Download Chemical Equations and Reactions

Chemical Equations and Reactions
Chemical Reaction: one or more substances are changed
into one or more different substance
Original substances- reactants
New stuff- products
Indication of a Chemical Reaction:
1) Evolution of heat or light
2) Production of a gas
3) Formation of a precipitate (a solid that is produced
during a chemical reaction
4) Color change
5) Odor production
Chemical Equations
• The law of conservation of mass must be satisfied
• Coefficients: small whole number that appears in front
of a formula in a chemical equation
• Word equation:
methane + oxygen
carbon dioxide + water
• Formula equation:
CH4(g) + O2(g)
CO2(g) + H2O(g) (not balanced)
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g) (balanced)
• Balancing: make sure that the number of moles of an
element are equal on both sides of the equation
• Catalyst: substance that a changes the rate of a
chemical reaction but can be recovered unchanged.
Symbols used in Chemical Equations
Yeilds
reversible reaction
(s) a solid
(l) a liquid
(g) a gas
(aq) aqueous solution (dissolved in water)
a gas
a precipitate
heat
reactants are heated D
MnO4 formula of a catalyst used to alter the rate
2 atm pressure at which the reaction is carried out
0C
temperature at which the reaction is carried out
H2
+
Cl2
2HCl
1 mol H2
1 mol Cl2
2 mol HCl
2.02 g H2
70.90 g Cl2
2 x 36.46 = 72.92 g HCl
Diatomic molecules: H2, F2, Cl2 , Br2, I2, O2, N2
Balancing:
1) write out word equation is the problem is a written
2) balance atoms one at a time
3) first balance atoms that are combined into a formula
4) then balance polyatomic ions
5) Balance H atoms and O atoms after all other elements
are balanced
6) Check the number for all atoms on both sides!! They
have to be equal
Types of Chemical Reactions
Combustion Reactions: substance combines with
oxygen, releasing a large amount of energy (light/ heat)
- Carbon Compounds + O2 : always get CO2 + H2O
C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)
Synthesis Reaction: (composition reaction) two or more
substances combine to form a new compound.
A + X
AX
• Reactions of elements with Oxygen and Sulfur:
- 2Mg(s) + O2(g)
2MgO(s)
• Metals and Halogens :
- 2Na(s) + Cl2(g)
2NaCl(s)
• Metal Oxides and Water:
CaO(s) + H2O(l)
Ca(OH)2(s)
Decomposition Reaction: single compound breaks down
into two or more simpler substances
- Opposite of synthesis reaction
- usually only takes place if energy (electricity/ heat) is
added
AX
A + X
• Electrolysis: decomposition of a substance by an
electric current
• If H in the compound you get H2O
If C in the compound you will get CO2
If O and no H or C, then you get O2
•
H2SO3
SO2 + H2O
Single Replacement (displacement) Reaction: when one
element replaces a similar element in a compound
A + BX
AX + B
Y + BX
BY + X
• Mg(s) + 2HCl(aq)
H2(g) + MgCl2(aq)
• Group 1 & 2 metals + H2O
metal hydroxides + H2(g)
• d-block metal + H2O
metal oxides + H2(g)
Double Replacement Reactions: ions of 2 compounds
exchange places in an aqueous solution to form new
AX + BY
AY + BX
• FeS(s) + 2HCl(aq)
H2S(g) + FeCl2(aq)
Solubility Rules
1. Alkali metals (gr. 1) and Ammonium compounds are soluble in water (aq).
2. Nitrates, acetates, and chlorates are soluble (aq).
3. Most chlorides, bromides, and iodides are soluble (aq), except those of
silver, mercury (I), and lead (s)
4. Most sulfates are soluble (aq), except those of barium, strontium, calcium,
lead, and mercury (s).
5. Most carbonates, phosphates, and sulfites are insoluble (s), except those of
alkali metals (gr. 1) and ammonium (aq).
6. Most sulfides are insoluble (s), except those of calcium, strontium, alkali
metals (gr.1) and ammonium (aq).
7. Most oxides are insoluble (s), except those of calcium, barium, and alkali
metals (gr. 1) and ammonium (aq)
8. Most hydroxides are insoluble (s), except those of barium, and alkali
metals (gr. 1) and ammonium (aq)
Solubility Examples
Ex: Predict compounds, solubility, and then balance
1. (NH4)2S (aq) + Cd(NO3)2 (aq) 
2.
Mercury (II) chloride (aq) + potassium sulfide (aq) 
3.
Sodium carbonate (aq) + calcium chloride (aq) 
Activity Series
• Activity: ability of an element to react
• the more an element reacts with other substances, the
greater the activity is.
• Metals: the greater the activity, the greater it loses
electrons (to form cations)
• Non-metals: the greater the activity, the greater it gains
electrons (to form anions)
• Activity series: a list of which elements a particular
element can replace in a single replacement reaction
- predicts whether or not a chemical reaction will occur
2Al(s) + 3ZnCl2(aq)
3Zn(s) + 2AlCl3(aq)
Co(s) + 2NaCl(aq)
no reaction
React vigorously
with Acids to
produce H2 gas
Li
K
MOST REACTIVE
Ba
Ca
Na
React vigorously with liquid
water to form H2 gas
Mg
Al
Zn
React with Acids to
produce H2 gas
Cr
Fe
Cd
Co
React slowly with liquid water but
readily with steam to form H2 gas
Ni
Sn
Pb
H2
Do not react with
Acids
Cu
Hg
Ag
Au
LEAST REACTIVE
Oxidation Numbers
- used for covalent bonds and instead of charges (ions)
1) Atoms of a pure element have an oxidation number
“0”. Pure Na or O2 have an oxidation # equal to “0”
2) More electronegative element has oxid # = to its
charge (if it were an anion).
Least electronegative element has oxid # = to its
charge (if it were a cation).
3) Oxygen is -2, except when it is O2, then it is -1
4) Hydrogen is +1, except when with a metal, it is -1
5) the sum of the oxid. #s is equal to zero (neutral),
except for a polyatomic ion, when the sum is equal to
the ion’s overall charge. (NO3-1)
Redox Reactions
• Reduction and Oxidation Reactions
• Reduction Reaction: gain electrons
– Gaining electrons results in a more negative charge and
therefore reduced! GER
– Cl2 + 2e2Cl- , the chlorine diatomic molecule goes
from oxidation state 0 to a chlorine ion with a –1 charge
• Oxidation Reaction: lose electrons
– Losing electrons results in less negative charge LEO
– Increase in oxidation state (charge) because more positive (ie.
Less negative)
– Na
Na+ + e- , sodium goes from oxidation state 0 to
a sodium ion with a +1 charge
• Reduction: when an atom/molecule turns
into a negative ion it is reduced, made more
negative.
• Oxidation: when an atom of/molecule turns
into a positive ion it is oxidized, made more
positive.
0
0
• H2 + Cl2
+1 -1
2HCl
• 2H2O + 2Cl2  4HCl + O2
• Academic: look in book for examples and
go over on board.
Stoichiometry
Grams X
(molar mass)
mol X
mol X
(mol ratio)
mol Y
mol Y
(molar mass)
grams Y
• Mole ratio: conversion factor that relates the amounts
in moles of any two substances involved in a
chemical reaction
• 2Al2O3(l)
4Al(s) + 3O2(g)
2 mol Al2O3
or
4 mol Al
4 mol Al
2 mol Al2O3
3 mol O2
or
2 mol Al2O3
2 mol Al2O3
3 mol O2
Limiting Reactants
• The reactant that limits the amounts of the other
reactants that can combine in a chemical reaction
(it also limits the amount of product that can form)
• look at moles, not grams
Excess reactant: The substance that is not used up
• To determine the limiting reactant- must convert to
moles
– moles for all substances must be for the same
reactant
– convert moles of Y to moles of X, then compare to
original moles of X
Percent Yeild
• The ratio of the actual yield to the theoretical yield
• percent yield = actual yield
x 100
theoretical yield
• Given: the actual yield of a product
the mass (grams) of some reactant
1) first change the mass of reactant to grams of
product using molar mass, then mole ratio, then
molar mass
2) This will be theoretical yield