Exam-useful questions from this chapter, grouped by marks, each with a full model answer. Try writing the answer yourself first, then check it against this one.
1 1-Mark Questions
Q. What does the arrow in a chemical equation show?
Answer: The arrow shows the direction of the reaction, from reactants towards products.
Q. What is a skeletal chemical equation?
Answer: An equation in which the number of atoms is not equal on both sides is called a skeletal (unbalanced) chemical equation.
Q. What does the symbol (aq) mean?
Answer: The substance is present as a solution in water.
Q. What is thermal decomposition?
Answer: When a decomposition reaction is carried out by heating, it is called thermal decomposition.
Q. What is a precipitate?
Answer: An insoluble solid formed in a reaction, which settles down in the solution, is called a precipitate.
Q. Rusting is an example of which kind of process?
Answer: Rusting is a process of corrosion, in which iron reacts with moisture and oxygen in the air and is slowly destroyed.
Q. Define oxidation in one line.
Answer: Oxidation is the gain of oxygen or the loss of hydrogen.
Q. Give one method to prevent rancidity.
Answer: Keeping food in an airtight container, so it does not come into contact with air (oxygen).
2 2-Mark Questions
Q. Why must a chemical equation be balanced? Explain with reason.
Answer: By the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction. So the number of atoms of every element present in the reactants must equal the number in the products, otherwise the equation would go against a basic law of nature.
Q. Explain the difference between a combination and a decomposition reaction, with one example each.
Answer: In a combination reaction, two or more substances combine to form a single product, e.g. . A decomposition reaction is the opposite: one reactant breaks down into two or more simpler products, e.g. .
Q. Explain the difference between exothermic and endothermic reactions, with one example each.
Answer: In an exothermic reaction, heat is released along with the formation of products, e.g. respiration. In an endothermic reaction, energy is absorbed, e.g. the thermal decomposition of or the reaction of barium hydroxide with ammonium chloride.
Q. What is the difference between a displacement and a double displacement reaction?
Answer: In a displacement reaction, a more reactive element displaces a less reactive one from its compound (e.g. ). In a double displacement reaction, there is an exchange of ions between the reactants (e.g. , where an insoluble precipitate of barium sulphate also forms).
Q. What was observed on heating copper powder in Activity 1.11? Write the equation.
Answer: Heating copper powder in air deposits a black coating on it, because the copper is oxidised by oxygen in the air to form copper(II) oxide: .
Q. Explain the chemical reaction that takes place in whitewashing a wall.
Answer: A solution of slaked lime (calcium hydroxide) is applied to the wall. It reacts slowly with carbon dioxide in the air to form a thin layer of calcium carbonate, , which gives the wall a shiny finish two to three days after whitewashing.
Q. What is photolytic decomposition? Give one example.
Answer: When a decomposition reaction is brought about by the energy of sunlight, it is called photolytic decomposition. Example: ; this reaction is used in black-and-white photography.
Q. What is observed in the electrolysis of water (Activity 1.7)?
Answer: Passing an electric current through acidified water produces bubbles at both electrodes: hydrogen gas at one, oxygen gas at the other, and the volume of hydrogen collected is exactly double the volume of oxygen.
3 4-Mark Questions
Q. Write the steps of balancing by the hit-and-trial method.
Answer: 1) Draw a box around each formula; nothing inside a box may be changed. 2) Count the atoms: Fe 1 and 3, H 2 and 2, O 1 and 4. 3) Choose the compound with the most atoms, , and balance its oxygen first: take . 4) Now balance hydrogen: take on the right. 5) Finally balance iron: take on the left. 6) Check: is balanced.
Q. Explain oxidation and reduction in terms of gain/loss of oxygen, with one example each.
Answer: Oxidation: a substance gains oxygen, e.g. , where copper is oxidised to copper oxide. Reduction: a substance loses oxygen, e.g. , where copper oxide is reduced to copper.
Q. Explain the difference between corrosion and rancidity, and give two prevention methods for each.
Answer: Corrosion: when a metal is attacked by substances around it, such as moisture and acids, it corrodes (e.g. rusting of iron). Prevention: painting iron articles, so that the iron does not come into direct contact with moisture and air. Rancidity: fats and oils in food are oxidised, changing their smell and taste. It is slowed down by adding antioxidants, keeping food in airtight containers, and flushing packets (such as chips) with nitrogen.
Q. Explain the four types of chemical reaction with a definition and one equation each.
Answer: 1) Combination: two or more substances combine to form a single product, . 2) Decomposition: one reactant breaks down into simpler products, . 3) Displacement: a more reactive element displaces a less reactive one from its compound, . 4) Double displacement: there is an exchange of ions between the reactants, .
Q. Explain the changes observed and the reaction when ferrous sulphate crystals are heated in Activity 1.5.
Answer: Heating green ferrous sulphate crystals () first drives off water and the colour of the crystals changes; then the compound decomposes into ferric oxide, sulphur dioxide and sulphur trioxide, with the characteristic odour of burning sulphur: . Ferric oxide is a solid, while and are gases. This is a thermal decomposition reaction.
Q. How are reaction conditions (temperature, pressure, catalyst) shown in an equation? Give examples.
Answer: Reaction conditions are written above and/or below the arrow. Example: , where the pressure is written on the arrow. In photosynthesis, , sunlight and chlorophyll are shown on the arrow.
Q. A shiny brown element ‘X’ turns black on heating in air. Name element ‘X’ and the compound formed, with the equation.
Answer: Element ‘X’ is copper (Cu), and the black compound formed is copper(II) oxide (CuO): . This is an oxidation reaction, as seen in Activity 1.11.
4 6-Mark Questions
Q. What is a chemical equation? Why must it be balanced? Check the equation and explain why it is already balanced.
Answer: A chemical equation represents a chemical reaction using chemical formulae. By the law of conservation of mass, mass can neither be created nor destroyed in a reaction, so the number of atoms of each element must be the same on both sides: only then is an equation balanced. Checking : Zn 1 = 1, H 2 = 2, S 1 = 1, O 4 = 4. Every element has equal atoms on both sides, so this equation is already balanced and needs no coefficients.
Q. Explain the four types of chemical reaction (combination, decomposition, displacement, double displacement) in detail with example, equation and one special feature each.
Answer: 1) Combination reaction: two or more substances combine to form a single product (); feature: in Activity 1.4 a large amount of heat is released, so this example is exothermic. 2) Decomposition reaction: one reactant breaks down into simpler products (); feature: the opposite of combination, and it needs energy (heat, light or electricity), so it is endothermic. 3) Displacement reaction: a more reactive element displaces a less reactive one from its compound (); feature: zinc and lead are more reactive than copper, so they displace copper. 4) Double displacement reaction: there is an exchange of ions between the reactants (); feature: here an insoluble precipitate forms, so it is also a precipitation reaction.
Q. Define oxidation and reduction, and explain how both can happen together in the same reaction. Give two examples with equations.
Answer: Oxidation is the gain of oxygen or the loss of hydrogen; reduction is the loss of oxygen or the gain of hydrogen. In , copper(II) oxide loses oxygen and is reduced while hydrogen gains oxygen and is oxidised: one reactant is oxidised while the other is reduced (a redox reaction). Example 1: , where carbon is oxidised to CO and ZnO is reduced to Zn. Example 2: , where HCl is oxidised to and is reduced to .
Q. Explain in detail the two everyday effects of oxidation, corrosion and rancidity, with example, cause and prevention methods.
Answer: Corrosion: when a metal is attacked by substances around it, such as moisture and acids, it corrodes; examples are rusting of iron (a reddish-brown coating), the black coating on silver and the green coating on copper. Corrosion damages car bodies, bridges, iron railings, ships and all objects made of metals, especially iron, and every year an enormous amount of money is spent to replace damaged iron. Prevention: painting iron articles, so the iron does not come into direct contact with moisture and air. Rancidity: when fats and oils are oxidised, they become rancid and their smell and taste change. It is slowed down by adding antioxidants, storing food in airtight containers, and flushing packets with nitrogen (e.g. chips).
5 Case Studies
(A) 1 mark Which metal was displaced in this experiment?
Answer: Copper (Cu) was displaced.
(B) 1 mark Which metal proved to be more reactive?
Answer: Iron (Fe) proved to be more reactive than copper.
(C) 2 marks Write the balanced chemical equation for this reaction.
Answer:
(D) 2 marks What type of reaction is this, and why?
Answer: This is a displacement reaction, because iron removed copper from its compound, copper sulphate, and took its place.
(A) 1 mark Name the white insoluble substance formed in this reaction.
Answer: Barium sulphate (BaSO4).
(B) 1 mark What is this type of product called?
Answer: It is called a precipitate.
(C) 2 marks Write the balanced chemical equation for this reaction, with state symbols.
Answer:
(D) 2 marks Why is this called a double displacement reaction?
Answer: Because there is an exchange of ions between the two reactants: ions pair with ions and ions with ions. The barium sulphate formed is insoluble, so it separates as a precipitate.