Transcript Chemical Equations & Reactions

Describing a Chemical Reaction
Indications of a Chemical Reaction
– Evolution of heat, light, and/or sound
– Production of a gas
– Formation of a precipitate
– Color change
ENERGY CHANGES IN REACTIONS:
determined by CHANGES that occur
IN chemical BONDING.
Chemical reactions involve the
BREAKING of chemical
BONDS in reactants and the
FORMATION of chemical
BONDS in products.
Example: In the combustion of propane, the bonds in propane and oxygen molecules
are broken, while the bonds in carbon dioxide and water molecules are formed.
C3H8 + 5 O2 ——> 3 CO2 + 4H20 + heat
LAW OF CONSERVATION OF ENERGY
•In an exothermic reaction, the chemical energy of the
reactants is CONVERTED INTO HEAT plus the chemical
energy of the products.
•In an endothermic reaction, HEAT plus the chemical energy
of the reactants IS CONVERTED into the chemical energy of
the products.
In both cases, the total amount of
ENERGY BEFORE AND AFTER the
reaction IS THE SAME.
Chemical Equations
• Reactants – the substances that exist before a
chemical change (or reaction) takes place.
• Products – the new substance(s) that are formed
during the chemical changes.
• CHEMICAL EQUATION indicates the reactants and
products of a reaction.
REACTANTS  PRODUCTS
Word Equations
• A WORD EQUATION describes chemical change using the
names of the reactants and products. REMEMBER
conservation of mass when converting to chemical equation
Write the word equation for the reaction of methane gas
with oxygen gas to form carbon dioxide and water.
methane
+
oxygen
Reactants
carbon dioxide
+
Products
water
TYPES OF CHEMICAL REACTIONS
1. Oxidation-Reduction (gain/loss of electrons)
a. Synthesis – 2 or more substances form a new
single substance
b. Decomposition – One substance breaks down into
simpler substances
c. Single-displacement – A single element displaces
another element from a compound
H2O
d. Combustion – A compound reacts with O2 to form
TYPES OF CHEMICAL REACTIONS
2. Double Displacement (reactions where two elements or
groups of elements in two different compounds exchange
places to form two new compounds)
a. Precipitation – Two soluble substances form an
insoluble precipitate
b. Acid-Base – An acid reacts with a base to form a
“salt” and water
c. Gas Evolution – A gas is formed
SYNTHESIS REACTION: two or more substances react
to FORM A SINGLE SUBSTANCE.
A + B → AB
SO2 + O2 → SO3
CO + O2 → CO2
P4O10 + H2O → H3PO4
CuBr + Br2 → CuBr2
DECOMPOSITION REACTION: a compound BREAKS
DOWN into two or more simpler substances.
AB → A + B
H2CO3 → CO2 + H2O
NCL3 → N2 + CL2
NaHCO3 → CO2 + Na2CO3 + H2O
SINGLE-REPLACEMENT REACTION: one element
TAKES THE PLACE of another element in a compound.
A + BC → B + AC
SiO2 + Mg → Si + MgO
Cu + HgNO3 → Cu(NO3)2 + Hg
Li + H2O → LiOH + H2
H2SO4 + Ca → CaSO4 + H2
COMBUSTION REACTION: a substance REACTS
RAPIDLY WITH OXYGEN, often producing heat, light,
CO2 and water (overlaps with other types).
CH4 + O2 → CO2 + 2H2O
C4H10 + O2 → CO2 + H2O
C10H22 + O2 → CO2 + H2O
CH4 + O2 → CO2 + H2O
DOUBLE-REPLACEMENT REACTION: two different
compounds EXCHANGE POSITIVE IONS and form TWO
NEW COMPOUNDS.
AB + CD → AD + CB
CaCl2 + K3PO4 → Ca3(PO4)2 + KCl
CaCO3 + HCl → H2CO3 + CaCl2
AgSO4 + NH4I → (NH4)2SO4 + AgI
BaO + HNO3 → Ba(NO3)2 + H2O
Al2(SO4)3 + Ca(OH)2 → Al(OH)3 + CaSO4
Phase Symbols Used in Chemical Equations
“Yields”; indicates result of reaction
Used to indicate a reversible reaction
(s)
A reactant or product in the solid state;
also used to indicate a precipitate
Alternative to (s), but used only to indicate a precipitate
(l)
A reactant or product in the liquid state
(aq)
A reactant or product in an aqueous solution
(dissolved in water)
(g)
A reactant or product in the gaseous state
Phase Symbols Used in Chemical Equations
Alternative to (g), but used only to indicate a gaseous product
D
2 atm
pressure
Reactants are heated
Pressure at which reaction is carried out, in this case 2 atm
Pressure at which reaction is carried out exceeds normal
atmospheric pressure
0 oC
Temperature at which reaction is carried out, in this case 0 oC
MnO2
Formula of catalyst, in this case manganese (IV) oxide,
used to alter the rate of the reaction
CH4 + 2 O2  CO2 + 2 H2O
Matter/Mass Conserved
Reactants
1 C atom
4 H atoms
4 O atoms
Timberlake, Chemistry 7th Edition, page 167
Products
1 C atom
4 H atoms
4 O atoms
Balancing Chemical Equations
Balanced Equation – one in which the number of
atoms of each element as a reactant is equal to the
number of atoms of that element as a product.
Coefficients are used to balance, NOT subscripts
What is the relationship between conservation of mass and
the fact that a balanced equation will always have the same
number of atoms of each element on both sides of an equation?
Determine whether the following equation is balanced.
H2 + Cl2  HCl
Balancing Chemical Equations
1. Write a word equation for the reaction.
2. Write the chemical equation for all
reactants and products, then use MINOH.
3. Balance metal coefficients first (M).
4. Next, look for polyatomic ions, and
balance coefficients as a group (I).
4. Then, balance non-metal coefficients (N).
5. Balance O and H last (OH).
Write a balanced equation for the reaction between iron (III)
oxide and hydrogen to form iron and water
1) Write a word equation for the reaction.
Iron (III) oxide + hydrogen  iron + water
2) Write the correct formulas for all reactants and products.
Fe2O3
+
H2 (g)

Fe
+
H 2O
3) Determine the coefficients that make the equation balance.
Fe2O3
+
3 H2 (g)

2 Fe
+
3 H 2O
Write a balanced equation for the reaction between chlorine
and sodium bromide to produce bromine and sodium chloride.
1) Write a word equation for the reaction.
chlorine + sodium bromide  bromine + sodium chloride
2) Write the correct formulas for all reactants and products.
Cl2 (g)
+
NaBr

Br2 (g)
+
NaCl
3) Determine the coefficients that make the equation balance.
Cl2 (g)
+
2 NaBr

Br2 (g)
+
2 NaCl
Write the balanced equation for the reaction between
aluminum sulfate and calcium chloride to form a white
precipitate of calcium sulfate.
1) Write a word equation for the reaction.
aluminum sulfate + calcium chloride  calcium sulfate + aluminum chloride
2) Write the correct formulas for all reactants and products.
Al2(SO4)3
+
CaCl2

CaSO4 (s)
+
AlCl3
3) Determine the coefficients that make the equation balance.
Al2(SO4)3
+
3 CaCl2

3 CaSO4 (s)
+
2 AlCl3
Printable
Version
of
Activity
Series
Activity Series
Element Reactivity
Ca
Foiled again –
Aluminum loses to Calcium
Li
Rb
K
Ba
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Ni
Sn
Pb
H2
Cu
Hg
Ag
Pt
Au
Halogen Reactivity
F2
Cl2
Br2
I2
Write a balanced equation for the reaction between nitrogen
trihydride and oxygen to form nitrogen monoxide and water.
1) Write a word equation for the reaction.
Nitrogen trihydride + oxygen  nitrogen monoxide + water
2) Write the correct formulas for all reactants and products.
NH3
+
O2 (g)

NO
+
H 2O
3) Determine the coefficients that make the equation balance.
4 NH3
+
5 O2 (g)

4 NO
+
6 H 2O
Coefficients are used to balance, NOT subscripts
H
Cl
Cl
H
H
H
H
Cl
H2 + Cl2  HCl (unbalanced)
reactants
H
Cl
2
2
H
H
Cl
Cl
Cl
H2 + Cl2  2 HCl (balanced)
reactants
products
1
1
Cl
H
Cl
2
2
products
2
2