Describing chemical reactions Balancing chemical equations

1. CHEMICAL EQUATIONS AND
REACTIONS
Chapter 8.1

2. Objectives:
1. List three observations that suggest that a
chemical reaction has taken place.
2. List three requirements for a correctly
written chemical equation.
3. Write a word equation and a formula
equation for a given chemical reaction.
4. Balance a formula equation by inspection.

3. Describing Chemical Reactions
 Chemical reaction
 Defined as the process by which one or more
substance are changed into one or more different
substances.
 Reactants: original substances
 Products: resulting substances
 According to law of conservation of mass:
total mass of reactants must equal total mass of products
 Chemical equation
 Defined as represents, with symbols and formulas, the
identities and relative amounts of the reactants and
products in a chemical reaction.
 Example: this chemical equation shows the reactant
ammonium dichromate yields the products nitrogen,
chromium(III) oxide, and water
(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + 4H2O(g)

4. Indications of Chemical Reactions
1. Evolution of heat and light
2. Production of gas
3. Formation of a precipitate
4. Color change
Chemistry
comes alive!

5. Characteristics of Chemical
Equations
1. Equation MUST represent known facts.
 Must be actual chemical reaction
2. MUST contain the correct formulas for the
reactants and products.
 Sodium Carbonate
3. The law of conservation of mass must be satisfied.
 Same number of atoms on reactant side as on the product
side.
 Coefficients are added where necessary
o Specifies relative number of moles of that substance
H2 + O2 H2O
Ex: Hydrogen gas reacts with oxygen gas to yield water
Na2CO3
2 2

6. H - H O = O
H – O
H
H – O
HH - H
H2 + O2 H2O2 2
4 H’s 2 O’s 2 O’s4 H’s
 Satisfies the law of conservation of mass
 Same atoms in reactants as the products

7. Word and Formula Equations
 Word equation - 1st step in writing chemical
equation
 Equation in which the reactants and products in a
chemical reaction are represented by words
 Only qualitative meaning (doesn’t give quantities)
 Example: methane + oxygen carbon dioxide + water
 read as react to yield or yield (produce or form)
 Equation is read as “methane and oxygen yield carbon
dioxide and water”
 formula equation - 2nd step in writing chemical
equation
 Represents the reactants and products of a chemical
reaction by their symbol
 Example:
 (g) represents that it is in gaseous state
CH4 (g) + O2 (g) CO2 (g) + H2O (g)

8.  balance the equation – last step in writing a
chemical equation
 Satisfy law of conservation of mass by inserting
coefficients
 # of atoms of reactant must = # of atoms of
products
CH4 (g) + O2 (g) CO2 (g) + H2O (g)
Reactant Product
C
H
O
C
H
O
1
4
2
1
2
3
Step 1:
Type &
number
Step 2:
Add
coefficients
till reactants
= products
2
4
4
2
4
Step 3:
When
reactants =
products its
balanced

9. Additional symbols used in
chemical reactions
Yields
Reversible reactions
(s) or solid state
(g) or gaseous state
(l) liquid state
(aq) aqueous (dissolved in water)
reactants are heated
2 atm Pressure
0⁰ C Temperature
MnO2 Catalyst
Examples:
3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)
2HgO(s) 2Hg(l) + O2(g)
C2H2(s) + H2(g) pressure C2H6(g)
Pt

10. Most common mistakes!!!!
1. Writing incorrect formulas for reactants or
products
2. Trying to balance using subscripts
o Can only use coefficients
Practice 1
1. Write the word and formula equations for the chemical reaction
that occurs when solid sodium oxide is added to water at room
temperature and forms sodium hydroxide (dissolved in the
water). Include symbols for physical states in the formula
equation. Then balance the formula equation to give a balanced
chemical equation.
Step 1: write word equation
Sodium oxide + water sodium hydroxide

11. Step 2: convert to formula equation
Na2O + H2O NaOH
Step 3: add all symbols
Step 4: balance
Reactant Product
Na
H
O
2
2
1
1
1
2
22
Na
H
O
2(s) (l) (aq)
2

12. Practice 2
The reaction of zinc with aqueous hydrochloric acid
produces a solution of zinc chloride and
hydrogen gas. Write the balanced chemical
equation for the reaction.
zinc + hydrochloric acid zinc chloride + hydrogen
Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)
Reactant Product
Zn
H
Cl
1
1
1
2
2
2
21
Zn
H
Cl
2

13. Practice 3
Solid aluminum carbide, Al4C3, reacts with water to
produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation
for this reaction.
Aluminum carbide + water + aluminum hydroxidemethane
Al4C3(s) + H2O(l) + Al(OH)3(s)CH4(g)
Reactant Product
Al
C
H
Al
C
H
O O
4
3
1
1
7
3
152
1 3
3 4
4
12
24
12
24
12

14. Practice 4
Aluminum sulfate and calcium hydroxide are used in a
water-purification process. When added to water, they
dissolve and react to produce two insoluble products,
aluminum hydroxide and calcium sulfate. These products
settle out, taking suspended solid impurities with them.
Write a balanced chemical equation for the reaction.
Aluminum sulfate + Calcium hydroxide + calcium sulfatealuminum hydroxide
Al2(SO4)3 (aq) + Ca(OH)2(aq) + CaSO4(s)Al(OH)3(s)
Reactant Product
Al
SO4
Ca
Al
SO4
Ca
OH OH
2
3
1
1
11
2 3
2
2
6
3
3
3
3
3
6