Unit 6 Reactions Notes
Balancing Equations
• Coefficient: tells the number of molecules – CAN BE CHANGED
• Subscript: tells the number of atoms of each element
– CAN NOT BE CHANGED
• Example:
3H20
3 molecules of water 6 atoms of hydrogen 3 atoms of oxygen
• Reactants : are what start with at the beginning of reaction • Products : are what end with after the reaction has taken place • Example
Law of Conservation of Mass
• Matter can not be created or destroyed
• # of atoms of each element in reactants MUST EQUAL # of atoms of each element in the products
• Balance the equation by adding Coefficients ONLY
• Example
C6H12O6 + 6O2 à 6CO2 + 6H2O
Diatomic Elements
• Diatomic molecule: a molecule containing only 2 atoms
• Some elements are never found as a single atom, these elements bond with another atom of the same element to fill their outer energy level with electrons • These are called diatomic elements
Lewis Dot Structures of Diatomic Elements
Memorize these Diatomic Elements
H2 N2 O2 F2 Cl2 Br2 I2
Review of Ions
In ionic compounds
Cation, positive ion, is written first Anion, negative ion, is written second Polyatomic Ions
A group of atoms that are covalently bonded together but have an overall charge If there is more than one of the polyatomic ions, remember to put parenthesis
around the group Example:
Ca+2 bonded with OH- is written as Ca(OH) 2
You should be able to recognize these polyatomic ions
Polyatomic Ions
Ammonium NH4+ Nitrite NO2
-Nitrate NO3- Sulfite SO3
2-Sulfate SO42- Hydroxide OH
-Cyanide CN- Phosphate PO
4
3-Carbonate CO32- Hypochlorite ClO
-Chlorite ClO2- Chlorate ClO3
-Perchlorate ClO4- Permanganate MnO4
-Chromate CrO4
2-Swap and Drop
Is a method to write an ionic compound so the overall charge is neutral The overall positive charge must equal the overall negative charge. Example:
Ca+2 bonds with F- written as CaF 2
Reverse Swap and Drop
Is a method to determine the charge of the individual ions of an ionic compound Example:
Fe2O3
The iron ion has a +3 charge: Fe+3
Reverse Swap and Drop Doesn’t always work
You must know the charge of one of the ions in the compound Then work backwards
Example: Fe2O3
You know that oxygen forms a -2 ion There are 3 oxygen ions (subscript)
The total charge for all oxygen ions is -6 (2 x -3 = -6)
In order for the overall charge of the compound to be neutral the overall Positive charge must be +6
There are 2 Fe ions (subscript)
Therefore each Fe ion must be +3 (6 ÷ 2 = 3)
Acids
Acids are always aqueous, (aq), which means they are dissolved in water Acids are ionic compounds between H+ and an anion
Be able to recognize and use the names and formulas for the following acids HCl hydrochloric acid H+ and Cl
-HBr hydrobromic acid H+ and Br
-HNO3 nitric acid H+ and NO3
-HClO4 perchloric acid H+ and ClO4
-H2SO4 sulfuric acid H+ and SO4-2
Water is actually made up of Hydrogen ion H+
Hydroxide ion OH
-= HOH, H2O
Types of Chemical Reactions
• There are thousands of different chemical reactions.
• Often times we need to predict the products of a chemical reaction • It is useful to classify reactions according to similarities and regularities • Several different ways to classify reactions
• We will learn one way that classifies 5 different types of reactions
5 Types of Reactions
1. Synthesis 2. Decomposition 3. Single Replacement 4. Double Replacement 5. Combustion
1. Synthesis
A + B à AB Example:
Fe + O2 à
-Write charges for each reactant: Fe+3 O
2--Write product, swap and drop: Fe2O3
-Balance equation:
2. Decomposition
AB à A + B Example:
CaCO3 à
-Break up the reactant into the 2 ions that make it up: Ca2+ CO 3
2--Write them as products -Balance equation:
CaCO3 à Ca + CO3
3. Single Replacement
A + BX à AX + B Example: Mg + HCl à
-Write the charges for each ion Mg+2 H+ Cl
--If there is a lone cation, swap it with the cation in the compound. MgCl2
-If there is a lone anion, swap it with the anion in the compound -Write the new lone cation or anion
H2 (diatomic)
-Balance the equation
Mg + 2HCl à MgCl2 + H2
4. Double Replacement
AX + BY à AY + BX Example: Pb(NO3)2 + KI à
-Write the charges for each ion Pb+2 NO
3- K+ I
--Switch partners
-the cation in one compound joins with the anion in the other compound and visa versa
Pb2+ I
-K+ NO 3
--Balance the charges of the ions by swap and drop PbI2 KNO3
-Write as products and balance equation Pb(NO3)2 + 2KI à PbI2 + 2KNO3
5. Combustion
CxHy + O2 à CO2 + H2O + Energy
Example:
CH4 + O2 à
-The reactants are always a carbon, hydrogen compound and oxygen -The products are always carbon dioxide, water and energy
-Balance equation
Will the Reaction Actually Happen?
Just because a reaction can be written on paper does not mean that it will actually occur in the lab.
There are 2 sets of rules to determine if the reaction will actually happen
Activity Series is used to determine if single replacement reactions will occur
Solubility Rules are used to determine if double replacement reactions will occur
Activity Series and Single Replacement Reactions
In Single Replacement reactions either
A single cation is replacing a cation in a compound OR A single anion is replacing an anion in a compound
The reaction will occur if the single ion that is trying to replace the ion in the compound is more reactive.
The higher the element is on the Activity Series List, the MORE reactive it is. A + BX à AX + B
Will only happen if A is higher than B on the Activity Series, otherwise no reaction Example:
K + MgBr2 à KBr + Mg
K is trying to replace Mg Like replaces like
Cations replace cations, anions replace anions
K is higher on the Activity Series than Mg, therefore it is more reactive YES this reaction will occur
Example:
Cu + HCl à CuCl2 + H2
Cu is trying to replace H
Cu is lower on the Activity Series than H, therefore it is less reactive NO RXN
Activity Series
Metals Decreasing Activity as
Move Down the Table
Halogens
Lithium Fluorine
Potassium Chlorine
Calcium Bromine
Sodium iodine
Magnesium Aluminum Zinc Chromium Iron Nickel Tin Lead
HYDROGEN* Copper
Solubility Rules and Double Replacement Reactions
Double Replacement reactions will only occur if at least one of the products is a solid
Use the Solubility Rules to determine if the product is a solid Not Soluble = a solid (s)
Soluble = dissolved in water (aq)
Solubility Rules Compounds containing these ions are
SOLUBLE in water (aq)
. . . unless they also contain these ions, which make them NOT SOLUBLE(s)
Ammonium NH4+
Potassium K+
Sodium Na+
Acetate C2H3O2- Fe3+, Al3+, Hg22+
Chlorate ClO3
-Chloride Cl- Ag+, Hg22+, Pb2+
Nitrate NO3
-Sulfate SO42- Ca2+, Ba2+, Pb2+, Sr2+, Hg22+
Compounds containing these ions are
NOT SOLUBLE in water (s)
. . . unless they also contain these ions, which make them SOLUBLE (aq)
Carbonate CO32- K+, Li+, Na+, NH4+
Hydroxide OH- K+, Li+, Ba2+
Oxide O-2
Phosphate PO43- K+, Na+, NH4+
Silicate SiO32- K+, Na+
Sulfide S2- K+, Na+, NH
4+
Sulfite SO32- K+, Na+, NH4+
Example:
Na2CO3 + Ca(OH)2 à NaOH + CaCO3
This reaction will occur if NaOH or CaCO3 or both are solids
NaOH is aq CaCO3 is a solid
Chemical Equilibrium
Theoretically every reaction can proceed in two directions Forward à and Reverse
Therefore all chemical reactions are considered reversible under suitable conditions.
Reversible reaction: a chemical reaction in which the products can react to re-form the reactants.
Example:
Chemical Equilibrium: when the RATE of the forward reaction equals the RATE of the reverse reaction and
the concentration of the reactants and products remain unchanged. -At chemical equilibrium the amount of products does not equal
the amount of reactants.
-Chemical equilibrium is talking about the RATE of the reaction.
Shifting Equilibrium
A system will remain at equilibrium until something occurs to change this condition. Henri Louis Le Chatelier was a chemist who developed a principle that allows us to
predict how a change in conditions will affect a system at equilibrium.
LeChatelier’s principle: When a system at equilibrium is disturbed by a stress, the system will attain a new equilibrium that minimizes the stress.
A stress that causes an equilibrium shift can be 1. pressure
2. change the concentration 3. temperature
If a stress happens, the reaction will be OUT of equilibrium
Then the reaction will shift to favor either the forward reaction OR the reverse reaction to get back into equilibrium.
Concentration
If you change the amount of reactants or products then the equilibrium will shift. Changing the amount of reactants
If you increase the amount of reactants, the reaction will shift to the right (forward)
If you decrease the amount of reactants, the reaction will shift to the left (reverse)
Changing the amount of products
If you increase the amount of products, the reaction will shift to the left (reverse)
If you decrease the amount of products, the reaction will shift to the right (forward)
Temperature
If the reaction is exothermic, heat is a product
If you increase the temperature, the reaction will shift to the left (reverse) If you decrease the temperature, the reaction will shift to the right (forward) If the reaction is endothermic, heat is a reactant
Pressure
Changing pressure affects equilibrium in which gases are involved. Increasing pressure, decreases volume
The reaction has less space, it will have to make less stuff
Reaction will shift to favor the side of the equation with less total molecules Decreasing pressure, increases volume
The reaction has more space, it will make more stuff
Reaction will shift to favor the side of the equation will more total molecules
Equilibrium Constant (K)
Ratio of the products (concentrations multiplied) over the ratio of the reactants (concentrations multiplied)
nA + mB ↔ xC + yD
-K is determined experimentally. A chemist measures the concentration of the reactants and products of a reaction at a certain temperature. -If K is a large number then the reactants are almost all converted to products
before equilibrium is reached.
-If K is a small number then the forward reaction happens only slightly before equilibrium is reached.
Pure solids and pure liquids are not included in calculating K. Because their concentrations do not change.
Example:
An equilibrium mixture of N2 , O2 and NO gases at 1500 K is determined to
consist of 6.4 x 10-3 mol/L of N
2, 1.7x10-3 mol/L of O2, and 1.1 x 10-5 mol/L of NO. What
is the equilibrium constant for the system at this temperature? The chemical equations is
