Symbols and Terminology Law of Conservation of Mass · Conservation of Mass: • In an isolated system the total amount of matter remains constant • Matter cannot be created or

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




1. Write out the chemical equation: reactants and

products.


Count

Atoms

Adjust

Coefficients

Recount

Atoms




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.


Lowest

Ratio

Leave room in front of

compounds to add coefficients

Balancing Equations: Ex 1

Chemical

Equation






Zn + HCl → ZnCl2 + H2

Count

Atoms


Chemical

Equation

Count

Atoms Reactants Products atom

Zn H Cl

1 1 1

1 2 2



Chemical

Equation

Count

Atoms


Reactants Products atom Zn H Cl

1 1 1

1 2 2

Balance single atom type


2



Adjust

Coefficients


Chemical

Equation

Count

Atoms


Reactants Products atom Zn H Cl

1 1 1

1 2 2



Adjust

Coefficients

2

4. Recount atom type in compound when coefficient

is changed: change tally Recount

Atoms

2 2


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.



Chemical

Equation






Count

Atoms

AgI + Na2S → Ag2S + NaI (aq) (s) (aq) (aq)


Chemical

Equation

Count

Atoms


Reactants Products atom Ag I

Na

1 1 2

2 1 1

S 1 1



Adjust

Coefficients




Chemical

Equation

Count

Atoms

2



Adjust

Coefficients

Recount

Atoms




Reactants Products atom

I Na 2 S 1 1

Ag 2 2 2

2

2 2


Chemical

Equation

2 AgI + Na2S → Ag2S + NaI (aq) (s) (aq) (aq) 2

Lowest

Ratio

2 1 1 2





Chemical

Equation






Count

Atoms

KClO3 → KCl + O2

D


(s) (s) (g)


Chemical

Equation

Count

Atoms Reactants Products atom

K Cl O

1 1 3

1 1 2

KClO3 → KCl + O2

D (s) (s) (g)

Reactants


Chemical

Equation

Count

Atoms



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


Chemical

Equation

Count

Atoms



Adjust

Coefficients


1 1

KClO3 → KCl + O2

D (s) (s) (g)

x 3 = ? 2

3

Recount

Atoms



6 6

6

2

2 2


Chemical

Equation

Count

Atoms



Adjust

Coefficients


KClO3 → KCl + O2

D (s) (s) (g) 3

Recount

Atoms



6 6

2

2 2

2 2

2


Chemical

Equation KClO3 → KCl + O2

D (s) (s) (g) 3 2 2

Lowest

Ratio

2 2 3





Chemical

Equation




NaOH + H2SO4 → Na2SO4 + H2O

Count

Atoms

(aq) (aq) (l) (aq)


Chemical

Equation NaOH + H2SO4 → Na2SO4 + H2O

Count

Atoms

(aq) (aq) (l) (aq)

Count

Atoms


O H

1 3

1 2

SO4-2 1 1

Na 1 2



Adjust

Coefficients




Chemical


Count

Atoms

(aq) (aq) (l) (aq) 2

Count

Atoms


O H

1 2

SO4-2 1 1

Na 2



Adjust

Coefficients

Recount

Atoms



2 4

2

2


Chemical


Count

Atoms

(aq) (aq) (l) (aq) 2 2

Count

Atoms


O H

SO4-2 1 1

Na 2



Adjust

Coefficients

Recount

Atoms



2 4

2 2 4


Chemical

Equation NaOH + H2SO4 → Na2SO4 + H2O (aq) (aq) (l) (aq) 2 2

Lowest

Ratio

2 1 1 2




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

Documents

Symbols and Terminology Law of Conservation of Mass · Conservation of Mass: • In an isolated system the total amount of matter remains constant • Matter cannot be created or