Chemical Reaction and Equations

Chapter 1: Chemical Reactions and Equations

Introduction

• Any process that involves the rearrangement of the structure of the substance or conversion of reactants into products is defined as a Chemical Reaction
• In a chemical reaction the nature and the identity of any substance change.
• Some common examples of such change from everyday life are:
• Formation of curd from milk. (Lactose present in milk is converted into lactic acid by the action of bacteria.)
• Rusting of iron nail or rod or pan when exposed to humid atmosphere. (Iron gets converted into oxides of iron i.e., rust when exposed to water and oxygen).
• Fermentation of grapes (glucose present in grapes gets converted into alcohol and carbon dioxide gas by action of yeast).
• Digestion of food in our body. (Complex food i.e., sucrose is broken down into simpler particles like glucose)
• Our respiration process generates energy.
• Glucose in the presence of oxygen broken down to release energy along with carbon dioxide and water.
• Photosynthesis done by plants.
• Carbon dioxide and water combine in presence of sunlight and chlorophyll to form glucose.

Occurrence of a chemical reaction?

Question: What helps us to determine whether a chemical reaction has taken place?

Answer: Any of the following observations helps us to determine whether a chemical reaction has taken place:

• Change in state.
• Change in colour.
• Evolution of gas
• Change in temperature.
• Formation of precipitate.
• Change in State: In several chemical reactions, we can observe change in the state of reactant(s).
  • An example of this reaction is combustion of wax. Solid wax on burning forms water (vapour) and carbon dioxide (gas).
•  Change in Colour
  • When lead nitrate (transparent solution) is mixed with potassium iodide (transparent solution) it forms yellow coloured precipitate of Lead Iodide.

• Iron (silvery shiny metal) upon rusting changes colour from silver to reddish brown.

• Change in Temperature

Some chemical reaction takes place with evolution of heat and some chemical reaction takes place by absorption of heat. Based on this, there are two types of reaction: Exothermic reaction and Endothermic reaction.

• Exothermic Reactions: Reactions in which energy is released in the form of heat are called Exothermic Reactions. You may observe an increase in the temperature of such reaction mixtures.

Examples –

1. All combustion reactions, for example burning of wood or methane etc.

1. Thermite reactions, which is used for welding cracks in railway track at far locations where welding machines cannot be taken.

1. Combination reactions are generally exothermic in nature.
   1. The decomposition of organic matters into compost is an example of exothermic reaction.
   1. Endothermic Reactions: Reactions in which energy or heat is absorbed are called Endothermic Reactions. You may observe a decrease in the temperature of such reaction mixtures.

Examples –

also, the reaction of photosynthesis –

• Evolution of any gas: Some reactions are accompanied with evolution of gases like , etc. For example, when zinc reacts with sulphuric acid it gives hydrogen gas.

• Formation of Precipitate: In some reactions insoluble products are formed When a soluble carbonate reacts with Barium, Barium Carbonate precipitate can be observed.
• Examples of chemical reaction
• Magnesium ribbon burns in air with a dazzling white light.

• Lead nitrate reacts to potassium iodide to form a yellow-coloured precipitate of lead iodide along with potassium nitrate.

• Zinc metal reacts with sulphuric acid with evolution of hydrogen gas.

• Chemical Equation
• The symbolic representation of chemical reaction using symbols and formulae is known as Chemical Equation.
• Substance(s) that undergo reaction are called reactants and substance(s) which are formed after completion of reaction are called products.
• In a chemical equation, reactant(s) are written in left-hand side whereas product(s) are written on the right-hand side of arrow symbol.
• For example:        

4.1 Balanced Chemical Equation

• A balanced chemical equation is the one where the number of atoms involved in reactants side is equal to number of atoms on product side.
• It goes in accordance with law of conservation of mass i.e., total mass of products must be equal to total mass of reactants.

Eq.1. Example of Balanced Chemical Equation

4.2 Steps to form Balanced Equation

• Let’s use the below equation and see how to balance it.

• Step 1: First, draw the boxes around each formula as shown below. The numbers inside these boxes must never be changed.

• Step 2: Find out the number of atoms of each element. For Example, on reactant side, 1 atom of Fe, 2 of H, and 1 of O and on product side we have, 3 atoms of Fe, 4 of O and 2 of H.
• Step 3: To make the process easier start to balance the equation with the compound having maximum number of atoms. So, in our example  has maximum number of atoms and therefore atoms of this compound will be balanced first. While balancing does not alter the formula of the compound (i.e., do not change any number inside the boxes).
• Step 4: One by one balance each element on reactant and product side.

• Step 5: After balancing number of atoms on both the side of the equation, finally check the correctness of the balanced equation by counting the number of atoms of each element on both the sides of equation.

• Step 6: Then write the symbols of the physical state of reactants and products as shown below-

• This above equation represents the balanced equation.
• Types of Chemical Reaction.
• Chemical reaction involves breaking and making of bonds between atoms to form new products/ substances.
• During any chemical reaction, type and number of atoms of reactants do not change, only their arrangement (bonds) changes to form new products.
• Some common types of chemical reactions which we will be studying in next sections are:
  • Combination reaction
  • Decomposition reaction
  • Displacement reaction
  • Double displacement reaction
  • Redox reaction (Reduction-Oxidation reaction)

• Combination Reaction:
• In combination reaction two or more reactants combine to form single product.
• Combination reaction is also known as synthesis reaction.
• We must remember that in combination reaction only one product is formed.
• A general representation of combination reaction is:

• For Example– calcium oxide (aka, quick lime) reacts with water to form calcium hydroxide (aka, slaked lime)

​

• Some more examples of combination reactions are
  • Burning of coal:
  • Formation of water:
• Combination reactions are generally exothermic in nature.
• Exothermic reaction release heats and raises the temperature of the surroundings.
  • For Example, Respiration is an example of exothermic reaction.

• Also, combustion of natural gas is example of exothermic reaction:

• The decomposition of dead organic material into compost is an exothermic reaction.
  • Decomposition Reaction 
• It is the type of reaction which involves breakdown of single reactant into simpler products.
• Decomposition reactions are of three types:
  • Thermal decomposition reaction
  • Electrolytic decomposition reaction or electrolysis
  • Photodecomposition reaction of photolysis
• If decomposition of reactant is carried out by heating, it is called thermal decomposition reaction.
  • Example 1: Decomposition of calcium carbonate into calcium oxide and oxygen

• Example 2: Decomposition of ferrous sulphate into ferric oxide, sulphur dioxide and sulphur trioxide.

• Example 3: Decomposition of lead nitrate into lead oxide, nitrogen dioxide and oxygen.

• If a reactant in aqueous solution or in molten state is decomposed by passing electricity, it is called electrolytic decomposition reaction or simply electrolysis.
  • Example: Electrolysis of water.

• If decomposition of reactant is carried out by sunlight (or any light), it is called photodecomposition reaction or photolysis.
  • Example 1: Decomposition of silver chloride into silver and chlorine in presence of sunlight.

• Example 2: Decomposition of silver bromide into silver and bromine gas in sunlight. This reaction is used in black-and-white photography in old cameras.

• Decomposition reactions require energy either in the form of heat, electricity of light, such reactions are called endothermic reactions.
• Endothermic reaction involves the absorption of the heat and thus it cools the surrounding.
  • Example: Reaction of barium hydroxide with ammonium chloride is a highly endothermic reaction.
  • Can you write the chemical equation for the above reaction?
  • Displacement Reaction 
• It is a reaction in which the more reactive element displaces the less reactive element in a compound.
• These reactions mainly occur in solution (aqueous) form.
• More active metal displaces or removes less active metal from its salt solution.
• Example 1: Iron nail immersed in copper sulphate (blue) solution turns brown in some time and blue colour of solution fades towards light green colour.

• Iron, being more reactive than copper, displaces copper from the aqueous solution of copper sulphate.
• Example 2:
• Example 3:

• Double Displacement Reaction 
• It is a type of reaction in which cations and anions in the reactants switch places to form new products.
• Also known as ion exchange reaction, as in this reaction ions of two reactants switch their places to form two new products.
• This reaction mainly occurs in solution (aqueous) form.
• Generally, one of the products formed is insoluble in water and is called precipitate.
• Example 1: On mixing aqueous solution of sodium sulphate and barium chloride and white coloured insoluble substance barium sulphate is formed along with soluble sodium chloride.

• Example 2: on mixing lead nitrate with potassium iodide, a yellow-coloured insoluble substance lead iodide is formed along with soluble potassium nitrate.

• The insoluble substance formed in the above reactions is known as precipitate.
• Reaction that produces a precipitate called precipitation reaction.
• Neutralization reactions are also examples of double displacement reactions.
• Reactions in which acids and bases reacts to form salt and water are called neutralization reaction.

• Redox Reaction 
• It is also known as Oxidation-reduction Reaction.
• In this type of reaction transfer of electrons occurs between the two species.
• Oxidation is defined as addition of oxygen or removal of hydrogen.
• Reduction is defined as removal of oxygen or addition of hydrogen.
• Oxidizing agent is the one which gains the electrons and is reduced in a chemical reaction.
• Reducing agent is oxidized in a chemical reaction and it loses the electrons.
• Fluorine is the strongest oxidizing agent.
• Other oxidizing agents are oxygen, chlorine, nitric acid, etc.
• Hydrogen, Formic acid, etc., are reducing agents.
• Below are some examples of redox reaction:

• Effects of oxidation in everyday life
• Oxidation has a damaging effect on metals and food items.
• Oxidation causes corrosion of metals and rancidity of oily food.
• Corrosion of metals
• Metals are prone to corrosion.
• It is a slow conversion of metals into some undesirable compounds.
• This may be due to reactions with oxygen, gases, acids etc.
• When iron reacts with atmospheric oxygen and moisture, a reddish-brown layer is formed on the surface of the iron, this process is known as Rusting.
• Rusting is just a special name given to corrosion of iron.
• Copper when left open in air corrodes and a greenish layer (basic copper carbonate) is formed on its surface.
• Blackening of silver (jewelry) when left open in the air is also an example of corrosion of metal.

Eq. 8. Equation for Iron Rusting

1. Rancidity
2. When food containing fats and oils are exposed to the air (oxygen), the oxidation of fat and oil occurs, this is known as rancidity.
3. Methods to Prevent Rancidity
4. Store cooking oils from direct sunlight.
5. Food should be placed at low temperature.
6. By adding antioxidants food can be protected from rancidity.

• BHA (Butylated-hydroxy-anisole) and BHT (butylated-hydroxy-toluene) are commonly used antioxidants.
• Packing material should replace the air with nitrogen.
• Minimize the use of salts in fried foods.

***