Chemical Reactions and Equations Class 10 Notes
Introduction
In our daily lives, physical changes involve alterations in appearance or state without creating new substances, like melting ice. On the other hand, chemical changes result in the formation of entirely new substances, such as cooking food or rusting iron. Recognizing these changes helps us understand when a chemical reaction occurs.
What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances, the Reactants, are converted to one or more different substances, the Products.
Chemical Equations
A chemical equation is a symbolic representation of a chemical reaction using the symbols and formulas of the substances involved. The substances that undergo chemical change in the reaction, Magnesium and Oxygen, are the Reactants. When magnesium burns in air, it combines with oxygen to form magnesium oxide. Chemical equations can be made more concise and useful if we use chemical formulae instead of words. Example: The chemical formula for water is H2O, which indicates that it is made up of two hydrogen atoms and one oxygen atom.
Multiple Choice Question
Try yourself: Which of the following is an example of a chemical change?
- A. Melting ice
- B. Boiling water
- C. Burning wood
- D. Cutting paper
Correct Answer: C - Burning wood is an example of a chemical change because it involves the combustion of wood, resulting in the formation of new substances such as ash and smoke. Melting ice is a physical change as it only involves a change in state from solid to liquid. Boiling water is also a physical change as it involves the conversion of liquid water to water vapor. Cutting paper is a physical change as it only alters the shape or appearance of the paper without forming any new substances.
1. Writing a Chemical Equation
Representation of a chemical reaction in terms of symbols and chemical formulae of the reactants and products is known as a chemical equation.
- For solids, the symbol is "(s)".
- For liquids, it is "(l)".
- For gases, it is "(g)".
- For aqueous solutions, it is "(aq)".
- For gas produced in the reaction, it is represented by "(↑)".
- For precipitate formed in the reaction, it is represented by "(↓)".
The reactants are on the left (LHS) of the arrow, while the products are on the right (RHS). A plus sign (+) links the different reactants and products together. A balanced account of a chemical reaction is a complete chemical equation, which symbolically depicts the reactants, products, and their physical states.
2. Balanced Chemical Equations
The Law of Conservation of Mass states that in a chemical reaction, atoms can't be created or destroyed. This means that the total number of atoms for each element in the starting materials (reactants) must be the same as in the end products, keeping the overall mass the same. A chemical equation is considered balanced when the number of atoms for each element on the reactant side is identical to the product side.
Steps for Balancing Chemical Equations
- To balance chemical equations, coefficients (numeric values preceding chemical symbols or formulas) are utilized. These coefficients represent the number of atoms or molecules involved. Adjust the coefficients as needed to make sure that the number of each type of atom is the same on both sides of the equation.
- For instance, in the equation Zn + HCl → ZnCl2 + H2, balancing involves adding coefficients to achieve equilibrium: Zn + 2HCl → ZnCl2 + H2. This balancing process often involves trial and error adjustments to ensure the equality of atoms on both sides.
Here are the steps to write a balanced chemical equation for Fe + H2O → Fe3O4 + H2:
- Step 1: Write the number of atoms of elements present in reactants and in products in a table.
- Step 2: Balance the atom which is maximum in number on either side of a chemical equation. Start by balancing the number of oxygen atoms, which has the maximum count on the right-hand side (RHS). To balance oxygen, multiply the number of oxygen atoms on the left-hand side (LHS) by 4. This makes it: Fe + 4 × H2O → Fe3O4 + H2.
- Step 3: Balancing oxygen created an imbalance in the number of hydrogen atoms. There are now eight hydrogen atoms on the LHS and two on the RHS. To balance hydrogen, multiply the number of hydrogen atoms on the RHS by 4. This makes it: Fe + 4 × H2O → Fe3O4 + 4 × H2.
- Step 4: There is only one iron atom on the LHS and three on the RHS. To balance iron, multiply the number of iron atoms on the LHS by 3. This makes it: 3 × Fe + 4 × H2O → Fe3O4 + 4 × H2.
The equation is now balanced, and the same number of atoms of each element is present on both sides. After balancing, the above equation can be written as follows: 3Fe + 4H2O → Fe3O4 + 4H2.
To make a chemical equation more informative, it is important to include the physical state of the substances involved. The symbol (g) is used to represent a gaseous state, (l) for liquid state, (s) for solid state, and (aq) for an aqueous solution. The conditions under which the reaction takes place are also important to include in the chemical equation. These conditions can be written above and/or below the arrow in the chemical equation. Including this information can help provide a clearer understanding of the chemical reaction taking place. Example: In photosynthesis, (g) indicates that carbon dioxide is a gas, (l) indicates that water is a liquid, and (aq) indicates that glucose is dissolved in water. Sunlight and chlorophyll are specifically mentioned as the sources of energy used in the reaction. This highlights the importance of these specific components in the photosynthesis process.
Multiple Choice Question
Try yourself: What is the balanced chemical equation for the reaction between hydrogen gas and oxygen gas to form water?
- A. 2H2 + O2 → 2H2O
- B. H2 + O2 → H2O
- C. H2 + 2O2 → 2H2O
- D. H2O → H2 + O2
Correct Answer: A. The equation tells us that two molecules of hydrogen gas (H2) react with one molecule of oxygen gas (O2) to produce two molecules of water (H2O). The coefficient 2 in front of the H2 and H2O represents the number of molecules involved in the reaction. The coefficient 1 in front of the O2 indicates that only one molecule of oxygen gas is involved in the reaction. Therefore the balanced equation is 2H2 + O2 → 2H2O.
Types of Chemical Reactions
Considering various factors, chemical reactions are classified into several categories.
1. Combination Reaction
It occurs when two or more reactants combine to form a single product. Or we say, such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.
Example:
(i) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat. CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat.
(ii) A solution of slaked lime produced by the reaction is used for white-washing walls. Calcium hydroxide reacts with carbon dioxide in the air to form calcium carbonate. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO3.
Note: Reactions that release heat when products are formed are known as exothermic chemical reactions.
Other Examples of Exothermic Reactions:
- (a) Burning of Coal: When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen: C(s) + O2(g) → CO2(g).
- (b) Formation of H2O: 2H2(g) + O2(g) → 2H2O(l).
- (c) Burning of natural gas: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g).
Note: Do you know that respiration is an exothermic process? We need energy to live, and we get it from the food we eat. When we digest food, it's broken down into simpler forms. For instance, rice, potatoes, and bread have carbohydrates, which are turned into glucose. This glucose then mixes with oxygen in our cells to produce energy: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy.
Let's Revise: Give an example of a combination reaction used in white-washing walls.
Ans: Calcium oxide reacts with water to form calcium hydroxide, which later reacts with carbon dioxide to form calcium carbonate used in white-washing.
2. Decomposition Reaction
A single reactant breaks down when exposed to heat, light, or electricity, resulting in the formation of two or more products. There are three types of decomposition reactions:
(a) Thermal decomposition
A decomposition reaction carried out by heating is called thermal decomposition.
- Example 1: Heating of Ferrous Sulphate: Ferrous sulphate crystals (FeSO4. 7H2O) lose water when heated and the green colour of ferrous sulphate crystals fades. It then decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3). Ferric oxide is a solid, while SO2 and SO3 are gases.
- Example 2: Decomposition of Calcium Carbonate: Heating calcium carbonate breaks it down into calcium oxide (lime) and carbon dioxide. This reaction is important in industries, with calcium oxide being used in cement production.
- Example 3: Heating of Lead Nitrate Powder.
(b) Photolytic decomposition
Where light is needed for the reaction. An example is the photolytic decomposition of H2O2.
- Example: Photolytic Decomposition of Hydrogen Peroxide: Hydrogen Peroxide undergoes photolytic decomposition when exposed to sunlight, breaking down into water and oxygen gases. This process is an example of photolytic decomposition, which is crucial in certain chemical reactions and can be influenced by light intensity.
- Example: Effect of sunlight on Silver Chloride: White silver chloride turns grey in sunlight. This occurs due to the decomposition of silver chloride into silver and chlorine when exposed to light. Silver bromide also behaves in the same way. The above reactions are used in black and white photography.
(c) Electrolytic decomposition
Where electricity is necessary for the reaction. An example is the electrolytic decomposition of H2O. Electrolysis of water is the decomposition into oxygen and hydrogen gas due to an electric current passed through the water.
Note: What form of energy is causing these decomposition reactions? Decomposition reactions need energy to break down the substances involved. This energy can come from heat, light, or electricity. These types of reactions, where energy is taken in rather than given off, are called endothermic reactions.
Let's Revise: What happens when ferrous sulphate is heated?
Ans: It loses water and decomposes into ferric oxide, sulphur dioxide, and sulphur trioxide.
Q: Are decomposition reactions endothermic or exothermic? Why?
Ans: They are endothermic because they require energy input to break compounds.
3. Displacement Reaction
A displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. Both metals and non-metals take part in displacement reactions.
Some Important Reactions are:
- Reaction of iron nails with copper sulphate solution: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s). The iron nail becomes brownish in colour and the blue colour of copper sulphate solution fades. In this reaction, iron has displaced or removed another element, copper from the copper sulphate solution.
- Other examples of displacement reactions are: Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
Multiple Choice Question
Try yourself: Which of the following is an example of a displacement reaction?
- A. Combustion of methane
- B. Photosynthesis in plants
- C. Rusting of iron
- D. Zinc reacting with copper sulphate
Correct Answer: D. A displacement reaction occurs when one element replaces another in a compound. In this case: Zinc reacting with copper sulphate is a classic example, where zinc displaces copper from its compound. The other options do not fit the definition of a displacement reaction: Combustion of methane involves a chemical change but is not displacement. Photosynthesis in plants is a process of converting light energy into chemical energy, not displacement. Rusting of iron is an oxidation process and does not involve displacement. Therefore, the correct example of a displacement reaction is when zinc reacts with copper sulphate.
4. Double Displacement Reaction
A double displacement reaction is a type of chemical reaction where two compounds react, and the positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.
Example: When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride. Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq).
Note: Double Displacement Reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reactions.
5. Endothermic and Exothermic Reaction
(i) Exothermic Reaction: An exothermic reaction is a type of chemical reaction in which energy is released from the reaction system into the surroundings, usually in the form of heat.
Example: Formation of Carbon dioxide. The chemical reaction can be depicted as: C(s) + O2(g) → CO2(g) + Heat.
(ii) Endothermic Reaction: An endothermic reaction is a type of chemical reaction in which energy is absorbed from the surroundings in the form of heat, light, or electricity.
Example: Photosynthesis. Plants absorb light energy from sunlight to convert carbon dioxide and water into glucose and oxygen. Reaction as: 6CO2 + 6H2O → C6H12O6 + 6O2.
Multiple Choice Question
Try yourself: Select the appropriate state symbols of the products given as X and Y in the following chemical equation by choosing the correct option from table given below: Zn(s) + H2SO4(aq) → ZnSO4(X) + H2(Y)
- A. X is (s) and Y is (l)
- B. X is (aq) and Y is (g)
- C. X is (aq) and Y is (s)
- D. X is (g) and Y is (aq)
Correct Answer: B. Zn(s): Zinc is a solid metal. H2SO4(aq): Sulfuric acid is an aqueous solution. ZnSO4(X): Zinc sulfate is an aqueous solution (aq). H2(Y): Hydrogen gas is a gas (g). It is type of a displacement reaction.
6. Oxidation and Reduction
A redox reaction happens when there is a change in the oxidation states of the substances involved. Oxidation is the loss of electrons or an increase in the oxidation state of a substance or its atoms, while reduction is the gain of electrons or a decrease in the oxidation state of a substance or its atoms.
(i) Oxidation is the process in which an atom, ion, or molecule loses one or more electrons. During oxidation, the oxidation state of the atom or molecule increases, since it becomes more positive or less negative. Some examples of oxidation include the reaction of iron with oxygen to form rust and the reaction of glucose with oxygen in cellular respiration.
(ii) Reduction is the opposite process of oxidation, in which an atom, ion, or molecule gains one or more electrons. Some examples of reduction include the reaction of silver ions with electrons to form silver metal, and the reaction of hydrogen ions with electrons to form hydrogen gas.
Oxidation of Copper to Copper Oxide
The surface of copper powder becomes coated with black copper(ll) oxide because oxygen is added to copper and copper oxide is formed. If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place, and copper is obtained. During this reaction, the copper(ll) oxide loses oxygen and is reduced. The hydrogen gets oxygen and is oxidized.
Examples:
- ZnO + C → Zn + CO
- MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
Multiple Choice Question
Try yourself: Which of the following statements is true about oxidation reactions?
- A. Oxidation is the loss of oxygen atom.
- B. Oxidation is the gain of oxygen atom.
- C. Oxidation and reduction both involve the gain of oxygen atom.
- D. Oxidation and reduction both involve the loss of oxygen atom.
Correct Answer: B. Oxidation reactions refer to the process where substances lose electrons. The following points clarify the true nature of oxidation: Oxidation is defined as the gain of oxygen atoms. It can also involve the loss of electrons or an increase in oxidation state. Reduction is the opposite, characterised by the loss of oxygen atoms or gain of electrons. Both oxidation and reduction must occur simultaneously in a chemical reaction. Therefore, Correct Answer - Option B.
- Redox Reaction, Oxidizing Agent, Reducing Agent: Oxidation and reduction always occur together in a chemical reaction and are often referred to as redox reactions.
- In a redox reaction, one substance is oxidized while another substance is reduced. The substance that is oxidized is called the reducing agent.
- It causes the reduction of another substance by donating electrons. The substance that is reduced is called the oxidizing agent. It causes the oxidation of another substance by accepting electrons.
- Example: In the reaction between zinc metal and hydrochloric acid, zinc is oxidized to form zinc ions, while hydrogen ions are reduced to form hydrogen gas: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g).
- In this reaction, zinc is the reducing agent and HCl acts as the oxidizing agent.
Have You Observed the Effects of Oxidation Reaction in Everyday Life?
1. Corrosion
- Numerous metals, prone to chemical activity, easily react with moisture, oxygen, and acids. Iron, for example, starts as shiny but eventually rusts, forming a reddish-brown powder due to oxidation.
- Metals, like iron, face corrosion as their oxides don't firmly attach, causing flaking, structural weakening, and breakdown. The process of metal deterioration due to environmental substances is called corrosion.
- Copper and silver items tarnish when exposed to air and water. Copper develops a green oxide layer, while silver acquires a black oxide covering. These oxide formations act as barriers, limiting further exposure and reducing corrosion.
Rusting: 4Fe(s) + 3O2(from air) + xH2O(moisture) → 2Fe2O3.xH2O(rust)
Corrosion of copper: Cu(s) + H2O(moisture) + CO2(from air) → CuCO3.Cu(OH)2(green)
Corrosion of silver: 2Ag(s) + H2S (from air) → Ag2S(black) + H2(g)
2. Rancidity
- The taste and odour of food materials containing fat and oil change when they are left exposed to air for a long time. This is called rancidity. It is caused by the oxidation of fats and oils present in food materials.
- Prevention: (i) Seal food in air-tight containers. (ii) Employ nitrogen packaging. (iii) Store in refrigeration. (iv) Add antioxidants or preservatives.
Let's Revise
Q: What roles do zinc and hydrochloric acid play in the redox reaction Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)?
Ans: Zinc is the reducing agent (oxidized to Zn²⁺), and hydrochloric acid is the oxidizing agent (H⁺ reduced to H₂).
Q: What is the primary cause of rancidity in food containing fats and oils?
Ans: Oxidation of fats and oils when exposed to air.
FAQs on Chapter Notes: Chemical Reactions & Equations
1. What's the difference between a physical change and a chemical reaction?
Ans. A physical change alters only appearance or state without forming new substances-like melting ice or dissolving salt. A chemical reaction creates entirely new compounds with different properties through breaking and forming bonds. For example, burning paper is chemical; tearing it is physical. Understanding this distinction is crucial for CBSE Class 10 Science exams.
2. How do I balance chemical equations step by step?
Ans. Start by writing the unbalanced equation, then count atoms of each element on both sides. Add coefficients (not subscripts) to equal the atom count on reactants and products sides. Check your work by recounting. Practice with simple equations before progressing to complex ones. Balancing ensures the law of conservation of mass holds true in every chemical reaction.
3. Why do some substances change colour during a chemical reaction?
Ans. Colour changes occur because chemical reactions break existing bonds and form new compounds with different electronic structures. These new substances absorb and reflect light differently than original reactants. Examples include copper turning green when exposed to air or iron rusting. This visible evidence helps identify whether a chemical change has actually occurred in your experiment.
4. What are the main types of chemical reactions and how do I identify them?
Ans. Chemical reactions fall into five categories: combination (two substances unite), decomposition (one substance splits), displacement (one element replaces another), double displacement (ions exchange partners), and redox (electron transfer). Identify them by observing what reactants combine or separate, whether precipitates form, or if colour and temperature change. Recognising reaction types helps predict products in CBSE chemistry problems.
5. How can I tell if a chemical equation is balanced correctly?
Ans. A balanced chemical equation has equal numbers of each element's atoms on both the reactant and product sides. Count atoms systematically using subscripts and coefficients. The total mass of reactants equals product mass, following the law of conservation of mass. Use flashcards and mind maps available on EduRev to practise identifying balanced versus unbalanced equations quickly during revision.