Download CHEMICAL REACTIONS

CHEMICAL REACTIONS

REVIEW OF CHEMICAL CHANGES

Occurs when a substance reacts and forms
one or more new substances

Examples: bake a cake, leaves change, food
digested, food spoils
HOW DO YOU KNOW WHEN A
CHEMICAL CHANGES HAS
OCCURRED?
LOOK FOR EVIDENCE!
Indicators of chemical reactions
Emission of light or heat
Formation of a gas
Formation of a precipitate
Color change
Emission of odor
CHEMICAL VS. PHYSICAL CHANGE



Not always easy to tell!
Water boiling to steam is still water
Horseshoe heat turns red is still iron
Are different substances present after the change
takes place?
When matter undergoes chemical change, the
composition of the matter changes. When it
undergoes physical change the composition of the
matter remains the same.





Chemical reaction is said to take place when
a chemical change happens
Describe what was present before & after the
reaction
Reactants: substances that undergo change
Products: new substances formed as result of
change
Example:
NaCl + BeF  NaF + BeCl2
Reactants
Products
CHECK YOUR UNDERSTANDING

1.
2.
3.
4.
5.
CHEMICAL OR PHYSICAL CHANGE?
Boiling water
Burning wood
Food scraps decomposing
Salt dissolving in water
Burning propane
USING EQUATIONS




Reactants  Products
Carbon + Oxygen  Carbon Dioxide
C + O2  CO2
Chemical equation: representation of a
chemical reaction in which the reactants and
products are expressed as formulas.
Symbols used in equations






(s) after the formula –solid Cu(s)
(g) after the formula –gas H2 (g)
(l) after the formula -liquid H2O(l)
(aq) after the formula - dissolved in
water, an aqueous solution. CaCl2 (aq)
used after a product indicates a gas (same
as (g)) O2
used after a product indicates a solid
(same as (s))
CaCo3 
CONSERVATION OF MASS



Mass is neither created or destroyed in a
chemical reaction
Mass of products always equals mass of
reactants
Charcoal burning into pile of ashes – but
CO2, too… pg. 193
BALANCING EQUATIONS

In order to show that mass is conserved
during a reaction, a chemical equation must
be balanced!

Coefficients: numbers that appear before
formula
Example: 2 H20 3 C6H12O6

STEPS IN BALANCING
1.Box in the compounds. Then, NEVER change
anything in the box.
N2H4 + O2  N2 + H2O
2. Set up an element inventory.
N
H
O
R
2
4
2
P
2
2
1
3. Add coefficients (the big numbers) in front of
the boxes.
4. Always update the inventory with every
coefficient.
PRACTICE






Write a balanced equation for the reaction
between copper and oxygen to produce copper
(II) oxide, CuO
Reactants: Cu, O2 Product: CuO
Cu + O2  CuO
Cu + O2  2 CuO
2 Cu + O2  2 CuO
Look back and ask, “Is your answer reasonable?”
TYPES OF REACTIONS







Reactions classified by type of reactant or the
number of reactants & products
SYNTHESIS
SINGLE REPLACEMENT
DOUBLE REPLACEMENT
DECOMPOSITION
COMBUSTION
RE-DOX (Oxidation – Reduction)
SYNTHESIS REACTIONS






Reaction in which 2 or more substances react
to form a single substance
Reactants can be compounds or elements
Products are always compounds
A + B  AB
2Na + Cl2  2NaCl2
2H2 + O2  2H2O
DECOMPOSITION REACTIONS






Opposite of synthesis
Reaction in which a compound breaks down
into 2 or more simpler substances
Reactant must be compound
Products can be elements or compounds
AB  A + B
CaCO3  CaO + CO2
SINGLE REPLACEMENT
REACTIONS




Reaction in which one element takes the
place of another element in a compound
A + BC  B + AC
Cu + 2 AgNO3  Cu(NO3)2 + 2 Ag
2K + 2H2O  H2 + 2KOH
DOUBLE REPLACEMENT
REACTIONS


Reaction in which 2 different compounds
exchange positive ions and form 2 new
compounds
AB + CD  AD + CB
ACTIVITY SERIES


Used to predict which
elements will replace others
Activity Series of Metals:
K
Ca
Mg
Zn
Fe
Pb
H
Cu
Ag

Reactivity decreases
down the series

Fe + CuSO4  FeSO4 + Cu

Can predict that iron
will replace copper
COMBUSTION REACTIONS



Reaction in which a substance reacts rapidly
with oxygen, often producing heat / light
Burning of natural gas (methane)
CH4 + 2O2  CO2 + 2 H2O
TIP: If the reactants include a fuel & O2and
the products include CO2 & H2O, it’s
probably a combustion reaction
REACTIONS AS e- TRANSFERS

Some chemical reactions are transfers of ebetween atoms
OXIDATION: any process in which an
element loses e-… an element is oxidized if it
loses e- Oxygen does not always have to be present


REDUCTION: process in which an element
gains e- during a chemical reaction…
element is reduced if it gains e-

Oxidation – Reduction always occur together
CHECK YOUR UNDERSTANDING


WHAT TYPE OF REACTIONS ARE THESE?
Are they balanced?
__ Na + ___ Cl2  ___ NaCl
__ CH4 + __ O2  __ CO2 + __ H2O
__ Na2S + __ AgNO3  _ NaNO3 + _ Ag2S
___ Mg + ___ HCl  ___ H2 + ___ MgCl2
ENERGY CHANGES IN REACTIONS



CHEMICAL ENERGY: energy stored in the
chemical bonds of a substance
Propane – C3H8
Has 10 single bonds
Chemical reactions involve the breaking of
chemical bonds in the reactants and
formation of chemical bonds in the products
EXOTHERMIC & ENDOTHERMIC
REACTIONS
During a chemical reaction, energy is either
released or absorbed
 EXOTHERMIC: chemical reaction that
releases energy into its surroundings
- Combustion
 ENDOTHERMIC: chemical reaction that
absorbs energy from its surroundings
- Add the energy term kJ into reactants

LAW OF CONSERVATION OF
ENERGY

Energy is neither created nor destroyed

WHAT DOES THIS LAW REMIND YOU OF?
REACTION RATES

Some reactions happen immediately, others
happen over time

REACTION RATE: rate at which reactants
change into products over time

Tells you how fast a reaction is going
FACTORS AFFECTING REACTION
RATES
1.Temperature: ^ temp will ^ rr
- Causes particles to move faster, collide, and
react
2. Surface area: increases the exposure of
reactants to one another, so more collisions,
and more reactions
3. Stirring: ^ exposure of reactants to each
other
4. Concentration: number of particles in a
given volume
5. Catalysts: substance that affects the rr w/o
being used up in the reaction
V2O5
-
2 SO2 + O2 --- 2SO3
EQUILLIBRIUM
State in which the forward & reverse paths of
a change take place at the same rate
 Physical equilibrium – when physical change
does not go to completion, a physical
equilibrium is established
Ex. Water evaporating in a sealed bottle
H2O(l) ↔
H2O(g)


Chemical equilibrium- most chemical
reactions are reversible to some degree…

Reversible reactions are those in which the
conversion of reactants into products and
conversion of products into reactants can
happen simultaneously

2SO2(g) + O2 ↔ 2SO3