1. Introduction to Chemical Reactions and Equations
There are two types of changes, one physical, which is a temporary and reversible change and the other one is a chemical change which is irreversible and permanent in most of the cases.
We encounter chemical reactions all the time in our daily lives.Digestion of food, burning of a candle, cooking etc involve chemical reactions.
A chemical reaction or a chemical change is a process in which one set of chemicals,called as reactants is transformed into another set called the products.
A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae.
Characteristics of Chemical Reactions
A chemical reaction is a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.
Example: Rusting of iron, the setting of milk into curd, etc.
Reactant: The chemical substance which takes part and undergoes a chemical change.
Product: The new substance which is formed after a chemical change.
Observations for a chemical change:
- Change in state of a substance.
- Change in colour of a substance.
- Change in temperature
- Evolution of heat
- Absorption of heat.
- Evolution of gas.
- Evolution of light and sound.
Chemical equation:
The representation of the chemical reaction using symbols of substances.
Example: A + B ⇨ C + D
In this equation, A and B are called reactants and C and D are called the products. The arrow shows the direction of the chemical reaction.
A chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.
Balanced Chemical Equation:
A chemical equation having an equal number of atoms on both sides of the reaction is called a balanced chemical reaction.
E.g., Zn + H2 SO4 ⇨ ZnSO4 + H2
Balancing equations:
- The same number of atoms of reacting elements on both sides of the equation.
- An element which appears in only one reactant and one product should be balanced first.
- Alternately, choose the compound with a maximum number of participating atoms.
- Never change the formulae of compounds, only coefficients should be added.
- Law of Conservation of Mass applies to all chemical reactions.
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another.
Types of chemical reactions are broadly classified as:
- Combination reaction
- Decomposition reaction
- Single-Displacement reaction
- Double Displacement reaction
In this chapter, we will discuss in detail about these chemical reactions.
Types of Chemical Reactions
Combination Reaction or synthetic reaction:
A combination reaction is a reaction where two or more reactants combine under suitable conditions to form a new substance. As a new substance is being formed, this type of reaction is also called as a synthesis reaction.
A + B ⇨ AB
Example:
Combination or Synthesis reactions span id="MathJax-Element-1-Frame" cass="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="\ce(X+Y>XY)">\ce(X+Y−>XY)\ce(X+Y−>XY) Example: span id="MahJax-Element-2-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="\ceH2+Cl2>2HCl">\ceH2+Cl2−>2HCl\ceH2+Cl2−>2HCl
Decomposition Reaction:
Reactions in which one compound decomposes in two or more compounds or elements are known as Decomposition Reactions. A decomposition reaction is just opposite of combination reaction.
AB ⇨ A + B
Decomposition reaction span id="MathJax-Elemen-3-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="\ce(XY>X+Y)">\ce(XY−>X+Y)\ce(XY−>X+Y)
Three Types of decomposition reactions are:
- Thermal decomposition: Thermal decomposition is a chemical reaction where a single substance breaks into two or more simple substances when heated.
- Electrolytic decomposition Electrolytic decomposition may result when an electric current is passed through an aqueous solution of a compound. A good example is the electrolysis of water.
- Photochemical decomposition Photochemical decomposition is a chemical reaction in which a substance is broken down into simple substances by exposure to light.
Displacement reaction:
Reactions in which atoms or ions move from one compound to other to form a new compound are known as Displacement reaction. Displacement reaction is also known as Substitution Reaction or Single displacement /Replacement Reaction. Both metals and non-metals take part in displacement reactions.
A + BC ⇨ AC + B
- Single-Displacement reaction:
span id="MathJax-Element-4-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="\ce(X+YZ>XZ+Y)">\ce(X+YZ−>XZ+Y)\ce(X+YZ−>XZ+Y) e.g. span id"MathJax-Element-5-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="\ceFe+CuSO4>FeSO4+Cu">\ceFe+CuSO4−>FeSO4+Cu\ceFe+CuSO4−>FeSO4+Cu Elements higher in reactive series can displace elements lower in series, vice versa not possible. Always redox reaction.
- Double-displacement reaction:
Double Displacement Reaction: Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.
Double-displacement reaction san id="MathJax-Element-6-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="\ce(XY+WZ>WZ+WY)">\ce(XY+WZ−>WZ+WY)\ce(XY+WZ−>WZ+WY)
Few common Double-displacement reactions are:
- Precipitate formation: san id="MathJax-Element-7-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="\cePb(NO3)2+2KI>2KNO3+PbI2">\cePb(NO3)2+2KI−>2KNO3+PbI2\cePb(NO3)2+2KI−>2KNO3+PbI2 (yellow lead iodide precipitate)
- Water formation (Acid-base reaction): span id="MathJax-Element-8-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="\ceHCI+NaOH>NaCI+H2O">\ceHCI+NaOH−>NaCI+H2O\ceHCI+NaOH−>NaCI+H2O
Exothermic and Endothermic reaction:
A reaction can either be endothermic or exothermic. An exothermic reaction is a chemical reaction that gives out energy (mostly in the form of heat and light) and an endothermic reaction is a reaction which takes in energy from the surroundings.
Example:
- Reactions which produce energy are called exothermic reaction. Most of the decomposition reactions are exothermic. A nuclear explosion is an exothermic reaction.
- Photosynthesis is an endothermic reaction which allows the plants to use sunlight to produce food and energy.