Short Answer Questions
Q1: What is a Chemical Reaction?
Ans: A chemical reaction refers to the process that leads to the transformation of one set of chemical substances into another. It involves the making or breaking of bonds between atoms, ions, or molecules. During a chemical reaction, reactants are transformed into products. For instance, when iron rusts, the iron (reactant) reacts with oxygen to form iron oxide (product).
Q2: What is a balanced chemical equation? Why is it necessary to balance a chemical equation?
Ans: A balanced chemical equation is one in which the number of atoms involved on the reactant side is equal to the number of atoms on the product side. This is based on the law of conservation of mass which states that matter cannot be created or destroyed. For example, the balanced chemical equation for the reaction between nitrogen and hydrogen to form ammonia is N2 + 3H2 → 2NH3. Balancing a chemical equation is essential because it follows the Law of Conservation of Mass, which states that the total number of atoms of each element in the reactants must be equal to the total number of atoms of that element in the products. Without balance, the chemical equation may incorrectly represent the actual reaction, leading to inaccurate predictions about the quantities of reactants needed or products formed.
Q3: What is the method of balancing chemical equations?
Ans: The hit-and-trial method is used to balance simple chemical equations. In this method, coefficients before the symbols/formulae of the reactants and products are adjusted in such a way that the total number of atoms of each element on both sides becomes equal. For example, consider the unbalanced equation H2 + O2 → H2O. Here, there are 2 H atoms on the left and only 1 on the right, and 2 O atoms on the left but only 1 on the right. By adjusting coefficients, we get the balanced equation as 2H2 + O2 → 2H2O. Now, there are 4 H atoms and 2 O atoms on both sides.
Q4: What are the characteristics of chemical reactions?
Ans: Characteristics of chemical reactions are as follows: (i) Evolution of a gas, (ii) Formation of a precipitate, (iii) Change in color, (iv) Change in temperature, and (v) Change in the state.
Q5: What is a Precipitation reaction? Give an example.
Ans: A precipitation reaction is a type of chemical reaction where two soluble salts in aqueous solution combine to form an insoluble salt (precipitate). This precipitate can be separated from the solution by filtration. For example, when solutions of silver nitrate and sodium chloride are mixed, a white insoluble salt, silver chloride (AgCl), precipitates out of the solution: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq).
Q6: What is a redox reaction?
Ans: A redox (reduction-oxidation) reaction is a type of chemical reaction that involves the transfer of electrons between two species. Redox reactions are of two types: (i) Oxidation and (ii) Reduction. An oxidation reaction is one in which a substance loses electrons, while a reduction reaction is one in which a substance gains electrons. For example, in the reaction between hydrogen and oxygen to form water (2H2 + O2 → 2H2O): Hydrogen (H2) is oxidized because it loses electrons (H2 → 2H+ + 2e-) and Oxygen (O2) is reduced because it gains electrons (O2 + 4e- → 2O2-).
Q7: In the equations given below, state giving reasons, for whether substances have been oxidized or reduced.
(i) PbO + CO → Pb + CO2
Ans: In this reaction, carbon monoxide (CO) is oxidized because it gains oxygen to form carbon dioxide (CO2). Lead oxide (PbO) is reduced to lead (Pb) because it loses oxygen. PbO acts as the oxidizing agent because it accepts electrons, and CO acts as the reducing agent because it donates electrons.
(ii) H2S + Cl2 → 2HCl + S
Ans: In this reaction, hydrogen sulfide (H2S) is oxidized to sulfur (S) because it loses hydrogen. Chlorine (Cl2) is reduced to hydrogen chloride (HCl) because it gains electrons. Cl2 acts as the oxidizing agent because it accepts electrons, and H2S acts as the reducing agent because it donates electrons.
Q8: What are the different ways that can make chemical equations more informative?
Ans: Chemical equations can be made more informative by:
- (i) By indicating the physical states of the reactants and products (e.g., Gaseous state is indicated by the symbol (g)).
- (ii) By indicating the heat changes that take place in the reaction (e.g., writing "Heat" on the products side for exothermic reactions).
- (iii) By indicating the "conditions" under which the reaction takes place.
Q9: A sample of water weed was placed in water and exposed to sunlight. Bubbles of gas are seen on the surface of the leaves.
(i) Name the gas evolved. (ii) Name the process taking place. (iii) Write a balanced equation of reaction taking place.
Ans:
(i) Oxygen.
(ii) Photosynthesis.
(iii) 6CO2 + 6H2O → C6H12O6 + 6O2.
Photosynthesis is the process in which plants use carbon dioxide and water to form glucose in the presence of energy from the sunlight and release oxygen. The bubbles of gas seen on the surface of leaves are of oxygen.
Q10: Give balanced equations, wherever possible, or where this is not possible, explain the following by means of examples:
- (i) A reaction which gives out heat: Coke on heating in air catches fire and liberates a large amount of heat: C + O2 → CO2 + Heat.
- (ii) A reaction that takes place with the help of sunlight: Carbon dioxide and water react in the presence of chlorophyll and sunlight to form glucose and oxygen: 6CO2 + 6H2O → C6H12O6 + 6O2.
- (iii) A reaction that is brought about by the electric current: Molten lead bromide decomposes into lead metal and bromide on the passage of electric current: PbBr2 → Pb + Br2.
- (iv) A reversible reaction: The reaction between red hot iron and steam is reversible: 3Fe + 4H2O ↔ Fe3O4 + 4H2.
- (v) A reaction with a solid and gas that produces heat: When magnesium burns in air or oxygen it liberates a large amount of heat: 2Mg + O2 → 2MgO + Heat.
Q11: Balance the following chemical equations:
(i) Mg + N2 → Mg3N2
Ans: On the reactant side, there is 1 Mg atom and 2 N atoms, while on the product side, there are 3 Mg atoms and 2 N atoms. By multiplying Mg by 3, the balanced equation becomes: 3Mg + N2 → Mg3N2.
(ii) KClO3 → KCl + O2
Ans: To balance the oxygen atoms, multiply O2 by 3 and KClO3 by 2 (2KClO3 → KCl + 3O2). Then multiply KCl by 2. The balanced equation is: 2KClO3 → 2KCl + 3O2.
Q12: Write the uses of decomposition reactions.
Ans: A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more simpler substances, usually under the influence of heat, light, or electricity. Examples of uses include:
- Metal Extraction: Used to extract metals like aluminum, sodium, and potassium from their ores through electrolysis.
- Chemical Synthesis: Employed to create specific chemicals by breaking down complex substances.
- Environmental Applications: Helps break down harmful substances into less toxic forms in waste treatment.
- Food Preparation: Baking soda decomposes to release carbon dioxide, helping dough rise.
- Explosives: Rapid energy release is used in explosive devices.
Q13: What do you understand by the term corrosion?
Ans: Corrosion is when metals like iron, copper, or silver slowly get damaged because of things around them like water or acids. For example, when iron reacts with air and water, it turns reddish-brown, forming rust. Silver might turn black, and copper might turn green. Corrosion weakens structures like cars, bridges, and fences, making them unsafe.
Q14: Can rancidity retard by storing foods away from the light?
Ans: Yes. Rancidity is a process where food products, particularly those containing fats and oils, deteriorate due to oxidation over time, leading to unpleasant smells and tastes. If food is stored away from light, this oxidation process slows down, retarding the development of rancidity.
Q15: Which of the following is a balanced chemical equation? (A) 2H2 + O2 -> H2O (B) H2 + O2 -> H2O (C) 2H2 + O2 -> 2H2O (D) H2 + 2O2 -> 2H2O
Ans: Correct Answer: C. A balanced chemical equation is one in which the number of atoms involved on the reactant side is equal to the number of atoms on the product side. In Option C, there are exactly 4 hydrogen atoms and 2 oxygen atoms on both sides.
Frequently Asked Questions (FAQs)
1. What's the difference between a chemical reaction and a physical change?
Ans. A chemical reaction produces new substances with different properties (e.g., burning wood creates ash and gases), while a physical change only alters appearance or state without forming new compounds (e.g., melting ice is still water).
2. How do I balance chemical equations step by step?
Ans. Count atoms of each element on both sides, then adjust coefficients (numbers before compounds) to match quantities—never change subscripts. It is recommended to balance metals first, then non-metals, then hydrogen and oxygen last.
3. Why do some reactions need specific conditions like heat or light?
Ans. This is due to activation energy—the minimum energy required. Some reactions need external energy (heat, electricity, light) to break molecular bonds and initiate the reaction. For example, photosynthesis requires sunlight.
4. What does the arrow symbol mean in a chemical equation?
Ans. The arrow (→) indicates reaction direction and products formed from reactants. Reversible reactions use a double arrow (↔) showing both forward and backward processes occur simultaneously.
5. How can I tell if a chemical equation is balanced correctly?
Ans. Count total atoms of each element on the reactant side and product side separately—both totals must match exactly. Imbalanced equations violate the law of conservation of mass.