Chapter Overview
Before your board exam, here is what you must know from Chapter 1. You will learn how chemical bonds break and form to produce new substances with distinct chemical properties. This summary covers chemical equations, balancing methods, indicators of chemical changes, five main types of reactions, energy changes, and real-world oxidation phenomena like corrosion and rancidity.
Easy Explanation
A chemical reaction is a process where starting substances, called reactants, undergo chemical changes by breaking old chemical bonds and forming new ones to produce distinct ending substances, called products.
When you burn a magnesium ribbon in air, it reacts with oxygen to form a white powder called magnesium oxide (MgO). Before burning, you must clean the magnesium ribbon with sandpaper to remove the protective layer of basic magnesium oxide that forms on its surface when exposed to moist air.
Think of a chemical reaction like rearranging building blocks. The total number of individual blocks (atoms) never changes, but the way they connect creates new structures with different properties.
To record a reaction efficiently, chemical formulas replace full word names. The Law of Conservation of Mass states that mass cannot be created or destroyed in a chemical reaction. Therefore, the total mass of reactants must equal the total mass of products. This requires every chemical equation to have an equal number of atoms of each element on both sides.
Important Concepts
- Indicators of a Chemical Reaction: A reaction has occurred if you observe a change in state, change in color, evolution of a gas, or change in temperature.
- Balancing Chemical Equations: Use the hit-and-trial method to balance equation coefficients. Never change the chemical subscripts of a formula. Physical state symbols are added as (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution (dissolved in water).
- Combination Reaction: Two or more substances combine to form a single product. Example: Calcium oxide (quicklime) reacts vigorously with water to form calcium hydroxide (slaked lime), releasing heat.
$CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat$ - Decomposition Reaction: A single reactant breaks down into two or more simpler products. These require energy input:
- Thermal Decomposition: Heat energy breaks the bonds. Heating green ferrous sulphate crystals ( ext{FeSO}4· 7H2O) turns them white as water is lost, followed by decomposition into red-brown ferric oxide, sulphur dioxide, and sulphur trioxide gas.
$2FeSO4(s) xrightarrow{Heat} Fe2O3(s) + SO2(g) + SO3(g)$ - Electrolytic Decomposition: Electric current decomposes a compound, such as the electrolysis of water producing hydrogen and oxygen gas in a 2:1 volume ratio.
- Photolytic Decomposition: Light energy breaks bonds. Silver chloride turns grey in sunlight.
$2AgCl(s) xrightarrow{Sunlight} 2Ag(s) + Cl2(g)$
- Thermal Decomposition: Heat energy breaks the bonds. Heating green ferrous sulphate crystals ( ext{FeSO}4· 7H2O) turns them white as water is lost, followed by decomposition into red-brown ferric oxide, sulphur dioxide, and sulphur trioxide gas.
- Displacement Reaction: A more reactive element displaces a less reactive element from its aqueous salt solution. When an iron nail is dipped in blue copper sulphate solution, the solution turns light green due to the formation of ferrous sulphate, and brown copper coats the nail.
$Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)$ - Double Displacement Reaction: An exchange of ions occurs between two compounds. Often, an insoluble solid called a precipitate is formed.
$Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)$ - Oxidation and Reduction (Redox): Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen. When copper powder is heated, it oxidizes to black copper(II) oxide (CuO). Passing hydrogen gas over heated CuO reduces it back to brown copper.
$CuO + H2 → Cu + H2O$ - Exothermic and Endothermic Reactions: Exothermic reactions release energy to the surroundings (such as burning natural gas, digestion, and respiration). Endothermic reactions absorb energy from the surroundings (such as most decomposition reactions).
Key Terms
Reactants: The starting chemical substances that undergo a chemical transformation in a reaction.
Products: The new substances formed as a result of a chemical reaction.
Balanced Chemical Equation: A chemical equation that has an equal number of atoms of each element on both reactant and product sides.
Precipitate: An insoluble solid compound formed during a reaction in an aqueous solution.
Corrosion: The gradual deterioration of metals caused by chemical reactions with atmospheric moisture, oxygen, acids, or gases. Example: Rusting of iron ( ext{Fe}2O3· xtext{H}2O).
Rancidity: The oxidation of fats and oils in food substances, leading to an unpleasant smell, altered taste, and spoilage.
Diagram/Visual Explanation
Understanding key experimental setups from NCERT activities:
- Action of Dilute Acid on Zinc Granules (Activity 1.2): Take zinc granules in a conical flask and add dilute hydrochloric acid or sulphuric acid. Bubbles of hydrogen gas evolve around the zinc granules. Placing a burning candle near the escaping gas produces a pop sound. Touch the flask to feel a rise in temperature, showing the reaction is exothermic.
- Electrolysis of Water: Two carbon electrodes are placed in a vessel filled with water acidified with a few drops of dilute sulphuric acid. Plastic test tubes filled with water are inverted over the electrodes. Passing electric current causes gas bubbles to form at both electrodes, displacing water. The volume of gas collected at the cathode (hydrogen) is double the volume collected at the anode (oxygen).
What You Must Remember
- Always balance chemical equations to comply with the Law of Conservation of Mass.
- Respiration is an exothermic reaction because glucose combines with oxygen in cells to release energy.
- Photolytic decomposition of silver chloride and silver bromide is used in black-and-white photography.
- Nitrogen gas is flushed into bags of potato chips to prevent oxidation and rancidity.
- Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen.
Quick Revision
A chemical reaction produces new chemical substances with different properties. Equations must be balanced step-by-step so that total atoms on the left equal total atoms on the right. Physical state notations must be included where appropriate.
Important Questions
- Why should a magnesium ribbon be cleaned before burning in air? (1 Mark)
Answer: To remove the protective coating of basic magnesium oxide formed by reaction with atmospheric oxygen, allowing the ribbon to burn readily. - Why is double the amount of gas collected in one test tube during the electrolysis of water compared to the other? Name the gases. (2 Marks)
Answer: Water contains hydrogen and oxygen in a 2:1 ratio by volume ( ext{H}2O). Hydrogen gas collects at the cathode in double the volume of oxygen gas collected at the anode. - Balance the following equation step by step: (2 Marks)
$Fe(s) + H2O(g) → Fe3O4(s) + H2(g)$
Answer:
Balance Fe atoms: 3 Fe on right, so write 3 on left:
$3Fe + H2O → Fe3O4 + H2$
Balance O atoms: 4 O on right, so write 4 before H2O:
$3Fe + 4H2O → Fe3O4 + H2$
Balance H atoms: 8 H on left, so write 4 before H2 on right:
$3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)$ - Identify the substances oxidized and reduced in the following reaction: (2 Marks)
$CuO(s) + H2(g) → Cu(s) + H2O(l)$
Answer: H2 is oxidized to H2O (gain of oxygen). CuO is reduced to Cu (loss of oxygen). - What happens when barium chloride solution is added to sodium sulphate solution? Write the chemical equation and identify the type of reaction. (3 Marks)
Answer: A white insoluble precipitate of barium sulphate is formed immediately.
$Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)$
This is a double displacement and precipitation reaction. - Explain decomposition reactions with one balanced chemical equation each for thermal, photolytic, and electrolytic decomposition. (5 Marks)
Answer: Decomposition is a reaction where a single compound breaks down into two or more simpler products.
1. Thermal Decomposition: 2Pb(NO3)2(s) xrightarrow{Heat} 2PbO(s) + 4NO2(g) + O2(g)
2. Photolytic Decomposition: 2AgBr(s) xrightarrow{Sunlight} 2Ag(s) + Br2(g)
3. Electrolytic Decomposition: 2H2O(l) xrightarrow{Electric current} 2H2(g) + O2(g)