Chemical Reactivity | General Chemistry 2

Chemical reactivity is studied in this chapter: chemical reactions and equations, balancing chemical equations, types of reaction (combination, decomposition, replacement, combustion), combustion analysis, acid-base theories, oxidation-reduction reactions

Chemical Reactions

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)

Balancing Chemical Equations

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

Types of Reaction

Commonly encountered reaction types are:
 

  • Combination: 2 or more reactants combine to form a single product
  • Decomposition: 1 substance breaks down to form 2 or more products
  • Single replacement: 1 more reactive element replaces another element in a reactant
  • Double replacement: the cations and anions of 2 compounds switch places
  • Combustion: a compound (usually an alkane) reacts with O2 to produce 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]

Br(l) + CaI(aq) → CaBr(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

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

Acid-Base Theories

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 Reactions

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

Check your knowledge about this Chapter

A chemical reaction is the transformation of one or more substances into different substances with different properties. Signs that a chemical reaction has occurred may include the evolution of gas, the formation of a precipitate, a change in temperature, a change in color, the emission of light, or a change in the energy of the system. These signs indicate that chemical bonds have been broken and new bonds have been formed, changing the composition of the original substances.

  • 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

It is important to balance chemical equations in order to obey the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction. Balancing equations ensures that the same number of each type of atom is present on both the reactants and products side of the equation, indicating that the mass of the system is constant.

 Balancing chemical equations involves the following steps:

  1. Start by writing the unbalanced chemical equation.

  2. Determine the initial number of atoms for each element on either side of the arrow.

  3. Adjust the coefficients of the compounds before changing the coefficients of the individual elements.

  4. As a final step, consider balancing the oxygen or hydrogen atoms.

  5. Finally, check the equation to ensure that the stoichiometric coefficients cannot be further reduced.

The main types of chemical reactions are:

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

To balance a combustion reaction: 

  • First, write the unbalanced equation with the organic compound and oxygen as reactants and carbon dioxide and water as products
  • Next, balance the number of carbon atoms by adjusting the coefficients in front of CO2 
  • Then, balance hydrogen by adjusting the water (H2O) coefficients
  • Finally adjust the oxygen molecules (O2) to balance the oxygen atoms on both sides of the equation.

If there is an odd number of oxygen atoms after balancing carbon and hydrogen, you may need to use fractional coefficients initially, which can be multiplied by two at the end to clear any fractions. 

In combustion analysis, the unknown organic compound is burned in the presence of excess oxygen, producing carbon dioxide and water as the main combustion products. By measuring the mass of carbon dioxide and water produced, the amounts of carbon and hydrogen in the original compound can be determined. This data, along with information on the mass of the original sample, can be used to calculate the empirical formula of the compound. If the molar mass of the compound is known, the molecular formula can also be determined.

  • According to Arrhenius, acids and bases are identified by their ability to produce H+ or OH ions in aqueous solutions.
  • In the Bronsted theory, substances are recognized as acids or bases based on their ability to donate or accept protons.
  • In the Lewis theory, acids are identified as electron pair acceptors, and bases as electron pair donors, regardless of the presence of protons.

The Bronsted-Lowry theory defines acids as proton donors and bases as proton acceptors, extending the concept beyond the Arrhenius definition that limited acids to substances that produce hydrogen ions in water and bases to substances that produce hydroxide ions in water. This broader definition allows substances to be classified as acids or bases in non-aqueous solutions and even in reactions where no water is present. Additionally, it accounts for the role of substances that can act as both acids and bases, known as amphiprotic species.

The oxidation state indicates the degree of oxidation of an atom in a chemical compound. Oxidation involves an increase in the oxidation state due to the loss of electrons, whereas reduction involves a decrease in the oxidation state due to the gain of electrons. By comparing the oxidation states of an element before and after a reaction, you can determine whether it has been oxidized (oxidation state increased) or reduced (oxidation state decreased).

In the reaction Zn + CuSO4 → ZnSO4 + Cu, zinc goes from an oxidation state of 0 in Zn to +2 in ZnSO4, indicating it has been oxidized, while copper is reduced from +2 in CuSO4 to 0 in Cu.

An oxidizing agent is characterized by its ability to accept electrons and undergo reduction in a redox reaction, while a reducing agent is defined by its ability to donate electrons and be oxidized in the process.