Summary
Key terms
Chemical formula: The chemical formula of a substance is a notation that uses atomic symbols with numerical subscripts to convey the relative proportions of atoms of each element in that substance. For example chemical fomula of Aluminium hydroxide, Al(OH)3. This compound consists of one aluminium atom and 3 oxygen atoms and 3 hydrogen atoms.
Oxidation reactions: In oxidation reactions, addition of oxygen, or removal of hydrogen takes place in a compound.
OR
In a broader sense, an oxidation reaction is the one where an atom or ion loses electrons.
Reduction reaction: In a reduction reaction, oxygen is removed from or hydrogen is added to a compound.
OR
A reduction reaction is a reaction where an atom or an ion gains extra electrons.
Redox reaction: Any chemical reaction in which the oxidation numbers (oxidation states) of the atoms are changed is an oxidation–reduction reaction. Such reactions are also known as redox reactions, which is shorthand for reduction–oxidation reactions.
Isomerization reactions: A Chemical reactions where rearrangements of atoms occur with in a substance without any change in the molecular formula are called isomerization reactions.
Example below shows the isomerization reaction:
Example below shows the isomerization reaction:
Photochemical reactions: The reactions, in which light energy is used to start the reaction and bring about a chemical change in a compoud. Such a reactions are known as photochemical reactions. Photosynthesis is a good example of this type of reaction.
Precipitate: A precipitate is a solid product which separates out from the solution during a chemical reaction. It can also be formed by passing a gas into an aqueous solution of a substance (like passing carbon dioxide into lime water).
A common example is that of mixing of two clear solutions:
A common example is that of mixing of two clear solutions:
- Silver nitrate (AgNO3)
- Sodium chloride (NaCl)
Exothermic reactions: A Chemical reaction in which release of energy is observed as the reaction proceeds. Generally, exothermic reactions occur spontaneously, when the reacting substances are brought closer.
Endothermic reactions: A Chemical reaction in which energy has to be supplied for the reaction to occur. The spontaneity is not present, as heat has to be supplied for the reaction.
Combustion:
Combustion or burning is a chemical reaction that occurs between a fuel and an oxidizing agent that produces energy, usually in the form of heat and light. Example:
CH4 [g] + 2O2 [g]
CO2 [g] + 2H2O [g] + energy
CH4 [g] + 2O2 [g]
CO2 [g] + 2H2O [g] + energy Neutralization: These reactions are special type of double replacement reaction. These reactions are exothermic reactions. Neutralization reaction is a chemical reaction in which an acid and a base react to form a salt. Water is frequently, but not necessarily, produced as well.
Key concepts
- A physical reaction (change) always accompanies with a change of state and no new products are formed. In a chemical reaction new products are formed but there may or may not be a change of state. In any chemical reaction there is breakage of existing bonds and formation of new bonds. In both physical and chemical changes accompany with a change in energy.
- A compound can be represented with a chemical formula which describes the simplest atom ratio, actual number of atoms in that compound.
- While writing a chemical formula, if it is an ionic compound, name of the cation written in first followed by name of the anion. The suffix – ide is written for an anion. When two or more non – metal compounds have different numbers of the same elements, prefixes are added to remove the ambiguity. Names of hydrates give the number of associated water molecule with a numerical prefix.
- A chemical reaction is represented by a chemical equation. An arrow (→) in a equation points from the reactants to the product indicates the direction in which the reaction is proceeding.
- A double arrow (
), the arrow faces products as well as reactants represents that the products recombine to give rise to reactants again. - An arrow pointing downward indicates the formation of a product in the form of a precipitate (↓)and an arrow pointing upward shows the release of a gaseous product (↑).
- The physical state of the chemicals may be written near the formula by using standard short forms such as (s) – – – solid ; (l) – – – liquid ; (g) – – – gas or vapor and (aq) – – aqueous solution.
- If heat is required to initiate the reaction it is written over the horizontal arrow or on the left side of the equation. If heat is given out, then it is written on the right side of the equation. If any catalyst is used for increasing the reaction rate, then it is written below the horizontal arrow.
- A balanced chemical equation means each atom shown in the equation must appear on both sides of the arrow for the same number of times.
- With the help of a balanced chemical equation we know the information about the elements present in the reaction, the number of atoms of each element taking part in the reaction, the formula of the reactants and the products, the number of moles of each reactant and product and the direction of the reaction.
- If we know the relative atomic mass of each element, we should be able to calculate the weight of each of the compounds in the reaction.
- A balanced equation does not tell us if the reaction is complete or not, what is the rate at which the reaction is proceeding, etc.
- When two reactants combine to give a new compound, then the reaction is known as combination reaction (or synthesis or addition reaction).
- When a single compound breaks up into more than one product, the reaction is known as decomposition reaction. Generally decomposition occurs when heat is supplied.
- When one element replaces (or displaces) another from a compound, the reaction is known as substitution reaction.
- An element or species which is more reactive displaces the least reactive element in a substitution or displacement reaction.
- Displacement reactions commonly occur in metallic salt solutions where a metal (reactive) displace the other metal in ionic form.
- When the reactants in a chemical reaction, exchange groups while making the products, the reaction is described as double decomposition reaction or double displacement reaction.
- In oxidation reaction, oxygen is added to, or hydrogen is removed from a compound or electron is removed from an element. In a reduction reaction, oxygen is removed from, or hydrogen is added to a compound or electron is added to an element. When oxidation occurs simultaneously reduction should occur, the combined reaction is called as a redox reaction.
- Combustion or burning is a type of redox reaction that occurs between a fuel and an oxidizing agent that produces energy, usually in the form of heat and light.
- Compounds having the same molecular formula but different arrangements of atoms are known as isomers. Chemical reactions where rearrangements of atoms occur within a substance without any change in the molecular formula to give an isomer of the reactant are called isomerization reactions.
- Neutralization reaction is a special type of double displacement reaction in which an acid and a base react to form a salt.
- In photochemical reactions, light energy is used to start the reaction and bring about a chemical change. Photosynthesis is a good example of this type of reaction.
- The conversion of reactants into products in a chemical reaction is often accompanied by some observable changes such as evolution of a gas, formation of a precipitate, change in color, change in temperature, change in state etc.
- Based on the change in heat of the reaction, chemical reactions are broadly classified into two types – exothermic reactions and endothermic reactions. In an exothermic reaction enthalpy of formation of products is less than the enthalpy of formation of reactants so there is an evolution of heat during reaction and reverse is the case where heat is absorbed for an endothermic reaction.