —
A 0
B 1
C 2
D 4
1 Mg + 2 HCl → MgCl2 + H2
The correct answer is B
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 Rust100 g + ?g 143g
What mass of
oxygen is needed?
Mass of the reactants = Mass of the products
(also # of atoms before = # of atoms after)
2H2 + O2 → 2H2O 2 x 2 H atoms + 2 O atoms = 2 (2 H atoms + 1 O atom)
4 x 1 = 4 amu + 2 x 16 = 16 amu = 2(2x1 amu + 16 amu)
36 = 36 amu
Conservation of Mass
Open vs. Closed systems
Na + N2 → Na3N
MgCl2 + Li2O → MgO + LiCl
Practice
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