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Chemical Reactions

There are just 26 alphabets in the English language, but the number of words are innumerable. You have studied about the 115 known elements and how they are classified for systematic and comprehensive study. Thousands of compounds are born by various chemical reactions involving these elements.

In the early days of chemistry, these substances were named in a variety of ways. Sometimes, the name referred to the use of the compound, such as baking soda. In others, it reflected a property, such as quicksilver. Or sometimes, it referred to the source of the substance as in wood alcohol. This system worked well when the number of compounds was less. It was realized that a standard system would not only provide better communication with Universities all over the world, but also allow a scientist to name and recognize a large number of substances without having to memorize it, by learning a few rules.

The study of chemistry, at least at the beginning, is like learning a foreign language. Names and formula are to chemistry as nouns and abbreviations are to a foreign language and just as in language study, some of the key rules and practices have to be committed to memory. A knowledge of valency is fundamental to writing a formula.

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What is a chemical reaction?

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A chemical reaction involves rearrangement of atoms from the reactants into new substances, called products.
A chemical reaction network consists of a set of chemical reactions and a set of chemical compounds. Each chemical compound is a node of the network. Each chemical reaction is a directed vertex of the network, connecting the chemical compounds involved in the reaction from the reactants towards the products.The new substances produced have chemical properties different from those of the reactants. A non spontaneous reaction can be made to occur by linking it to an energy source. Reactants collide when sufficient energy is provided and the products are formed.
Five basic types of chemical reactions include combustion, synthesis, decomposition, displacement and double displacement reaction.

Displacement Reaction

When a piece of aluminium foil is added to a solution of copper (II) chloride, reddish copper metal starts to form on the aluminum foil, and some of it falls to the bottom of the container. Aluminium is displacing copper in the solution to form copper metal.
This reaction is single displacement reaction, where Aluminium is replacing copper from its solution.

2Al(s) + 3CuCl2 (aq) 2AlCl3 (aq) + 3Cu(s)

An activity series predicts displacement reactions. Any element on the list will displace elements below it from compounds that are in solution.

Double displacement reactions can be presented by net ionic equations indicating only the ions that are exchanged.
2KI + Pb(NO3)2 PbI2 + 2KNO3 ,
all in aqueous solution.

Decomposition Reaction

When calcium carbonate is heated, carbon dioxide gas escapes, and then calcium oxide is left behind. This reaction is said to be decomposition where one compound breaks into either elements or simple compounds.

CaCO3 CaO + CO2

Synthesis Reaction

In a synthesis reaction, complex molecules are made from simpler substances.
Example
6CO2 + 6H2O C6H12O6 + 6O2

Plants can synthesize complex molecules, such as starch, caffeine chlorophyll, etc, from simple molecules, like carbon dioxide and water.

Combustion Reaction

Oxygen combines with other elements in a combustion reaction. Many substances react with oxygen producing oxides. They are exothermic reactions, releasing large amount of energy in the form of light, sound, etc. Such reactions are called as combustion reactions.

CH4 + 2O2 CO2 + 2H2O + 803 kJ
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Chemical Reactions Examples

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Some examples of common chemical reactions are

Example:
1

CH3OH CO + 2H2

In the above chemical reaction, we see that methanol has disintegrated into two small compounds, carbon monoxide and hydrogen.
This reaction is a displacement reaction.

2K + 2HOH 2KOH + H2

Example:
2

Potassium replaces hydrogen from its solution, water. So, since one element has replaced another in the reaction, this reaction is a single replacement or displacement reaction.

Chemical Reaction Experiments

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Many chemical reaction experiments are carried out in the chemical industry. Some examples of chemical reaction experiments are:
  1. Sulfuric acid manufacture by contact process
  2. Ammonia manufacture by Haber's process
  3. Nitric acid manufacture by Ostwald process
  4. Chloro alkali industry
  5. Extraction of metals and the production of oil

Signs of a Chemical Reaction

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  1. A formula equation uses atomic symbols to describe the rearrangement of atoms of the reactants to form products in a chemical reaction.
  2. Notations in a chemical equation can indicate conditions required for a reaction to occur as well as the state of matter for the substances in the reaction.
  3. The coefficients in a balanced equation can be interpreted in terms of formula units, number of molecules, atoms or ions or as a mole ratio.
  4. Chemical equations may describe energy changes in a reaction. ΔH for example, indicates the energy absorbed as heat for the overall reaction.

Evidence of a chemical reaction

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  1. An evidence that a chemical reaction has occurred can be seen in many ways.
  2. Some reactions proceed to completion with a change in color of the reaction mixture.
  3. Hydrogen or oxygen can evolve from the reaction mixture. So, evolution of a gas can also indicate that the chemical reaction is complete, or some reaction is happening.
  4. Many reactions may occur with a loud noise. So, in most chemical reactions, the evidence of a chemical reaction taking place is seen clearly.
  5. Say, when we react a mixture of acid, a colorless substance with a base which is also colorless, we can hear some sort of hissing noise coming up from the reaction mixture which is the evidence of a chemical reaction.

Rate of Chemical Reactions

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The rate of a chemical reaction may be defined as the change in concentration of a reactant in a given time interval. It is the measure of the speed of a reaction. It is expressed in moles/volume.time.
It is expressed as

Average reaction rate = $\frac{Total\ change\ in\ concentration}{Time\ taken\ for\ the\ change}$

Exothermic Chemical Reaction

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A chemical reaction is usually accompanied by energy changes. Energy may be needed for a reaction to occur or it may be released in certain reactions like combustion.

Depending upon how a reaction is proceeding with respect to energy changes, there are two types of reactions

Endothermic Chemical Reaction


Reactions that take in heat, or a reaction where energy has to be supplied for the reaction to occur, are said to be exothermic reactions.

These can be represented as
Reactants + energy Products.

Endothermic Reaction


Reactions like combustion reaction, occur with the release of a lot of energy. Such reactions where energy is released when a reaction is completed are said to be exothermic reactions.

They can be represented as :

Reactants Products + energy in KJ

The energy released in an exothermic reaction, can be of any form. Some reactions proceed with a loud bang, so energy here is released in the from of sound. Some reactions give out a lot of light, so energy is released in the form of light.

Combustion Chemical Reaction

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Combustion normally refers to combining a substance, such as an element, with oxygen. Combustion can be complete, involving an excess of oxygen, or incomplete, using limited supply of oxygen.

Example: Combustion of hydrocarbons with excess oxygen. A hydrocarbon, on combustion gives carbon dioxide and water.

Oxygen is a reactive molecule. So, it combines with almost all elements. Combustion can also be defined as burning of substances in the presence of air or oxygen.
More topics in Chemical Reactions
Atoms and Molecules Conservation of Matter
Chemical Reactions Experiments
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