Transcript A Balanced Chemical Equation

Chapter 5
Chemical Quantities and Reactions
5.4
Chemical Equations
Copyright © 2009 by Pearson Education, Inc.
1
Chemical Equations
A chemical equation gives
• the formulas of the reactants on the left of the arrow.
• the formulas of the products on the right of the arrow.
Reactants
Product
O2 (g)
CO2 (g)
C(s)
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2
Symbols Used in Equations
Symbols in chemical
equations show
TABLE
• the states of the
reactants.
• the states of the
products.
• the reaction
conditions.
3
Chemical Equations are Balanced
In a balanced
chemical reaction
• no atoms are
lost or gained.
• the number of
reacting atoms
is equal to the
number of
product atoms.
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4
A Balanced Chemical Equation
In a balanced chemical equation,
• the number of each type of atom on the reactant side is
equal to the number of each type of atom on the product
side.
• numbers called coefficients are used in front of one or
more formulas to balance the number of atoms.
Al + S
2Al + 3S
2 Al
3S
=
=
Al2S3
Al2S3
2 Al
3S
Not Balanced
Balanced using coefficients
Equal number of Al atoms
Equal number of S atoms
5
A Study Tip: Using Coefficients
When balancing a chemical equation,
• use one or more coefficients to balance atoms.
• never change the subscripts of any formula.
N2 +
O2
NO
Not Balanced
N2 +
O2
N2O2
Incorrect formula
N2 +
O2
2NO
Correctly balanced
using coefficients
6
Learning Check
State the number of atoms of each element on the
reactant side and the product side for each of the
following balanced equations.
A. P4(s) + 6Br2(l)
B. 2Al(s) + Fe2O3(s)
4PBr3(g)
2Fe(s) + Al2O3(s)
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Solution
A. P4(s) + 6Br2(l)
4 P atoms
12 Br atoms
4PBr3(g)
=
=
B. 2Al(s) + Fe2O3(s)
2 Al atoms
2 Fe atoms
3 O atoms
=
=
=
4 P atoms
12 Br atoms
2Fe(s) + Al2O3(s)
2 Al atoms
2 Fe atoms
3 O atoms
8
Learning Check
Determine if each equation is balanced or not.
A. Na(s) + N2(g)
B. C2H4(g) + H2O(l)
Na3N(s)
C2H6O(l)
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Solution
Determine if each equation is balanced or not.
A. Na(s) + N2(g)
Na3N(s)
No. 2 N atoms on reactant side; only 1 N atom
on the product side. 1 Na atom on reactant side;
3 Na atoms on the product side.
B. C2H4(g) + H2O(l)
Yes. 2 C atoms
=
6 H atoms
=
1 O atom
=
C2H6O(l)
2 C atoms
6 H atoms
1 O atom
10
Guide to Balancing a Chemical
Equation
11
Steps in Balancing an Equation
To balance the following equation,
Fe3O4(s) + H2(g)
Fe(s) + H2O(l)
• work on one element at a time.
• use only coefficients in front of formulas.
• do not change any subscripts.
Fe:
Fe3O4(s) + H2(g)
3Fe(s) + H2O(l)
O:
Fe3O4(s) + H2(g)
3Fe(s)
+ 4H2O(l)
H:
Fe3O4(s) + 4H2(g)
3Fe(s)
+ 4H2O(l)
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Balancing Chemical Equations
1. Write the equation with the correct formulas.
NH3(g) + O2(g)
NO(g) + H2O(g)
2. Determine if the equation is balanced.
No, H atoms are not balanced.
3. Balance with coefficients in front of formulas.
2NH3(g) + O2(g)
2NO(g) + 3H2O(g)
Double the coefficients to give even number of O atoms.
4NH3(g) + 7O2(g)
4NO(g) + 6H2O(g)
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Balancing Chemical Equations
4. Check that atoms of each element are equal in reactants
and products.
4NH3(g) + 5O2(g)
4 N (4 x 1 N)
12 H (4 x 3 H)
10 O (5 x 2 O)
4NO(g) + 6H2O(g)
=
=
=
4 N (4 x 1 N)
12 H (6 x 2 H)
10 O (4 O + 6 O)
14
Equation for a Chemical Reaction
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15
Checking a Balanced Equation
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Reactants
1 C atom
4 H atoms
4 O atoms
=
=
=
Products
1 C atom
4 H atoms
4 O atoms
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Learning Check
Check the balance of atoms in the following:
Fe3O4(s) + 4H2(g)
3Fe(s) + 4H2O(l)
A. Number of H atoms in products.
1) 2
2) 4
3) 8
B. Number of O atoms in reactants.
1) 2
2) 4
3) 8
C. Number of Fe atoms in reactants.
1) 1
2) 3
3) 4
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Solution
Fe3O4(s) + 4H2(g)
3Fe(s) + 4H2O(l)
A. Number of H atoms in products.
3) 8 (4H2O)
B. Number of O atoms in reactants.
2) 4 (Fe3O4)
C. Number of Fe atoms in reactants.
2) 3 (Fe3O4)
18
Learning Check
Balance each equation and list the coefficients in the
balanced equation going from reactants to products:
A. __Mg(s)
1) 1, 3, 2
B. __Al(s)
1) 3, 3, 2
+ __N2(g)
2) 3, 1, 2
+ __Cl2(g)
2) 1, 3, 1
__Mg3N2(s)
3) 3, 1, 1
__AlCl3(s)
3) 2, 3, 2
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Solution
A. 3) 3, 1, 1
3Mg(s) + 1N2(g)
B. 3) 2, 3, 2
2Al(s) + 3Cl2(g)
1Mg3N2(s)
2AlCl3(s)
20
Equations with Polyatomic Ions
2Na3PO4(aq) + 3MgCl2 (aq)
Mg3 ( PO4(s) + 6NaCl(aq)
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21
Balancing with Polyatomic Ions
Na3PO4(aq) + MgCl2(aq)
Balance PO43- as a unit
2Na3PO4(aq) + MgCl2(aq)
2 PO43Check Na + balance
6 Na+
Mg3(PO4)2(s) + NaCl(aq)
=
=
Balance Mg and Cl
2Na3PO4(aq) + 3MgCl2(aq)
3 Mg2+
=
6 Cl=
Mg3(PO4)2(s) + NaCl(aq)
2 PO436 Na+
Mg3(PO4)2(s) + 6NaCl(aq)
3 Mg2+
6 Cl22
Learning Check
Balance and list the coefficients from reactants to products.
A. __Fe2O3(s) + __C(s)
__Fe(s) + __CO2(g)
1) 2, 3, 2, 3
2) 2, 3, 4, 3
3) 1, 1, 2, 3
B. __Al(s) + __FeO(s)
__Fe(s) + __Al2O3(s)
1) 2, 3, 3, 1
2) 2, 1, 1, 1
3) 3, 3, 3, 1
C. __Al(s) + __H2SO4(aq)
__Al2(SO4)3(aq) + __H2(g)
1) 3, 2, 1, 2
2) 2, 3, 1, 3
3) 2, 3, 2, 3
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Solution
A. 2) 2, 3, 4, 3
2Fe2O3(s) + 3C(s)
4Fe(s)
B. 1) 2, 3, 3, 1
2Al(s) + 3FeO(s)
3Fe(s) + 1Al2O3(s)
C. 2) 2, 3, 1, 3
2Al(s) + 3H2SO4(aq)
+ 3CO2(g)
1Al2(SO4)3(aq) + 3H2(g)
24