# Module 3: Reactive Chemistry

## Types of Reactions

### Decomposition Reactions

• Decomposition reactions involve breaking down one compound into 2 or more simpler substances
• Decomposition is an ENDOTHERMIC reaction, meaning it reuqires heat input
• An example of a decomposition reaction is carbonate decomposition:

$$\color{lightgreen}{CuCO_3}$$$$\rightarrow$$$$\color{lightblue}{CuO+CO_2}$$

$$\color{lightgreen}{\text{Green: Reactants}}$$

$$\color{lightblue}{\text{Light Blue: Products}}$$

#### Decomposition by light

• Some compounds will decompose when exposed to light
• An example is Silver Nitrate ($$AgNO_3$$):

$$\color{lightgreen}{AgNO_3}$$$$\rightarrow$$$$\color{lightblue}{2Ag + 2NO_2 +O_2 }$$

• Light-based decomposition is the basis of film photography

### Combustion Reactions

• Combustion reactions occur when something burns
• Combustion reactions are EXOTHERMIC (i.e. light, sound, heat are usually produced)
• Oxygen (or any oxidizer) is always a component of a combustion reaction
• An example of a combustion reaction is burning Propane:

$$\color{lightgreen}{2C_3 H_{8(g)} +7O_{2(g)}}$$$\rightarrow$$$\color{lightblue}{2C_{(s)} + 2CO_{(g)}+ 2CO_{2(g)} +8H_2 O_{(g)}}$$

• Some combustion reactions only have $$CO_2$$ and $H_2 O$ as products
• These are known as “complete combustion reactions”
• An example of a complete combustion reaction is burning Methane:

$$\color{lightgreen}{CH_4 +2O_2}$$$\rightarrow$$$\color{lightblue}{CO_2 +2H_2 O}$$

### Precipitation Reactions

• When soluble ionic compounds are dissolved in water, the lattice “dissolves”, and the ions are separated
• If two solutions are mixed together, it’s really just 4 different ions suspended in water
• However, certain combinations of ions will form an insoluble compound when mixed
• These compounds will form a PRECIPITATE, a small ionic crystal lattice
• This is known as a precipitation reaction
• An example is mixing sodium sulfide and copper sulfate solutions:

$$\color{lightgreen}{Na_2 S_{(aq)}+CuSO_{4(aq)}}$$$$\rightarrow$$$$\color{lightblue}{CuS_{(s)}+Na_2SO_{4(aq)}}$$

#### Solubility Rules

The solubility rules are used to determine which compound is the precipitate.

Ion Soluble? Exceptions
$NO_{3}^-$
$ClO_{4}^-$
$Cl^-$ $Ag, Hg_2 , Pb$
$I^-$ $Ag, Hg_2 ,Pb$
$SO_{4}^{2-}$ $Ca, Ba, Sr, Hg, Pb, Ag$
$CO_{3}^{2-}$ Alkalis and Ammonium
$PO_4 ^{3-}$ Alkalis and Ammonium
$OH^-$ Alkalis, $Ca, Ba, Sr$
$S^{2-}$ Alkalis, Alkaline Earths, Ammonium
$Na^+$
$NH_4 ^+$
$K^+$

#### NAGSAG and PMS (Mnemonics)

• NAGSAG can be used to remember common soluble ions:

N - Nitrates ($NO_{3}^-$)

A - Acetates ($C_2 H_3 O_2 ^-$)

G - Group 1 ($Li^+ , Na^+ , K^+ , etc.$)

S - Sulfates ($SO_{4}^{2-}$)

A - Ammonium ($NH_4 ^+$)

G - Group 17 ($F^- , Cl^- , Br^- , I^- , etc.$)

• PMS can be used to remember exceptions:

P - $Pb^{2+}$ (Lead)

M - Mercury

S - Silver

### Corrosion Reactions

• Corrosion is a reaction involving a metallic element being converted into a more chemically stable form (e.g. an oxide, hydroxide, or sulfide)
• Combustion and Corrosion are both types of “oxidization reactions”
• Corrosion is EXOTHERMIC, although not as much as combustion
• An example of corrosion is iron rusting:

$$\color{lightgreen}{4Fe+3O_2}$$$$\rightarrow$$$$\color{lightblue}{2Fe_2 O_3}$$

### Acids and Bases

#### Neutralization Reactions

• When an acid and base are added together, they “neutralise” each other
• This creates water and an ionic compound known as a “salt”
• The general formula for acid-base reactions is:

$$\color{lightgreen}{\text{Acid}+\text{Base}}$$$$\rightarrow$$$$\color{lightblue}{\text{Salt}+H_2 O}$$

#### Acid-Metal Reactions (Displacement Reactions)

• Many metals will react with Acids to produce a “salt” and Hydrogen gas ($$H_2$$)
• The general formula for Acid-Metal reactions is:

$$\color{lightgreen}{\text{Acid}+\text{Metal}}$$$$\rightarrow$$$$\color{lightblue}{\text{Salt}+H_2}$$

#### Acid-Carbonate Reactions

• When an acid reacts with a carbonate compound, the products are always $$\ce{CO2}$$, $$H_2 O$$, and a salt
• The general formula is:

$$\color{lightgreen}{\text{Acid}+\text{Carbonate Compound}}$$$$\rightarrow$$$$\color{lightblue}{\text{Salt}+H_2 O + CO_2 }$$

### Redox Reactions

• Redox is short for “Reduction-Oxidization”
• Redox reactions occur between 2 substances, where electrons are LOST by one (the reductant), and GAINED by the other (the oxidant)
• An easy way to remember this is with AN OIL RIG CAT:
• AN - at the ANode,
• OIL - Oxidization Involves Loss of electrons
• RIG - Reduction Involves Gain of electrons
• CAT - at the CAThode

#### Rules

1. Metals are always reductants, Metal IONS are always Oxidants
2. Oxygen has an oxidation state of $2-$ (unless in a peroxide)
3. Hydrogen has an oxidation state of 1+ (except in metal hydrides)
4. Free elements have an oxidation state of 0
5. The oxidation state of an ion is equal to it’s charge
6. In compounds, the sum of all oxidation states is 0
7. The halogens (F, Cl, Br and I) typically have an oxidation state of 1- in their ionic compounds. In molecular compounds their oxidation number is typically 1- or 7-.
8. When naming ionic compounds in which variable oxidation states of metal ions are present, the oxidation state is shown in roman numerals.

$\text{Example: }\ce{FeCl2}$

$\require{color}\text{Iron has an oxidation state of 2+}$

$\text{Therefore, the compound is called Iron } \colorbox{lightgray}{(II)} \text{ Chloride}$

1. When Hydrogen ($\ce{H2}$) is burned in Oxygen ($\ce{O2}$), water ($\ce{H2O}$) is formed

## Rates of Reaction

### Activation Energy $(E_a )$

• Activation energy is the minimum amount of energy required to initiate a reaction
• Activation energy is measured in $kJ/mol$ 1

$\color{green}k=Ae^{\frac{-E_{a}}{RT}}$

$k:\text{Reaction Rate Coefficient}$

$A:\text{Frequency Factor of the reaction}$

$e:\text{Euler’s Number (}e\text{ on calculator, approx. 2.7182)}$

$R:\text{Universal Gas Constant}$

$T:\text{Temperature }(K)$

• According to this equation, the rate of reaction increases with temperature
• However, there are some cases where the activation energy is negative, and so higher temperatures DECREASE the rate of reaction

#### Catalysis

• Catalysis is the process of increasing the rate of a chemical reaction by introducing a catalyst

• A catalyst is any substance that lowers the activation energy of a reaction WITHOUT MODIFYING THE PRODUCTS

• Catalysts are not consumed by the reaction, and do not change the equilibrium constant of the reaction.

• As a result, catalysts should be written in both the products and reactants of a chemical equation
• Catalysts which trigger a reaction are known as Activators
• The SI unit for Catalysis is the Katal $(Kat)$

$1Kat=1mol/s$

• There are 3 main kinds of catalysts:

• Heterogenous Catalysts are those which exist in a different phase from the reaction being catalyzed. For example, solid catalysts the catalyze a reaction in a mixture of liquids and/or gases are heterogeneous catalysts. Surface area is critical to the functioning of this type of catalyst.
• Homogenous Catalysts exist in the same phase as the reactants in the chemical reaction. Organometallic catalysts are one type of homogeneous catalyst.
• Enzymes are protein-based catalysts. They are one type of biocatalyst. Soluble enzymes are homogeneous catalysts, while membrane-bound enzymes are heterogeneous catalysts.

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