Download Chemical Reactions

Unit 6 – Chemical Reactions
Chemical equations, Molar mass,
Rates of reactions, Limiting
reactants
Intro Vocabulary
Chemical reaction is the changing of substances
to other substances by breaking bonds in
reactants and forming new bonds in the products.
-when some chemicals come into contact, they
break apart, join, or rearrange to form new
chemicals (always to become more stable)
-produce new substances with new properties
Chemical equations are shorthand
representations of chemical reactions.
Some more vocabulary
Reactants are the elements or compounds
that enter into a reaction
Products are the elements or compounds
that are formed as a result of a chemical
reaction
Arrow () means yields, produces or
forms
Reactant(s) yield product(s)
A + B  AB
Al2(SO4)3 + Ca(OH)2  Al(OH)3 + CaSO4
Skeleton equation (Unbalanced equation)
Consists of symbols and subscripts
 Symbols: element, yield (), combining (+)
 Subscript: small number found below the
element symbol representing the number of
atoms of each element present
Al2(SO4)3 + Ca(OH)2  Al(OH)3 + CaSO4
Name the reactants (everything left of arrow)
 Aluminum sulfate & Calcium hydroxide
Types & number of atoms in each reactant
 Aluminum sulfate
Al = 2
S=3
O = 12
 Calcium hydroxide
Ca = 1
O=2
H=2
Al2(SO4)3 + Ca(OH)2  Al(OH)3 + CaSO4
Name the products (everything right of arrow)
 Aluminum hydroxide & Calcium sulfate
Types & number of atoms in each reactant
 Aluminum hydroxide
Al = 1
O=3
H=3
 Calcium sulfate
Ca = 1
S=1
O=4
Al2(SO4)3 + Ca(OH)2  Al(OH)3 + CaSO4
Notice that the number of atoms is not
the same on both sides of the equation.
Al = 2 S = 3 O = 14 Ca = 1 H = 2
On the reactant side
Al = 1 S = 1 O = 7 Ca = 1 H = 3
On the product side
Balancing equations why & how
Why do we balance equations?
 Law of conservation of mass
-atoms are not created or destroyed in an
ordinary chemical reaction, just
rearranged to form new substances
What is used to balance chemical equations?
 Coefficients
-the number before the chemical formula
(the number is written normal size – not
superscript or subscript)
Steps to Balancing Equations
1. Determine the number of atoms of each
element in reactants and products
2. Balance by adding coefficients
A. Polyatomic ions (if same poly. ion
on both sides  balance as a chunk)
B. Metals
C. Nonmetals
D. “O” & “H”
3. Recheck your count!!!
2 H2 + O2  2 H2O
coefficient
subscript
H
O
Only
1 hydrogen
2
oxygen
atom
“in”“out”
(4each
“out”)
(2 type
“in”)of atom
Recount
total number
of
 place 2 in front of H2O
* Only add coefficients, NEVER Δ subscripts
Can you ever make just 1 molecule of water?
2NaOH  Na2O + H2O
Na
O
H
Only
1
sodium
atom
“in”
(2
“out”)
Recount
total2number
type of atom
 place
in frontofofeach
NaOH
2 Fe + 3 O2  2 Fe2O3
4
Fe
O
Recount
total atom
number
each
of atom
Only 2
1 iron
oxygen
atom
“in”of
“in”
(2
(4
“out”)
(3 type
“out”)
 change
place 2 2ininfront
frontofofFeFe
a4
2Ointo
3
and place 3 in front of O2
Counting molecules/compounds
How many molecules of each of the following
compounds are present in this equation?
Al2(SO4)3 + 3Ca(OH)2  2Al(OH)3 + 3CaSO4
 Reactants:
Al2(SO4)3 = 1 (when only 1 = no number)
Ca(OH)2 = 3 (large 3 in front)
 Products:
Al(OH)3 = 2 (large 2 in front)
CaSO4 = 3 (large 3 in front)
2H20  2H2 + O2
How many molecules of oxygen are
formed when 2 molecules of water are
broken down?

one
How many molecules of water would be
required to form 3 molecules of oxygen?

Six
The number of molecules does not have
to be the same!

2 molecules in, 3 molecules out.
Using state symbols
When writing chemical equations, the state of each
product or reactant may be labeled with the following
abbreviations
(s) = solid
(g) = gas
(l)
= liquid
(aq) = aqueous (solid dissolved in a
liquid usually water)
NOTE: If the states of matter are not included, you will
NOT need to include them. If the states of matter are
present, you MUST include them!
7 Diatomic elements
7 elements can not exist as single elements –
must exist in pairs if it is JUST that element
HONClBrIF
These 7 are always H2, O2, N2, Cl2, Br2, I2, F2
Never just write H, O, N, Cl, Br, I, F without
being bonded to another element.
H20 is okay – WHY?
Because O is bonded to another element
Steps to using word equation to form
formula equations:
1) Write formulas / symbols
2) Check for diatomic elements
3) Add state symbols (if given)
4) Balance (if can’t balance, then
recheck formulas!!)
Writing chemical equation from
word equations
Na+1
Br-1
Solid sodium bromide reacts with chlorine
gas to yield solid sodium chloride and
Na+1
bromine gas.
Cl-1
Recheck
Balance
Check
for
diatomics
(HONClBrIF)
Write
Add
formulas
state
symbols
& element
symbols
2 NaBr(s) +
Cl 2 (g)  2 NaCl (s) +
Na
Br
Cl
Br2 (g)
Another word equation
Solid aluminum metal reacts with oxygen gas
to form solid aluminum oxide.
O-2
Al+3
Check
for
diatomics
(HONClBrIF)
Write
formulas
& element
symbols
Add
state
symbols
Balance
Recheck
2 Al (s) + 3 O2 (g)  2 Al2O3 (s)
4
Al
O
Writing Word Equations
Na2O(s) + CO2(g)  Na2CO3(s)
Solid sodium oxide combines with (reacts
with / and) carbon dioxide gas to form
(yields/produces) solid sodium carbonate.
NaCl(s) + AgNO3(aq)  NaNO3(aq) + AgCl(s)
Solid sodium chloride and (combines with /
reacts with) aqueous silver nitrate forms
(yields / produces) aqueous sodium nitrate
and solid silver chloride.
5 Basic Types of Reactions
Synthesis Reaction
 Two or more substances combine to form
a single substance.
 Also known as a combination reaction.
 A + B  AB
 always forming 1 product
 Example: 2K + Cl2  2KCl
5 Basic Types of Reactions
Decomposition Reaction
 A single compound is broken down into
two or more products.
 AB  A + B
 always having 1 reactant
 Example: CaCO3  CaO + CO2
5 Basic Types of Reactions
Single Replacement Reaction
 one element replaces another element in a
compound (also called single
displacement)
 AB + C  AC + B
 Always a compound + element as reactants
 Example:
Mg + Zn(NO3)2  Mg(NO3)2 + Zn
(Mg is Cation so replaces the
cation in the compound)
5 Basic Types of Reactions
Double Replacement Reaction
the positive ions are exchanged between two
reacting compounds (also called double
displacement)
 AB + CD  AD + CB
 Always a compound + compound as reactants
 Example: BaCl2 + K2CO3  BaCO3 + 2KCl
(Ba & K are the cation that switch
places forming the new compounds)

5 Basic Types of Reactions
Combustion Reaction
 an element or a compound reacts with
oxygen often producing energy as heat and
light
 CxHy + O2  CO2 + H2O
 Always has oxygen as a reactant
 Is an exothermic reactions (gives off heat)
 Example: CH4 + 2O2  CO2 + 2H2O
Calculating Gram Formula Mass
Formula mass can be calculated in amu’s or
g’s of a substance by multiplying the number
of atoms of each element by the mass in
amu’s or g’s of the element. Then add the
values together. (YES, sig figs COUNT!!!)
Example: CaSO4
(# atoms each element x mass = total mass of element in compound)


1 Ca
1S
4O
x
x
x
40.078g
= 40.078
32.066g
= 32.066 +
+
15.9994g = 63.9976

Then add masses of all elements together
136.1416 = 136.142
The Mole
To make a quantity that is measurable in lab, we use
the mole.
In chemistry one mole is equal to 6.022 x 1023
particles (Avogadro’s number).
The gram formula mass of any compound is the
mass of 1 mole of the compound in grams.
1 mole = 6.0022 x 1023 is similar to
 12 eggs = 1 dozen
 52 weeks = 1 year
 1 gross = 144
Limiting Reactants
The limiting reactant is the reactant that determines
the maximum amount of product that is formed.
The limiting reactant will be completely used up in a
reaction. This makes the reaction stop.
The other reactant will have some unchanged so it is
said to be the excess reactant.
For example, if you need to make 10 chicken
sandwiches. You have 10 slices of bread and 10
pieces of chicken. If each sandwich requires 2 slices
of bread and 1 piece of chicken, which is the limiting
reactant? Excess reactant?
Rates of Reactions
The reaction rate is the change in
concentration of reactants and products in a
certain amount of time.
Rate at which the reactants disappear and the
products appear.
Combining two substances (causing a
reaction) means forcing their particles to hit,
or collide with, one another
Collision Theory states that molecules must
collide in order to react
Activation Energy
The activation energy is the energy needed to
start the reaction.
When particles collide with sufficient energy – at
least equal to the activation energy – existing bonds
may be disrupted and new bonds can form
Endothermic reaction – the energy of the product
is greater than that of the reactants (energy is
absorbed into the reaction)
Exothermic reaction – the energy of the products
is lower than that of the reactants (energy is
released from the reaction)
Factors Affecting Reaction Rates
Nature of Reactants
 Depends on the state of particular reactants
and the complexity of the bonds that have to
be broken and formed in order for the
reaction to proceed
 The more bonds to be broken then the
longer the reaction takes
 A reaction between two gases will be
quicker than a reaction between two
liquids or two solids.
Factors Affecting Reaction Rates
Temperature
 The higher the temperature at which a
reaction occurs, the faster the particles will
move
 The faster the particles move the higher the
frequency of collisions
 For example, food spoils faster at room
temperature than when it is refrigerated.
Factors Affecting Reaction Rates
Concentration
 Deals with how many particles are there
 An increase in concentration means that there are
more particles within a given volume and thus
smaller spaces between the reacting particles.
 Thus, the higher the concentration of reactants, the
greater the frequency of collisions among their
particles.
 For example, the more people there are in a room
the more people you will bump into as you walk
through the room.
Factors Affecting Reaction Rates
Surface Area
 Surface area deals with the number of
particles that are exposed for reaction.
 The larger the surface area the greater the
number of particles that are exposed for
reaction.
 For example, many small pieces of coal will
burn faster than a lump of coal (small pieces
have more particles exposed to react with
more oxygen particles)
Factors Affecting Reaction Rates
Catalysts
A catalyst is a substance that increases the rate of
the reaction without itself being used up in the
reaction (doesn’t appear as a reactant or a
product)
 Catalysts lower the activation energy required for
a reaction to occur.
 Thus a catalyst creates a different pathway from
reactants to products – one that requires less
energy.
 Catalysts in the body are enzymes – there to
speed up reactions in the body that are essential
to life.
