Chemical Reactions
• Physical Change - alters the form or appearance of
a substance but does not change it into a new,
different substance
• Chemical Change (aka chemical reaction) - forms
one or more new substances with properties
different from those of the original substances
• Evidence for Chemical Reactions
1. color change
2. odor
Potassium Iodide and Lead Nitrate form Lead Oxide
Barium Chloride and Sodium Sulfate form Barium Sulfate
Mercury Acetate and Sodium Iodide form Mercury Iodide
3. formation of a precipitate, a solid that forms when
two liquids are mixed*
4. production of a gas (seen as bubbles in liquid)*
5. energy changes
• energy is absorbed in an endothermic reaction
• energy is released in an exothermic reaction
• Chemical formula - tells you how many atoms of
each element are in a compound
• example: CO2 (carbon dioxide) has one carbon atom
and two oxygen atoms
• Chemical Equation - a short, easy way to show a
chemical reaction, using symbols
• The substances you begin with are called the
reactants.
• The new substances formed from the reaction are
called the products.
• → = yields
• The subscript (small number to the right of the
element’s symbol) tells you how many atoms of
that element are in the molecule.
• The coefficient (number to the left of the element’s
symbol) tells you how many molecules of that
compound or element are present.
• Count the Atoms
• C6H12O6 = ___ C atoms
= ___ H atoms
= ___ O atoms
= ___ total atoms
• Count the Atoms
• 3H2O
= ___ H atoms
= ___ O atoms
= ___ total atoms
• Count the Atoms
• 5H2SO4 = ___ H atoms
= ___ S atoms
= ___ O atoms
= ___ total atoms
• Count the Atoms
• (NH4)2SO4
= ___ N atoms
= ___ H atoms
= ___ S atoms
= ___ O atoms
= ___ total atoms
• Conservation of Mass – matter is not created or
destroyed during a chemical reaction
• Balancing Chemical Equations
• a chemical equation must show the same number
of each type of atom on both sides of the equation
• When balancing a chemical equation, you can
change the coefficient, but you can’t change the
subscript as that would form a new substance.
H2O vs. H2O2
“Two scientists walk into a bar . . .”
Steps to balancing a chemical equation:
1. Write the equation.
H2 + O2 → H2O
Steps to balancing a chemical equation:
1. Write the equation.
2. Count the atoms of each type of element on both
the reactant and product sides of the equation.
H2 + O2 → H2O
H=
O=
H=
O=
Steps to balancing a chemical equation:
1. Write the equation.
2. Count the atoms of each type of element on both
the reactant and product sides of the equation.
3. Use coefficients to balance the number of atoms
on both sides of the reaction.
__H2 + __O2 → __H2O
H=
O=
H=
O=
Steps to balancing a chemical equation:
1. Write the equation.
2. Count the atoms of each type of element on both
the reactant and product sides of the equation.
3. Use coefficients to balance the number of atoms
on both sides of the reaction.
4. Look back and check that the equation is balanced.
2H2 + O2 → 2H2O
Let’s try another one:
CH4 + O2 → CO2 + H2O
Count the atoms:
C=
H=
O=
C=
H=
O=
Change the coefficients to balance the hydrogen
atoms:
Now, change the coefficients to balance the oxygen
atoms:
__CH4 + __O2 → __CO2 + __H2O
C=1
H=4
O=2
C=1
H=2
O=3
Check to make sure the equation is balanced:
CH4 + 2O2 → CO2 + 2H2O
C=
H=
O=
C=
H=
O=
• Kahn Academy – Balancing Chemical Equations
• https://www.youtube.com/watch?v=TUuABq95BB
M
• Balance the Equations
__Mg + __O2 → __MgO
__FeS + __HCl → __FeCl2 + __H2S
• Five types of chemical reactions
• Synthesis - two or more simple substances combine
to form a more complex substance
• A + B → AB (two or more reactants; one product)
• ex: formation of water
• Decomposition - a compound breaks down into
simpler substances
• AB → A + B (one reactant; two or more products)
• ex: hydrogen peroxide
decomposes to form
water and oxygen
2H2O2 → 2H2O + O2
• Single Replacement - one element replaces another
in a compound
• A + BC → B + AC
C + 2Cu2O → 4Cu + CO2
• Double Replacement - two elements in different
compounds trade places
• AB + CD → AD + CB
FeS + 2HCl → FeCl2 + H2S
(iron sulfide + hydrochloric acid → iron chloride + hydrogen sulfide)
• Combustion - oxygen combines with a carbon
compound to form water and carbon dioxide
• C#H# + O2 → CO2 + H2O
• exothermic
• Activation Energy - the minimum amount of energy
needed to get a chemical reaction started
• Activation Energy - the minimum amount of energy
needed to get a chemical reaction started
• Rates of Chemical Reactions - chemists can speed
up or slow down chemical reactions by changing
one or more of the following factors:
• Surface Area - increasing the surface area where a
reaction takes place will increase the rate of the
reaction
• Chewing food provides
increased surface area;
faster digestion
• brain is folded to increase surface area; more
chemical reactions can take place simultaneously
• grinding a reactant into a powder creates more
surface area; increase the rate of a reaction
• Temperature - increasing the temperature will
increase the rate of the reaction; decreasing the
temperature will decrease the rate of the reaction
• Concentration - the amount of a substance in a
given volume (its strength)
• increasing the concentration of the reactants
increases the rate of the reaction
• Catalyst - a material that increases the rate of a
chemical reaction by lowering the activation energy
• Elephant Toothpaste
• an enzyme is a biological catalyst
• Inhibitor - decreases the rate of a reaction by
preventing reactants from coming together