Chemical Reactivity | General Chemistry 2

Chemical reaction:

A process in which one or more substances (the reactants) are converted to one or more different substaces (the products)
 

Chemical equation

A chemical reaction is represented by a chemical equation:

• The chemical formulas of the reactants are on the left-hand side 
• The chemical formulas of the products are on the right-hand side
• Reactants and products are separated by an arrow
• The chemical formula of each substance is separated from the others by a '+' sign

Physical states can be specified in parentheses after the chemical formula of each substance as (s), (l), (g) or (aq)  for solid, liquid, gas, and aqueous (dissolved in water), respectively
 

Mg (s) + Cl₂ (g) → MgCl₂ (s)

According to the atomic theory, atoms are neither created nor destroyed in a chemical reaction ⇒ the chemical equation must be balanced (same number of each type of atom on both sides) by changing the coefficients of the reactants and/or products, and never by changing their formulas
 

Balance the following chemical equation: Li + Br₂ → LiBr

2 Li + Br₂  → 2 LiBr   (balanced)
2 Li + Br₂  → LiBr₂   (incorrect: the nature of the product is different ⇒ LiBr₂ ≠ LiBr)

How to balance chemical equations:

1. Write the unbalanced chemical equation

2. Determine how many atoms of each element are present on either side of the arrow

3. Change the coefficients of the compounds before changing the coefficients of the elements

4. Balance the oxygen or hydrogen atoms last

5. At the end, check your equation to make sure you can't reduce the stoichiometric coefficients

Commonly encountered reaction types are:
 

• Combination: 2 or more reactants combine to form a single product
• Decomposition: 1 substance splits apart to form 2 or more products
• Single replacement: 1 element more reactive replaces another element in a reactant
• Double replacement: the cations and anions of 2 compounds switch places
• Combustion: a compound (generally an alkane) reacts with O₂ producing CO₂, H₂O and energy (heat and light)

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

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

Br₂ (l) + CaI₂ (aq) → CaBr₂ (aq) + I₂ (s)     [single replacement]
Fe (s) + H₂SO₄ (aq) → FeSO₄ (aq) + H₂ (g)     [single replacement]

NaCl (aq) + AgNO₃ (aq) → NaNO₃ (aq) + AgCl (s)     [double replacement]
BaCl₂ (aq) + Na₂SO₄ (aq) → 2 NaCl (aq) + BaSO₄ (aq)     [double replacement]

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

Combustion analysis:

An analysis used to determine the empirical formula of chemical compounds and mainly organic molecules (which contain carbon, hydrogen, and sometimes oxygen)
 

Principle:

The sample is burned in a stream of oxygen gas; all the elements present are converted into CO₂ and H₂O. The masses of water and carbon dioxide formed are measured. If oxygen is present in the original sample, it must be determined by mass difference

Arrhenius theory:

• Arrhenius acid: a substance which has hydrogen in its formula and which dissociates in water to give H⁺ ions ⇒ Arrhenius acid increases H⁺ concentration when added to water
• Arrhenius base: a substance which has OH in its formula and which dissociates in water to give HO^- ions ⇒ Arrhenius base increases HO^- concentration when added to water

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

Brönsted theory:

• Brönsted acid: a substance that can donate a proton H⁺ (proton donor)
• Brönsted base: a substance that can accept a proton H⁺ (proton acceptor)

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

NH₃ (aq) + H₂SO₄ (aq) → NH₄⁺ (aq) + HSO₄^- (aq)
NH₃ is a base, H₂SO₄ is an acid.
NH₄⁺ / NH₃ and H₂SO₄ / HSO₄^- are 2 conjugate acid-base pairs

Lewis theory:

• Lewis acid: a species that can accept a pair of electrons (electron-pair acceptor) ⇒ Lewis acid must be electron deficient or have a vacant orbital: BF₃, AlCl₃, SO₂
• Lewis base: a species that can donate a pair of electrons (electron-pair donor) ⇒ Lewis base must have a lone pair of electrons: NH₃, H₂O

When a Lewis base donates a pair of electrons 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 reaction (redox reaction):

A chemical reaction in which electrons are transferred from one reactant to another. The oxidation state of an atom, molecule or ion changes during a redox reaction.

• Oxidation: the particle becomes more positively charged (loss of electrons) ⇒ the oxidation state increases
• Reduction: the particle becomes less positively charged (gain of electrons) ⇒ the oxidation state decreases

Oxidizing vs. reducing agent:

Oxidizing agent: a species that can accept electrons ⇒ an oxidizing agent is reduced in a redox reaction
Reducing agent: a species that can donate electrons ⇒ a reducing agent is oxidized in a redox reaction
 

2 Fe (s) + 3 Cl₂ (aq) → 2 Fe³⁺ (aq) + 6 Cl^- (aq) is an oxidation-reduction reaction

Fe becomes Fe³⁺ during this reaction ⇒ it donates electrons ⇒ it is the reducing agent
Cl₂ becomes Cl^- during this reaction ⇒ it gains electrons ⇒ it is the oxidizing agent