Transcript chemical equations

1. Chemical reaction = process by which
one or more substances are changed
into one or more different substances
(atoms are rearranged)
a) reactants = original substances(written to
the left of the arrow in a chemical equation)
b) products = resulting substances(written
to the right of the arrow in a chemical
equation)
Reactants
CH4 + 2O2
Yields
Products
CO2 + 2H2O
c) Indications of a chemical reaction
(chemical change):
– evolution of heat and/or light
– production of a gas (often seen
as bubbles)
– color change
– formation of a precipitate when
two solutions are mixed
(Precipitate = a solid that
separates from a solution)
d) catalyst = a substance that speeds up a
chemical reaction but is not permanently
consumes in the reaction.
–Catalysts DO NOT appear as reactants
or products!
–Ex: manganese dioxide is a catalyst in
the following reaction:
2H2O2
MnO2
O2 + 2H2O
1. Chemical equation = a representation of
the reactants and products in a chemical
reaction
a) Word equation = a chemical equation in
which the reactants and products in a
chemical reaction are represented by
words
• Ex:
sodium + water
reactants
 sodium hydroxide +
hydrogen
products
b) Formula equation = a chemical equation
in which the reactants and products in a
chemical reaction are represented by
chemical symbols or chemical formulas
• Ex:
Na
+
H2O
reactants

NaOH
+
products
H2
c) Atom inventory (for a chemical equation)
= a “bookkeeping” technique designed to
count the number of each type of atom
(ion) represented on each side of a
chemical equation
d) Subscript = whole number that appears
to the right and below a chemical symbol in
a chemical formula (if no number is written
it is assumed to be “1”)
– Indicates the number of atoms or ions
– Ex:
H2O
subscript
subscript (assumed to be “1”)
• Coefficient = whole number that appears
in front of a reactant or product in a
balanced chemical equation (if no number
is written it is assumed to be “1 1”) )
– Indicates the number of molecules or
moles
• Ex:
2 CO2
coefficient
• Ex:
Al2S3
coefficient (assumed to be “1”)
f) Balanced chemical equation = a formula
equation where coefficients are added so
equal numbers of each type of atom (ion)
are represented on each side of the
equation
• Ex:
2 Na (s)
+
reactants
2 H2O (l)

2 NaOH(aq)
products
+
H2(g)
2 Na (s)
+
2 H2O (l)

2 NaOH(aq)
+
• 2 molecules of solid sodium reacts with 2
molecules of water to yield 2 molecules of
aqueous sodium hydroxide and 1 molecule of
hydrogen gas.
OR
• 2 moles of solid sodium reacts with 2 moles of
water to yield 2 moles of aqueous sodium
hydroxide and 1 mole of hydrogen gas.
•
H2(g)
4 Fe + 3 O2

2 Fe2O3
• What are the reactant(s)?=
4 Fe + 3 O2
___________________________
• What are the product(s)?
2 Fe2O3
_____________________________
3
• # of moles of O2 =______________
3
• # of molecules of O2=_____________
4
# of atoms of Fe in 2Fe2O3 = ________
6
• # of atoms of O in 3 O2 =_________
Symbol
Meaning
“yields”;indicates result of a
reaction
Indicates a reversible
reaction


(s)
A reactant or product in the
Solid state

(l)
(aq)
(g)
Alternative to (s);used only for a
Precipitate (solid) falling out of
solution
A reactant or product in the liquid
statet
A reactant or product in aqueous
Solution (dissolved in water)
A reactant or product in the
Gaseous state


2atm
Alternative to (g);used only for a
Gaseous product
Reactants are heated
Pressure at which the reaction is
Carried out, in this case 2 atm
Temperature at which reaction is
O°C Carried out, in this case O°C
A catalyst is used to speed up the
Reaction rate, in this case MnO2
MnO2
Would be used to speed up the
Reaction rate.
1. The equation must represent the known
facts with all reactants and products
identified.
2. The equation must contain the correct
formulas of the reactants and products.
• A chemical reaction obeys the “Law of
Conservation of Matter”
• The Law of Conservation of Matter =
matter cannot be either created or
destroyed in ordinary chemical or physical
means.
• The Law of Conservation of Matter must
not be violated!
– This means you must balance
equations. (Equations are balanced by
adding coefficients, NOT subscripts.)
4. Why do we balance equations?
–The Law of Conservation of Matter
must not be violated!
5. Atomic theory can be used to explain why
chemical reactions obey the “Law of
Conservation of Matter”.
– All atoms present at the start of a
chemical reaction are present at the end
of that chemical reaction (atoms are
simply rearranged during a chemical
reaction).
•
–Since no atoms are created or destroyed
in a chemical reaction no matter is
created or destroyed in a chemical
reaction.
–Since no matter is created or destroyed
in the chemical reaction the mass of the
reactants must equal the mass of the
products in a chemical reaction.
•
6. A balanced chemical equation details how
a chemical reaction obeys the “Law of
Conservation of Matter”.
1. Whether the reaction will actually occur.
Chemical equations can be written for
reactions that do not necessarily take
place.
2. The speed of a reaction.
3. The pathway atoms or ions take in
moving from reactants to products.
1. Write a word equation
2 2. Write a formula equation
a) remember naturally occurring
diatomic elements:
• H2, O2, N2, Cl2, Br2, I2, F2
b) unless otherwise noted, other elements
are represented by the element
symbol; ex: magnesium = Mg
c) remember the formulas for acids:
• HCl = Hydrochloric Acid
• HNO3 = Nitric Acid
• H3PO4 = Phosphoric Acid
• CH3COOH = Acetic Acid (NOTE: the
positive ion (H+) is at the end)
• H2CO3 = Carbonic Acid
• H2SO4 = Sulfuric Acid
d) remember the rules for writing formulas
for ionic compounds (ions)
• METALS & NONMETALS!!
e) remember the rules for writing
formulas for molecular compounds
(prefixes)
• ONLY NONMETALS!!
f) remember the formula for water, H2O
• HOH (hydrogen hydroxide)
3. Write a balanced chemical equation by
adding coefficients, NOT subscripts (this
will require trial and error, the following
guidelines may be helpful)
a) balance the different types of atoms one
at a time
b) first, balance the atoms of elements that
are combined and that appear only once
on each side of the equation
c) balance polyatomic ions that appear on
both sides of the equation as single units
d) balance H atoms and O atoms after
atoms of all other elements have been
balanced
4. Conduct an atom inventory throughout the
process. Be sure to conduct an atom
inventory at the end of the process to be
certain you have the correct answer.
5 5. Be certain that the coefficients represent
the SMALLEST possible whole number
ratio of reactants and products.
6. Common problems when balancing
equations:
a) You must write correct formulas!
b) once a correct formula is written,
subscripts CANNOT be changed!
a) Hydrogen reacts with oxygen to produce water.
b) Zinc reacts with hydrochloric acid to produce a
solution of zinc chloride and hydrogen gas.
c) Aluminum sulfate reacts with calcium hydroxide
to form aluminum hydroxide and calcium sulfate.
• 2 or more substances combine to form one
new substance
• General format:
A
+ B
AB
Ex:
NH3 + HCl
2 Na + C12
NH4Cl
2 NaCl
• Synthesis Practice: For each of the
following, predict the products and write a
word equation. Next, write a formula
equation and a balanced chemical
equation. Finally, write the type of reaction
represented by each chemical equation.
a) magnesium + oxygen 
b) sodium + fluorine 
c) magnesium + fluorine 
d) lithium + chlorine 
• HOMEWORK!!
e) barium + oxygen 
• HOMEWORK!!
f) potassium + oxygen 
HOMEWORK!!
• Substances break up or decompose into
simpler substances
• General format:
AB
• Ex:
A + B
H2CO3
H2O + CO2
2 Al2O3
4 Al + 3 O2
• Decomposition Practice: For each of the
following, predict the products and write a
word equation. Next, write a formula
equation and a balanced chemical
equation. Finally, write the type of reaction
represented by each chemical equation.
a) water 
b) mercury(II) oxide 
c) magnesium chloride 
• HOMEWORK!!
d) silver oxide 
HOMEWORK!!
• One element replaces another in a
compound
• General format:
A + BC
Ex: Cl2 + 2 KBr
2 Na + 2 H2O
AC + B
2 KCl + Br2
2 NaOH + H2
• Single Displacement Practice: For each
of the following, predict the products and
write a word equation. Next, write a
formula equation and a balanced chemical
equation. Finally, write the type of reaction
represented by each chemical equation.
a) aluminum
+ lead(II) nitrate 
b) lithium
+ water 
c) aluminum
+ sulfuric acid 
d) chlorine
+ potassium bromide 
e) fluorine
+ sodium chloride 
f) aluminum
+ silver nitrate 
HOMEWORK!!
g) barium
+ water 
HOMEWORK!!
h) lithium
+ hydrochloric acid 
HOMEWORK!!
• Positive and negative ions are exchanged
• General format:
AB + CD
CB + AD
Ex: Ba(OH)2 + H2SO4
AgNO3 + NaCl
BaSO4 + 2 H2O
AgCl + NaNO3
• Double Displacement Practice: For each
of the following, predict the products and
write a word equation. Next, write a
formula equation and a balanced chemical
equation. Finally, write the type of reaction
represented by each chemical equation.
a) potassium iodide
+ lead(II) nitrate 
b) sulfuric acid
+ sodium hydroxide 
c) silver nitrate + sodium phosphate
d) nitric acid + potassium hydroxide
HOMEWORK!!
e) sulfuric acid + potassium hydroxide
• HOMEWORK!!
f) calcium phosphate + acetic acid
HOMEWORK!!
_____________ 1) 2 H2O2
Decomposition
2 H2O +O2
Synthesis
____________2) 4 Fe + 3 02
2 Fe2O3
Single
___________3)
Cl2 + 2 NaBr
2 NaCl +Br2
Displacement
Single
Displacement
_____________4) Zn +H2SO4
Double
p
Displacement
5) Pb(NO )
____________5
3 2
+K2CrO4
ZnSO4 +H2
PbCrO4 + KNO3
• Reaction rate = the rate (speed) of
disappearance of a reactant or the rate of
appearance of a product in a chemical
reaction
a) A chemical reaction that takes a long
time to happen has a slow reaction
rate.
b) A chemical reaction that takes a short
time to happen has a fast reaction rate.
1) Nature of reactants
a) Reactions occur at different rates
depending on the reactants.
b) ) Some reactions occur almost
instantaneously (such as doublereplacement reactions).
c) Other reactions occur slower.
2) Concentration = a measure of the
number of particles of matter per unit
volume
a) An increase in the concentration of the
reactants generally increases reaction
rate.
b) A decrease in the concentration of the
reactants generally decreases reaction
rate.
3. Temperature = a measure of the average
kinetic energy of the particles in a sample
of matter
a) An increase in temperature increases
reaction rate.
b) A decrease in temperature decreases
reaction rate.
4. Presence of a catalyst = a substance that
increases the rate of a chemical reaction
without being permanently changed
a) A catalyst will increase reaction rate.
b) A catalyst is not permanently changed
in a chemical reaction.
1. Assumptions of collision theory:
a) Reacting molecules must collide with
each other.
b) Reacting molecules must collide with
sufficient energy.
c) Reacting molecules must collide in an
orientation that can lead to
rearrangement of the atoms.
2) Use collision theory to explain why an
increase in concentration increases
reaction rate:
a) ) At a higher concentration more reactant
molecules are present.
b) When more reactant molecules are present
more frequent collisions occur.
c) When more frequent collisions occur there
are more chances for molecules to react (so
reactions happen faster).
3. Use collision theory to explain why an
increase in temperature increases reaction
rate:
a) Molecules with a higher kinetic energy move
faster, collide more often, and have more
energetic collisions.
b) A higher temperature means that the
average kinetic energy of the reacting
molecules is increased; at a higher
temperature there are more molecules with a
higher kinetic energy.
c) At a higher temperature there are more
frequent collisions giving molecules more
chances to react (so reactions happen
faster).
d) At a higher temperature, more molecules
possess enough kinetic energy to react so
a higher percentage of collisions result in
molecules reacting (so reactions happen
faster).
•