Chemical reactions

Introduction

chemical reactions are the processes in which chemical composition changes
- the changes can be either:
  - physical - evaporation, sublimation, condensation
    - atomic composition stays the same
  - chemical - breaking bonds and creating new ones
    - atomic composition changes
chemical equation is a way to represent a chemical reactions based on the lowest mathematical molar ratios of the different substances that react together
- on both sides of a chemical equation:
  - the charge is conserved (law of conservation of energy)
  - the mass is conserved (law of conservation of mass)
reaction scheme is an unbalanced chemical equation (mass nor charge is conserved)
- the scheme is balanced using stoichiometrich coefficients
reactants are the substances that come into a chemical reaction
- they are written on the left side of a chemical equation
products are the substances that come out of a chemical reactions as the result of all the processes of the chemical reaction
- they are written on the right side of a chemical equation
stoichiometry is the collections of different calculations that can be performed with a balanced chemical equation
- the calculations are based on different laws (ideal gas law, vide ultra)
limiting reagent is the reactant that is in scarcity
excess reagent is the reactant that is in abundance (= is left unreacted due to a pressence of the limiting reagent)

Net ionic equations

ionic equations help us understand what exactly happens in an aqeous solution
they take into account every dissociation and formation
it also helps us identify spectator ions (ions, which do not interfere with the reactions in any significant way)
when a newly formed substance is insoluble, it is called the precipitate (verb to precipitate)
example:
- reaction scheme: $A l C l_{3} (a q) + N a O H (a q) ⟶ N a C l (a q) + A l (O H)_{3} (s)$
- chemical equation: $A l C l_{3} (a q) + 3 N a O H (a q) ⟶ 3 N a C l (a q) + A l (O H)_{3} (s)$
- net ionic equation: $A l^{3 +} + 3 C l^{-} + 3 N a^{+} + 3 O H^{-} ⟶ 3 N a^{+} + 3 C l^{-} + A l (O H)_{3} (s)$
- solute: $N a C l$
  - spectator ions: $N a^{+}$ and $C l^{-}$
- precipitate: $A l (O H)_{3}$

Types of chemical reactions

synthesis (combination reaction) is a reaction where two or more substances react to form a single new substance $A + B ⟶ A B$
analysis (decomposition reaction) is a reaction where one substance breaks up into two or more new substances $A B ⟶ A + B$
substitution (single-replacement reaction) is a reaction where a part of a molecule is replaced by a new substance $A B + C ⟶ A C + B$
double-replacement reaction is a reaction where parts of two molecules switch forming two new substances $A B + C D ⟶ A C + B D$
homogeneous reaction is a reaction where all the reactants are the same phase
heterogeneous reaction is a reaction where all the reactants are not the same phase

Redox reactions

oxidation-reduction reaction is a reaction where one atom undergoes oxidation and another undergoes reduction
oxidation is the process of increasing an atom’s oxidation state (i.e. the charge it would have if it was ripped out of the molecule it is in)
- this is caused by ridding of electrons
- a substance (or atom) that is capable of oxidizing other substances is called the reducing agent
reduction is the process of decreaseing an atom’s oxidation state
- this is caused by accepting electrons
- a substance that is capable of reducing other substances is called the oxidizing agent
examples of such reactions are: the formation of glucose and release of oxygen in plants (carbon reduces, oxygen oxidizes), corrosion of metals (metal oxidizes, oxygen reduces), neutralization of hypermangan ( $2 K M n O_{4} + 16 H C l ⟶ 5 C l_{2} + 2 M n C l_{2} + 2 K C l + 8 H_{2} O$ )
the balance of the change of oxidation states must be conserved
balancing redox equaions is more complicated than the balancing of simple equation where no substance undergoes oxidation or reduction
1. the oxidizing and reducing agents are identified
2. the total change in oxidation states is noted and switched between the agents (if the two numbers are divisible by one another, the division is made)
3. the numbers from the second step are written as stoichiometric coefficients to the reaction to the corresponding substances which undergo the oxidation or reduction
4. the two sides of the reaction scheme are then checked and the conservation of mass is controlled, if any inequiality is found, the stoichiometric coefficient are adjusted
disproportionation is a reaction, where one atom undergoes both oxidation and reduction, thus appearing in two different molecules of the product mixture in two different oxidation states
comproportionation is a reaction, where one atom in two different molecules of the reactant mixture undergoes both oxidation and reduction appearing in only one molecule in the product mixture in one new oxidation state