The first chapter in the CBSE Science textbook is Chemical Reactions and Equations. First, let us have an idea about both these terms with a little explanation.
A Chemical Reaction is the process which leads to the transformation of one chemical to another due to the re-arrangement of the molecular or ionic structure of the chemical involved. In simpler words, a Chemical Reaction occurs when two or more then two chemicals are involved to make a new chemical known as product and the phenomenon is known as a Chemical Reaction.
A Chemical Equation on the other hand is the symbolic representation of the Chemical Reaction. With the help of a chemical equation one can get an idea about the reaction and the product that will be formed after a chemical reaction. A Chemical Equation is very important as it can explain about the product that will be formed upon reaction of two or more chemicals without actually performing the whole reaction in the laboratory.
A chemical change is the result of a chemical reaction and it can be described as the change in which one or more than one new substances are formed. In other words, a chemical change can also be called as a chemical reaction.
Types of Chemical Change
› Combination – The term combination means combining of two or more substance and thus in a combination reaction two or more substances combine to form a completely new substance. For example, the rusting of iron is caused when oxygen reacts with iron to form rust. Another example is the burning of candle where the combination of oxygen and wax produces heat and light in the form of fire along with Carbon Dioxide (CO2).
› Decomposition – The term decomposition means the break down of a substance while forming two or more than two new substances. Decomposition is the opposite of combination. The example of Decomposition is the break down of dung into methane, oxygen, nitrogen and other gases. Another example is the formation of Hydrogen and Oxygen when energy is absorbed by water.
› The properties of a chemical change are as follows:
- The change in energy is involved.
- The change in mass takes place in a chemical change.
- Permanent or irreversible changes takes place in a chemical change.
- One or more than one new substances are formed in a chemical change.
› Examples of a Chemical Change are as follows:
- Baking of Cake
- Rusting of Iron
- Turning of Milk into Curd
- Ripening of Fruits
- Burning of Wood
Q) Why is the Rusting of Iron is considered as a Chemical Change?
The rusting of Iron (Fe) is considered as a chemical change because of the following reasons:
- Upon rusting a new substance is formed i.e. Iron Oxide (FeO).
- The rusting of an Iron is a permanent change.
- The formation of rust also makes the iron bulky and thus there is an increase in mass.
- There is also a change in energy which is can only be seen under advanced equipment.
Exothermic Reaction and Examples
The word Exothermic is made out of two words that is Exo+Thermic. Here Exo means Release or Give out and Therm means Heat or Energy. Hence, an Exothermic Reaction can be defined as a chemical reaction that releases energy in the form of Heat or Light. In other words, if the reaction of two or more substances releases energy in the form of heat or light is known as an Exothermic Reaction.
The examples of an Exothermic Reaction are as follows:
- Combustion of Fuel – Combustion is an exothermic reaction in which a fuel is burnt in the presence of oxygen to give out heat and energy. For example the burning of Methane in the presence of oxygen gives out carbon dioxide and water along with some heat.
CH4 (Methane) + 02 (Oxygen) → CO2 (Carbon Dioxide) + H20 (Water) + Heat
- Combustion of Glucose – The combustion of glucose gives out carbon dioxide with water and some amount of energy.
C6H1206 (Glucose) + 6O2 (Oxygen) → 6CO2 (Carbon Dioxide) + 6H20 (Water) + Energy
Q) Why is respiration an Exothermic Reaction?
Respiration is the process by which a living organism be it your pet dog, cat or a Human takes in oxygen and releases carbon dioxide. The chemical equation of respiration is as follows.
C6H12O6 (Glucose) + 6O2 (Oxygen) → 6CO2 (Carbon Dioxide) + 6H20 (Water) + Energy
As we can see, when the oxygen is taken by the lungs, gets combined with glucose to form carbon dioxide, water and energy. This is the energy that makes our body function properly. Thus, respiration is an exothermic reaction because the energy is released.
Endothermic Reaction and Examples
The word Endothermic can be broken down into two different words that is Endo and Thermic. Here Endo means to gain or to absorb and Therm means Heat or Energy. Hence, an Endothermic Reaction can be defined as a chemical reaction that absorbs energy in the form of Heat or Light. In other words, if the reaction of two or more substances can only occur in the presence of heat or light then the reaction will be known as Endothermic Reaction.
The examples of an Endothermic Reaction are as follows:
- The formation of Nitric Oxide upon the reaction of Nitrogen and Oxygen that requires the absorption of approx. 180.5 k/j of heat for every mole of N2 and O2.
N2 (Nitrogen) + O2 (Oxygen) + 180.5 kJ → 2NO (Nitric Oxide)
- The dissolving of ammonium chloride in water is an endothermic reaction. The salt dissociates into ammonium and chloride ions. The chemical equation for the same is as follows.
NH4CL(s) + H20(l) → NH4CL(aq) – Heat
Other examples of the Endothermic Reaction are Melting of Ice, the evaporation of water into water vapor or steam, the baking of cake etc.
The chemical formula of a substance is the symbolic representation of the actual number of atoms present in one molecule of that substance. The formula of one molecule of Sodium Chloride is NaCl that means one molecule of Sodium Chloride is made up of 1 atom of sodium and 1 atom of chloride. Similarly, the formula for one molecule of Methane is CH4 that means one molecule of Methane is made up of one atom of carbon and 4 atoms of hydrogen.
Other examples of Chemical Formula are as follows:
- Glucose (C6H1206) – One molecule of glucose is C6H12O6 that means one molecule of glucose is made up of 6 atoms of carbon, 12 atoms of hydrogen and 6 toms of oxygen.
- Alcohol (C2H60) – One molecule of alcohol is C2H60 that means one molecule of alcohol is made up of 2 carbon, 12 hydrogen and 6 oxygen.
- Sulfuric Acid ( H2S04) – One molecule of sulfuric acid is H2SO4 that means one molecule of sulfuric is made up of 2 hydrogen, 1 sulfur and 4 oxygen.
The representation of a chemical change in terms of symbols and formulae of the reactants and products is known as chemical equation of the reaction. A chemical equation plays an important role in a chemical reaction as it tells us about the nature of the reaction along with the products formed.
Other information that a chemical equation provides are the relative number of moles of the reactants and products, relative masses of the reactants and products along with the relative volumes of the gaseous reactants and products.
How to Write a Chemical Equation?
The steps to write a chemical equation correctly are as follows:
› Step 1 – Write the Skeletal Equation – A skeletal equation is also known as word equation which shows the change of reactants to products through an arrow pointing rightwards (→). The arrow is also known as “to form”, “to yield” or “to give” and shows the direction of the reaction.
› Step 2 – The reactants are placed on the left hand side (LHS) of the arrow and the products on the right hand side (RHS). The different reactants as well as products are connected by a plus sign (+). A proper chemical equation is comprised of reactants, products along with the physical states of the products and the reactants.
› Step 3 – Balance the Chemical Equation – An equation in which number of atoms of each element is equal on both the sides of the equation is known as balanced chemical equation. The number of atoms involved in a chemical reaction of the reactants and the products are equal at the both sides of the equation.
› For example: Whenever H2O is present on any side, the number of hydrogen on both the sides should be an even number (2 atoms of hydrogen in water). If there are 4H2O, then 4 × 2 = 8 hydrogen atoms. If there are 7H2O, then 7 × 2 = 14 hydrogen atoms i.e., all are even numbers.
On the reactant side, there must be an even number in front of HCl. As a result, the number of chlorine atoms will also be even. But on the product side, the number of chlorine atoms is odd (i.e. KCl = 1, MnCl2= 2, Cl2 = 2. i.e., 1 + 2 + 2 = 5). The only odd number of chlorine atoms is in KCl. Let us change it into the simplest even number possible that is 2.
∴ KMnO4 + HCL → 2KCL + MnCl2 + H20 + Cl2
Since number of K atoms in 2 KCl = 2, place 2 in front of KMnO4 to balance K atom.
2KMnO4 + HCL → 2KCL + MnCl2 + H20 + Cl2
In 2 KMnO4, there are 2 K, 2 Mn, and 8 O. So add these numbers in front of K, Mn and O, (K is already done).
∴ 2KMnO4 + HCL → 2KCL + 2MnCl2 + 8H20 + Cl2
If there are 8 H2O on the product side, there should be 16 H (8 x 2) on the reactants side as well.
∴ 2KMnO4 + 16HCL → 2KCL + 2MnCl2 + 8H20 + Cl2
Now the only unbalanced one is Chlorine. On the left hand side, there are 16 Cl. On the right hand side, firstly, there are 2 Cl in 2 KCl + 4 Cl in 2 MnCl2, making total of 6(2 + 4). Subsequently 10 more Cl atoms are to be accounted for. So place 5 in front of Cl2 to make it 10 (5 x 2).
∴ 2KMnO4 + 16HCL → 2KCL + 2MnCl2 + 8H20 + 5Cl2
Note: This type of balancing the chemical equations is known as the Hit and trial method.
Steps to Balance a Chemical Equation
› Step 1 – Write down the correct skeletal equation.
› Step 2 – Balance the compounds with maximum number of atoms first followed by the compound with the second highest number of atom.
› Step 3 – Now, balance the elements appearing only once on both the sides of the equations and then proceed by balancing the elements that are appearing more than once.
› Step 4 – The elementary substances (pure chemical substance e.g. Fe, Ag, Au, Zn etc.) must be balance in the last.
› Step 5 – In case the equation is not getting balanced, then multiplying the whole equation with any appropriate number should be done to make all the coefficients whole numbers.
Benefits of a Chemical Equation
- A chemical equation tells us the names of the reactants and the products.
- A chemical reaction also tells us about the formula of the reactants and the products.
- It tells us about the relative number of moles of the reactants and products.
- It tells us about the relative masses of both the reactants and products.
- In case of gaseous substance, it tells us about the volumes of the reactants and the products.
Limitations of a Chemical Equation
- A chemical equation in incapable of telling whether the reaction is complete or not.
- A chemical equation has no information regarding the speed of the reaction.
- No information about the concentration of the substances is shown. Such as Diluted or Concentrated.
- It does not show any change in the color from the reactants to the products, thus mentioning the colors separately could be done.
- It does not show if the reaction is endothermic or exothermic in nature.
Types of Chemical Reactions
There are total five types of chemical reaction that we have discussed below, namely Combination Reaction, Decomposition Reaction, Displacement Reaction, Double Displacement Reaction and Redox Reaction.
1) Combination or Synthesis Reactions
The reactions in which two or more substances combine to form a single new substance.
Types of Combination reactions:
- Combination of two elements to form a new compound. For example burning of hydrogen in air or oxygen to produce water.
2H2(g) + O2(g) → 2H20(l)
- Combination Reactions involving an Element and a Compound. For example burning of carbon monoxide in oxygen to form carbon dioxide.
2CO (g) + O2(g) → 2CO2 (g)
- Combination Reactions involving Two Compounds. For example combination of ammonia and hydrogen chloride to produce ammonium chloride.
NH3(g) + HCl(g) → NH4Cl(s)
2) Decomposition Reactions
The Decomposition Reactions are opposite to combination reactions. When a compound breaks down into two or more simple substances when heat or electricity is applied then the reaction is known as decomposition reaction. There are two types of decomposition reactions that is thermal decomposition (when substance undergoes decomposition due to heat) and electrolytic decomposition (when a substance undergoes decomposition due to electricity).
- Electrolysis: It is the decomposition of a substance when an electric current is passed through it.
- Photolysis: When a substance is decomposed in the presence of light is known as photolysis..
› Examples of Electrolytic Decomposition
- The breakdown of water into oxygen and hydrogen in an electrolytic cell in the presence of small quantity of sulfuric acid when a direct current is passed through it.
2H20(l) ——Electrolytic Decomposition—→ 2H2(g) + O2(g)
- The formation of lead and bromine takes place when an electric current is passed through molten lead bromide.
PbBr2(molten) ——Electrolytic Decomposition—→ Pb(s) + Br2(l)
› Examples of Thermal Decomposition
- Mercuric Oxide in the presence of heat undergoes Thermal Decomposition and yields mercury and oxygen.
2HgO(s) ——Thermal Decomposition—→ 2Hg(l) + O2(g)
- The heating of Blue colored crystals of Copper Nitrate gives Black colored copper oxide, reddish brown fumes of nitrogen dioxide and oxygen which is colorless.
2Cu(NO3)2(s) ——Thermal Decomposition—→ 2CuO(s) + 4NO2(g) + 02(g)
3) Displacement or Substitution Reaction
A Displacement Reaction or Single Displacement reaction is a chemical reaction in which one element which is more reactive replaces the other less reactive element. Elements which are higher in the Activity Series (the list of metals in the order of decreasing chemical activity, that means metals with higher chemical activity will be place higher than the metals with the lower chemical activity) displace those elements which are placed below them. More electro+ve elements displace lesser electro+ve elements. Conversely, higher electro-ve elements will displace lesser electro-ve elements. Both metals and non-metals take part in the displacement reaction.
› Examples of Displacement Reaction are as follows:
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)↓
Mg(s) + H2SO4(aq) → MgSO4(aq) + H2O(g)
2KI(aq) + Cl2(g) → 2KCl(aq) + I2(s)
4) Double Displacement or Metathesis Reaction
A double displacement reaction or metathesis reaction is the chemical reaction in which two compounds react with one another to form two different compounds by the mutual exchange of ions. The basic structure for the double displacement reaction is given below.
A+B– + C+D– → A+D– + C+B–
› Examples of the Double Displacement Reaction are as follows:
- The reaction of Sodium Chloride with Silver Nitrate to give out Sodium Nitrate and Silver Chloride (White)
Na+Cl–(aq) + Ag+NO3–(aq) → Na+NO3–(aq) + Ag+Cl–(s)↓
- Hydrochloric acid reacts with lead nitrate o give out nitric acid and lead chloride (White).
2H+Cl–(aq) + Pb2+(NO3)2–(aq) → 2H+NO3–(aq) + Pb2+Cl2–(s)↓
- The reaction of Zinc Sulfate and Barium Chloride gives zinc chloride and barium sulfate (White).
Zn2+SO42-(aq) + Ba2+Cl2-(aq) → Zn2+Cl2-(aq) + Ba2+SO42-(s)↓
There are two types of Double Displacement Reaction as follows:
- Precipitation Reaction – A precipitate is a white substance which is insoluble in water and in all the three above reactions a white precipitate (ppt) is formed. Thus, a chemical reaction which produces a precipitate is known as a precipitation reaction.
- Neutralization Reaction – The word neutralization means the neutralization of acid or base to form salt and water. Thus, neutralization is a type of double displacement reaction in which the two reactants that are acids and bases reacts together to give salt and water as the products. This happens because the positive charge of the hydrogen ion in the acids and the negative charge of hydroxyl ions in base lose their electrical charge to form a covalent molecule of water.
Na+OH–(Base) + H–Cl–(Acid) → Na+Cl–(Salt) + H2O(Water)
5) Oxidation – Reduction Reactions or Redox Reactions
- Oxidation Reaction – It is a chemical reaction in which the oxygen is added or hydrgen is removed from a substance. For example, Magnesium oxide is formed when the oxygen is added to magnesium, similarly Iodine is liberated when hydrogen is removed from hydro-iodic acid. In other words, the process of addition of electro-ve element or radical or removal of electro+ve element or radical. There is a loss of electrons in oxidation reaction.
The examples of Oxidation Reaction are as follows:
i) Addition of Oxygen in Magnesium to form Magnesium Oxide.
2Mg(s) + O2 → 2MgO(s)
ii) Removal of Hydrogen from hydro-iodic acid to liberate iodine.
2HI(g) → 2H2(g) + I2(g)
- Reduction – It is a chemical reaction opposite to the oxidation reaction. A reduction reaction takes place when the oxygen is removed from a substance or hydrogen is added to a substance. For example, Copper metal is formed when oxygen is removed from copper oxide, similarly Hydrogen is added to chlorine gas to form hydrogen chloride gas. In other words, the process of addition of electro+ve element or radical or removal of electro-ve element or radical. There is a gain of electrons in reduction reaction.
The examples of Reduction Reaction are as follows:
i) Removal of oxygen from copper oxide to form Copper metal.
CuO(s) + CO(g) → Cu(s) + CO2(g)
ii) Addition of Hydrogen to chlorine to form Hydrogen chloride gas.
H2(g) + Cl2(g) → 2HCl(g)
- Redox Reaction – When both the oxidation and reduction reaction occur simultaneously then that reaction is knows as Redox Reaction.
The example of Redox Reaction is as follows:
Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)
The example of Non Redox Reaction is as follows:
NaCl(aq) + AgNO3(aq) → AgCl(s)↓ + NaNO3(aq)
- Oxidizing Agents – The substances that are responsible for oxidation reaction are known as oxidizing agents. For example Bromine (Br2), Chlorine (C2), Oxygen (O2) etc.
- Reducing Agents – The substances that are responsible for reduction reaction are known as reducing agents. For example Carbon (C), Ammonia (NH3), Hydrogen (H2) etc.
The Effects of Oxidation Reaction in Everyday Life
The two major effects of Oxidation reaction in daily life is Corrosion and Rancidity. Let us study about both the terms in detail.
The corrosion is a chemical reaction in which metals gets affected by substances like moisture, air, acids etc. present in the surroundings and react with them to form an oxide layer on them. For example, iron undergoes corrosion and forms a reddish brown layer known as rust and the process is known as rusting. Corrosion causes decay or decomposition of metals and eventually destroys them.
- Iron undergoes oxidation and forms a reddish brown layer know as rust and the process of corrosion of iron is known as rusting.
- Copper gets oxidized in the presence of moisture to form a thin green layer of copper oxide due to corrosion.
- Silver is oxidized and turn black in the presence of moisture that is the result of oxidation of silver and the thin black layer is silver oxide.
Rancidity is another daily life example of oxidation. Whenever a food with oils or fats is left in the open for a long time then it undergoes decomposition and there is an unpleasant change in the flavor/smell of the food and the process is known as Rancidity. Long exposure to the surrounding makes the air to decompose the food that breaks down the fat causing the food to become stale and unsuitable for consumption.
- Factors that are responsible for the oxidation of fat include salt, moisture, light, bacteria and other microscopic organisms present in the atmosphere.
- There are substances that make delay the fat oxidation or rancidity are called antioxidants. The example of antioxidants are BHA, Vitamin E, C etc.
- To increase the life span of foods containing fats air tight/vacuum sealed containers are used. These helps in the slow down of oxidation and keeps the food safe for consumption for a long time.