Download Chemical Reactions

Describing Chemical
Reactions
Prentice-Hall Chapter 11.1
Dr. Yager
Objectives

Describe how to write a word equation

Describe how to write a skeleton equation

Describe the steps for writing a balanced
chemical equation
Word Equations
To write a word equation, write the names of
the reactants to the left of the arrow separated
by plus signs. Write the names of the products
to the right of the arrow, also separated by plus
signs.
Reactant + Reactant  Product + Product
Methane + Oxygen  Carbon dioxide + Water
iron + oxygen  iron(III) oxide
The Word Equation
(Not generally used)
Reactants → Products
Example:
Iron + Oxygen → Iron (III) oxide
or
Iron + Oxygen → Iron (II) oxide
How do you tell the difference?
Hydrogen Peroxide  Water and Oxygen
Chemical Equations
A chemical equation is a representation of
a chemical reaction where the formulas of
the reactants (on the left) are connected by
an arrow with the formulas of the products
(on the right).
Skeletal Equations
A skeleton equation is a chemical equation
that does not indicate the relative amounts of
the reactants and products.
Here is the equation for rusting:
Fe + O2  Fe2O3
Skeleton Equation
Reactants → Products
Fe + O2 → Fe2O3
How do you get Fe2 from Fe and O3 from O2?
We do this by balancing equations.
4Fe + 3O2 → 2Fe2O3
You cannot make or destroy matter!
It also helps to know if it is solid, liquid or gas.
4Fe(s) + 3O2(g)→ 2Fe2O3(s)
Catalyst
A catalyst helps or speeds up a reaction, but is
not used up in the reaction.
Example:
H2O2 → H2O + O2
This happens faster in the presence of MgO2:
MgO2
H2O2 → H2O + O2
Without Catalyst
With Catalyst
Balancing the Equation
To write a balanced chemical equation:

First, write the skeleton equation.
 Then use coefficients to balance the equation
so that it obeys the law of conservation of
mass.
Balancing the Equation
4Fe + 3O2 → 2Fe2O3
The coefficients in yellow balance the equation.
Law of the Conservation of Matter: There are
the same number and types of atoms on the left
side as on the right side.
4Fe = 2Fe2
3O2 = 2O3
The Reaction to Make Water
Balanced chemical equation:
2 H2 (g) + O2 (g)
2 x 2 “H” = 4 “H”
1 x 2 “O” = 2 “O”
→
2 H2O (l)
2 x 2 “H” = 4 “H”
2 x 1 “O” = 2 “O”
Steps:
1. Write the unbalanced equation.
2. Inventory the atoms in reactants and products.
3. Balance the equation.
Strategies:
o Balance elements left-to-right on the Periodic
Table (metals first, then nonmetals)
o Leave H and O until last.
1. Write the unbalanced equation.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
2. Inventory reactants and products.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
Zn 1
Zn 1
Cl
1
Cl 2
H
1
H
2
3. Balance the equation by changing coefficients.
__Zn (s) + __HCl
2
(aq) → __ZnCl2 (aq) + __ H2 (g)
Zn
1
Zn 1
Cl
1 2
Cl 2
H
1 2
H 2
BALANCED !
BALANCED !
Balance this reaction:
__CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l)
Balance this reaction:
__CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l)
C
1
C
1
H
4
H
2
O
2
O
3
2 2 (g)  __CO2 (g) + __H
2 2O (l)
__CH4 (g) + __O
C 1
C
1
H 4
H
2 4
O 2 4
O
3 4
Balance This Equation
2
2
3 2 (g)
__KClO
(s) + __O
3 (s)  __KCl
K
1 2
K 1 2
Cl
1 2
Cl 1 2
O
3 6
O 2 6
2Al(OH)3(s) + 3 H2SO4(aq)  Al2(SO4)3(aq) + 6H2O(l)
Al 1 2 2
Al 2 2
S 1 1 3
S 3 3
H 5 8 12
H 2 6x2=12
O 7 10 18
O 13 12+6=18
Practice Problems
1. _AgNO3 + _H2S → _Ag2S + _HNO3
2. _Zn(OH)2 + _H3PO4 → _Zn3(PO4)2 + _H2O
3. _FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2
4. _CuS + _O2 → _Cu + _SO2
5. _Fe2O3 + _H2 → _Fe + _H2O
Practice Problems
1. 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
2. _Zn(OH)2 + _H3PO4 → _Zn3(PO4)2 + _H2O
3. _FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2
4. _CuS + _O2 → _Cu + _SO2
5. _Fe2O3 + _H2 → _Fe + _H2O
Practice Problems
1. 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
2. 3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O
3. _FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2
4. _CuS + _O2 → _Cu + _SO2
5. _Fe2O3 + _H2 → _Fe + _H2O
Practice Problems
1. 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
2. 3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O
3. 2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2
4. _CuS + _O2 → _Cu + _SO2
5. _Fe2O3 + _H2 → _Fe + _H2O
Practice Problems
1. 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
2. 3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O
3. 2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2
4. 1CuS + 1O2 → 1Cu + 1SO2
5. _Fe2O3 + _H2 → _Fe + _H2O
Practice Problems
1. 2AgNO3 + 1H2S → 1Ag2S + 2HNO3
2. 3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O
3. 2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2
4. 1CuS + 1O2 → 1Cu + 1SO2
5. 1Fe2O3 + 3H2 → 2Fe + 3H2O
1. Propane gas reacts with oxygen to produce
water vapor and carbon dioxide. Choose
the correct word equation for this reaction.
a) propane + carbon dioxide  water + oxygen
b) propane + oxygen + water  carbon dioxide
c) propane + oxygen + water + carbon dioxide
d) propane + oxygen  water + carbon dioxide
1. Propane gas reacts with oxygen to produce
water vapor and carbon dioxide. Choose
the correct word equation for this reaction.
a) propane + carbon dioxide  water + oxygen
b) propane + oxygen + water  carbon dioxide
c) propane + oxygen + water + carbon dioxide
d) propane + oxygen  water + carbon dioxide
2. Which of the following is a skeleton
equation?
a) H2 + CO  CH3OH
b) 2H2 + CO  CH3OH
c) 2H2 + CO2  CH3OH
d) hydrogen + carbon monoxide  methanol
2. Which of the following is a skeleton
equation?
a) H2 + CO  CH3OH
b) 2H2 + CO  CH3OH
c) 2H2 + CO2  CH3OH
d) hydrogen + carbon monoxide  methanol
3. What coefficient for H2SO4 is required to
balance the following equation?
Ca3(PO4)2 + ____ H2SO4  3CaSO4 + 2H3PO4
a) 1
b) 2
c) 3
d) 4
3. What coefficient for H2SO4 is required to
balance the following equation?
Ca3(PO4)2 + ____ H2SO4  3CaSO4 + 2H3PO4
a) 1
b) 2
c) 3
d) 4