symbols and terminology law of conservation of mass · conservation of mass: • in an isolated...
TRANSCRIPT
Balancing Chemical
Equations
Previous lecture:
Order in Matter
Balancing Chemical Equations
• Review of the Chemical Equation
– Symbols and Terminology
– Law of Conservation of Mass
• Basics Rules for Balancing Equations
• 4 Example Problems
• Separate Lecture: Advanced Concepts
in Equation Balancing
Chemical Equations
A Chemical equation is the means of
communicating a chemical change
• It uses symbols and formulas to represent the
elements and compounds involved in the
chemical reaction.
Chemical equations represent the sentences in the
language of chemistry.
2H2 + 1O2
2H2O
Chemical equations represent the sentences in the
language of chemistry.
2H2 + 1O2
Chemical Equations
2H2O
• Subscripts give the number of atoms in the
compound
• Coefficients give the number of compounds
in the reaction
Reactants are the species present before the reaction
Products are the species present after the reaction
Reactants → Products
• Reactants: The (+) means reacts with
• Products: The (+) means also produces
• The arrow (→) means “yield(s)” or “react(s) to produce”
2H2 + 1O2 2H2O
• The arrows (↔) or ( ) means the reaction goes in both directions (is in equilibrium)
States and Phases Examples
• Solid (s)
• Liquid (l)
• Gas (g)
• Aqueous solution (aq)
Symbols and Terminology
Ag(s)
H2O(l)
H2(g)
CuSO4(aq)
2H2(g) + 1O2(g) 2H2O(l) Additional symbols
• Gas escaping (↑)
• Solid precipitation (↓)
• Heat produced/consumed (D)
• Catalyst → Pt
Conservation of Mass:
• In an isolated system the total amount of matter remains constant
• Matter cannot be created or destroyed. It can change form
Law of Conservation of Mass
2H2(g) + 1O2(g) 2H2O(l)
mass reactants = mass products
atoms in reactants = atoms in products
balanced equation
Balancing Equations: Rules
Chemical
Equation
4. Recount atom types in compound when coefficient
is changed: change tally
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
1. Write out the chemical equation: reactants and
products.
• Include state of matter if asked for:
Count
Atoms
Adjust
Coefficients
Recount
Atoms
2. Count total number of each atom type for
reactants and for products
• Start a tally for each atom type on each side
5. Verify that compound coefficients are in lowest
whole number ratio.
• Reduce by a common factor if necessary
Lowest
Ratio
Balancing Equations: Rules
Chemical
Equation
4. Recount atom types in compound when coefficient
is changed: change tally
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
1. Write out the chemical equation: reactants and
products.
• Include state of matter if asked for:
Count
Atoms
Adjust
Coefficients
Recount
Atoms
2. Count total number of each atom type for
reactants and for products
• Start a tally for each atom type on each side
Compounds will never change!
• NEVER change subscripts!
Equation is balanced when
the two tallies are equal
If polyatomic ion is same
both sides, count as a whole Balance single atom type
compound(s) LAST: N2, O2, H2
5. Verify that compound coefficients are in lowest
whole number ratio.
• Reduce by a common factor if necessary
Lowest
Ratio
Leave room in front of
compounds to add coefficients
Balancing Equations: Ex 1
Chemical
Equation
2. Count total number of each atom type for
reactants and for products
• Start a tally for each atom type on each side
Compounds will never change!
• NEVER change subscripts!
Zn + HCl → ZnCl2 + H2
Count
Atoms
Balancing Equations: Ex 1
Chemical
Equation
Count
Atoms Reactants Products atom
Zn H Cl
1 1 1
1 2 2
Zn + HCl → ZnCl2 + H2
Balancing Equations: Ex 1
Chemical
Equation
Count
Atoms
Zn + HCl → ZnCl2 + H2
Reactants Products atom Zn H Cl
1 1 1
1 2 2
Balance single atom type
compound(s) LAST: N2, O2, H2
2
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Balancing Equations: Ex 1
Chemical
Equation
Count
Atoms
Zn + HCl → ZnCl2 + H2
Reactants Products atom Zn H Cl
1 1 1
1 2 2
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
2
4. Recount atom type in compound when coefficient
is changed: change tally Recount
Atoms
2 2
Balancing Equations: Ex 1
Chemical
Equation Zn + HCl → ZnCl2 + H2 2
Lowest
Ratio
1 2 1 1
2 4 2 2 2 2 2 2
5. Verify that compound coefficients are in
lowest whole number ratio.
• Reduce by a common factor if necessary
Balancing Equations: Ex 2
Chemical
Equation
2. Count total number of each atom type for
reactants and for products
• Start a tally for each atom type on each side
Compounds will never change!
• NEVER change subscripts!
Count
Atoms
AgI + Na2S → Ag2S + NaI (aq) (s) (aq) (aq)
Balancing Equations: Ex 2
Chemical
Equation
Count
Atoms
AgI + Na2S → Ag2S + NaI (aq) (s) (aq) (aq)
Reactants Products atom Ag I
Na
1 1 2
2 1 1
S 1 1
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Balance single atom type
compound(s) LAST: N2, O2, H2
Balancing Equations: Ex 2
Chemical
Equation
Count
Atoms
2
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Recount
Atoms
4. Recount atom type in compound when coefficient
is changed: change tally
AgI + Na2S → Ag2S + NaI (aq) (s) (aq) (aq)
Reactants Products atom
I Na 2 S 1 1
Ag 2 2 2
2
2 2
Balancing Equations: Ex 2
Chemical
Equation
2 AgI + Na2S → Ag2S + NaI (aq) (s) (aq) (aq) 2
Lowest
Ratio
2 1 1 2
5. Verify that compound coefficients are in
lowest whole number ratio.
• Reduce by a common factor if necessary
Balancing Equations: Ex 3
Chemical
Equation
2. Count total number of each atom type for
reactants and for products
• Start a tally for each atom type on each side
Compounds will never change!
• NEVER change subscripts!
Count
Atoms
KClO3 → KCl + O2
D
• Include state of matter if asked for:
(s) (s) (g)
Balancing Equations: Ex 3
Chemical
Equation
Count
Atoms Reactants Products atom
K Cl O
1 1 3
1 1 2
KClO3 → KCl + O2
D (s) (s) (g)
Reactants
Balancing Equations: Ex 3
Chemical
Equation
Count
Atoms
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Reactants Products atom K Cl O
1 1 3
1 1 2
KClO3 → KCl + O2
D (s) (s) (g)
LCM 3, 2: x 2 = ? 6 3
3
Balancing Equations: Ex 3
Chemical
Equation
Count
Atoms
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Reactants Products atom K Cl O
1 1
KClO3 → KCl + O2
D (s) (s) (g)
x 3 = ? 2
3
Recount
Atoms
4. Recount atom type in compound when coefficient
is changed: change tally
6 6
6
2
2 2
Balancing Equations: Ex 3
Chemical
Equation
Count
Atoms
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Reactants Products atom K Cl O
KClO3 → KCl + O2
D (s) (s) (g) 3
Recount
Atoms
4. Recount atom type in compound when coefficient
is changed: change tally
6 6
2
2 2
2 2
2
Balancing Equations: Ex 3
Chemical
Equation KClO3 → KCl + O2
D (s) (s) (g) 3 2 2
Lowest
Ratio
2 2 3
5. Verify that compound coefficients are in
lowest whole number ratio.
• Reduce by a common factor if necessary
Balancing Equations: Ex 4
Chemical
Equation
2. Count total number of each atom type for
reactants and for products
• Start a tally for each atom type on each side
NaOH + H2SO4 → Na2SO4 + H2O
Count
Atoms
(aq) (aq) (l) (aq)
Balancing Equations: Ex 4
Chemical
Equation NaOH + H2SO4 → Na2SO4 + H2O
Count
Atoms
(aq) (aq) (l) (aq)
Count
Atoms
Reactants Products atom
O H
1 3
1 2
SO4-2 1 1
Na 1 2
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Balance single atom type
compound(s) LAST: N2, O2, H2
Balancing Equations: Ex 4
Chemical
Equation NaOH + H2SO4 → Na2SO4 + H2O
Count
Atoms
(aq) (aq) (l) (aq) 2
Count
Atoms
Reactants Products atom
O H
1 2
SO4-2 1 1
Na 2
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Recount
Atoms
4. Recount atom type in compound when coefficient
is changed: change tally
2 4
2
2
Balancing Equations: Ex 4
Chemical
Equation NaOH + H2SO4 → Na2SO4 + H2O
Count
Atoms
(aq) (aq) (l) (aq) 2 2
Count
Atoms
Reactants Products atom
O H
SO4-2 1 1
Na 2
3. Balance atom types, one at a time, by adjusting
coefficient in front of compound
Adjust
Coefficients
Recount
Atoms
4. Recount atom type in compound when coefficient
is changed: change tally
2 4
2 2 4
Balancing Equations: Ex 4
Chemical
Equation NaOH + H2SO4 → Na2SO4 + H2O (aq) (aq) (l) (aq) 2 2
Lowest
Ratio
2 1 1 2
5. Verify that compound coefficients are in
lowest whole number ratio.
• Reduce by a common factor if necessary
RECAP
• A chemical equation is the means of communicating a
chemical change • Reactants are the species present before the reaction • Products are the species present after the reaction • Conservation of mass: in an isolated system the total amount of
matter remains constant • Balancing Equations: Rules 1. Write out the chemical equation: reactants and products. NEVER change
subscripts!
2. Count total number of each atom type for reactants and for products: Start a tally for each atom type on each side
3. Balance atom types, one at a time, by adjusting coefficient in front of compound
4. Recount atom type in compound when coefficient is changed: change tally
5. Verify that compound coefficients are in lowest whole number ratio