Chemical Reactivity | General Chemistry 2

Chemical reactivity is studied in this chapter: precipitation, acid-base reactions, oxidation-reduction reactions, combination and decomposition, replacement reactions, combustion.


Precipitation: formation of a solid, called precipitate.
A solid is formed if ions of an insoluble salt are present.


AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
Formation of a precipitate: AgCl.



Types of Equations:

- Molecular equation: shows the formulas of the compounds.
     AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)

- Ionic equation: shows the ions of the compounds.
     Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) → AgCl (s) + Na+ (aq) + NO3- (aq)

- Net ionic equation: shows only the ions that form a solid.
     Ag+ (aq) + Cl- (aq) → AgCl (s)

Acid - Base

Arrhenius definitions of acids and bases:

- Acids: substances that have H in their formulas and that dissociate in water to give H+ ions.
- Bases: substances that have OH in their formulas and that dissociate in water to give HO- ions.


HCl is an Arrhenius acid: HCl (aq) → H+ (aq) + Cl- (aq)
NaOH is an Arrhenius base: NaOH (aq) → Na+ (aq) + HO- (aq)



Brönsted acid-base theory:

- Acids: substances that can donate a proton (proton donor)
- Bases: substances that can accept a proton (proton acceptor)
The acid molecules HA gives an H+ to the base molecule B: HA + B → A- + HB+

Conjugate acid-base pair (HX / X-): acid-base pair that differ only in the presence or absence of a proton


Ex: NH3 (aq) + H2SO4 (aq) → NH4+ (aq) + HSO4- (aq)
NH3 is a base, H2SO4 is an acid.
NH4+ / NH3 and H2SO4 / HSO4- are two conjugate acid-base pairs



 Lewis acid-base theory:

- Acids: electron-pair acceptors ⇒ must be electron deficient or have an orbital that can accept electrons: BF3, AlCl3, SO2
- Bases: electron-pair donors ⇒ must have a lone pair of electrons: NH3
When a Lewis base gives electrons from a lone pair to a Lewis acid, a covalent bond forms between the molecules and the product is called an adduct.


NH3 + BF3 → NH3BF3
NH3BF3 is an adduct

Oxidation - Reduction Reactions

Oxidation number:

- for an ion: charge of the ion
- for an atom: hypothetical charge if it was an ion
⇒ group IA: + 1; group IIA: + 2; oxygen: - 2; halogens: - 1; H with a nonmetal: + 1; H with a metal: - 1

Oxidation-reduction reactions (Redox reactions):
reactions in which the oxidation number of an atom, molecule or ion changes.
- Oxidation: particle becomes more positively charged (loss of electrons) ⇒ oxidation number increases
- Reduction: particle becomes less positively charged (gain of electrons) ⇒ oxidation number decreases

Oxidizing agent: substance that is reduced in a redox reaction
Reducing agent: substance that is oxidized in a redox reaction

Balancing Redox Reactions:

1) Divide the reaction into half-reactions
2) Balance atoms (other than O and H)
3) Balance O atoms by adding H2O molecules
4) Balance H atoms by adding H+ ions
5) Balance charge by adding electrons
6) Multiply each half-reaction by an integer to have: 
number of electrons lost = number of electrons gained
7) Add the half-reactions together

Under basic conditions:
8) Add HO- ions to react with all H+ ions: HO- + H+ → H2O

Combination et Decomposition Reactions

Combination reactions:
2 or more elements (or simple compounds) combine to form 1 product.

2 Na (s) + Cl2 (g) → 2 NaCl (s)
CO2 (g) + H2O (l) → H2CO3 (aq)


Decomposition reaction:
one substance splits into 2 or more simpler substances.

CaCO3 (s) → CaO (s) + CO2 (g)
MgSO4 (s) → MgO (s) + SO3 (g)

Replacement Reactions

Single replacement:
1 element takes the place of a different element in a reacting compound.
An element will displace another element in a compound if it is more active than the element in the compound.

Br(l) + CaI(aq) → CaBr(aq) + I2 (s) 
[activity of halogen gases decreases down the column]

Fe (s) + H2SO4 (aq) → FeSO4 (aq) + H2 (g)


Double replacement:
2 elements in the reactants exchange place.


NaCl (aq) + AgNO3 (aq) → NaNO3 (aq) + AgCl (s)
BaCl2 (aq) + Na2SO4 (aq) → 2 NaCl (aq) + BaSO4 (aq)

Combustion Reactions

Combustion reaction:
a compound (generally an alkane) reacts with O2 and produces CO2, H2O and energy (heat and light).

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)
2 C2H6 (g) + 7 O2 (g) →4 CO2 (g) + 6 H2O (g)


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