change in temperature.<\/li>\n<\/ol>\nQuestion 9.
\nWhat are reactants and products of a chemical reaction? Explain with an example.
\nAnswer:
\nThe substance or the substances that take part in a chemical reaction are called reactants. The substance or the substances formed during a chemical reaction are called products.<\/p>\n
For example, lead nitrate reacts with potassium iodide forming lead iodide and potassium nitrate. In this reaction, the starting materials namely lead nitrate and potassium iodide are the reactants. The substances that are formed during the reaction namely lead iodide and potassium nitrate are the products.<\/p>\n
Question 10.
\nWhat is a chemical equation?
\nAnswer:
\nThe representation of a chemical reaction in the form of symbols and formulae of the substances involved in it, wherein the reactants are written on the left-hand side and the products on the right-hand side is called a chemical equation.<\/p>\n
Question 11.
\nWhat is the simplest way of representing a chemical reaction?
\nAnswer:
\nA chemical reaction is represented in the simplest manner using a word equation.<\/p>\n
Question 12.
\nWhat is a word equation in Chemistry? Explain with an example.
\nAnswer:
\nA chemical reaction that is represented by writing the complete names of the reactants and products rather than their symbols or formulae is known as a word equation. It is simply a chemical equation written in words.<\/p>\n
In a word equation, the names of the reactants are written on the left hand side and the names of the products are written on the right hand side. The reactants and products are separated by an arrow directed to the right (towards the products).<\/p>\n
In case the reaction has more than one reactant, + sign is put between the reactants. This sign on the reactants’ side indicates \u2018reacts with\u2019 or \u2018combines with\u2019. Similarly, + sign is put between the products when there is more than one product. Here the + sign indicates \u2018and\u2019. The arrow sign indicates \u2018produces\u2019 or \u2018yields\u2019.<\/p>\n
Consider the reaction between zinc and dilute hydrochloric acid. During this reaction, zinc chloride and hydrogen gas are formed. This can be represented by a word equation as follows:
\n<\/p>\n
<\/p>\n
Question 13.
\nWhat does the arrow in a chemical equation indicate? What does a chemical equation represent?
\nAnswer:
\nThe arrow in a chemical reaction indicates the direction of the reaction. A chemical equation represents a chemical reaction.<\/p>\n
Question 14.
\nDistinguish between a skeletal equation and a balanced equation.
\nAnswer:
\n1. Skeletal equation:
\nA chemical equation in which the number of atoms of different elements is not equal on the two sides of the equation is called an unbalanced chemical equation. It is also called skeletal equation.<\/p>\n
2. Balanced equation:
\nA chemical equation in which the number of atoms of different elements is equal on the two sides of the equation is called a balanced chemical equation.<\/p>\n
Question 15.
\nWhat is a balanced chemical equation? Why should chemical equations be balanced?
\nAnswer:
\nA balanced chemical equation is one in which the total number of atoms of each element are equal on both sides of the equation. A chemical equation is balanced so that the numbers of atoms of each type involved in a chemical reaction are the same on the reactant and product sides of the equation.<\/p>\n
A balanced chemical equation satisfies the law of conservation of mass, i.e., the total mass of the reactants is equal to the total mass of the products.<\/p>\n
<\/p>\n
Question 16.
\nWrite the steps involved in writing a balanced chemical equation.
\nAnswer:
\nStep 1:
\nKnow the reactants and products of the given chemical reaction and write down the word equation for the reaction. Note that the reactants must be written on the left hand side while the products must be written on the right hand side.<\/p>\n
Step 2:
\nThe second step is to write the skeletal equation. Write down the symbol and formula of each of the reactants and products.<\/p>\n
Step 3:
\nEnclose the formula of each reactant and product in separate boxes and do not change the formula or the subscripts of anything inside the boxes while balancing the equation.<\/p>\n
Step 4:
\nList the number of atoms of different elements on LHS (reactants) and RHS (products) separately.<\/p>\n
Step 5:
\nStart balancing with the compound (either on the reactants side or on the products side) that contains the maximum number of atoms. After choosing the compound, select the element that has the maximum number of atoms. To balance the atoms, put a small whole number coefficient before the formula of the compound.<\/p>\n
Step 6:
\nBalance all the elements one by one similarly. Check for the correctness of the balanced equation by counting and comparing the number of atoms of each element on both sides of the equation.<\/p>\n
Step 7:
\nWrite the physical state of each of the reactants and products.<\/p>\n
<\/p>\n
Question 17.
\nIron reacts with water (steam) forming iron oxide and hydrogen gas. Write a balanced chemical equation for this reaction.
\nAnswer:
\nStep 1:
\nLet us write the word equation:
\nIron + Water \u2192 Iron oxide + Hydrogen<\/p>\n
Step 2:
\nLet us write the skeletal equation for the reaction using symbols and formulae:
\nFe + H2<\/sub>O \u2192 Fe3<\/sub>O4<\/sub> + H2<\/sub><\/p>\nStep 3:
\nLet us put all the formulae\/symbols in separate boxes.
\n<\/p>\n
Step 4:
\nLet us list the number of atoms of each element on both sides of the equation:
\n<\/p>\n
Step 5:
\nWe shall start with the compound having the highest number of atoms namely iron oxide. In this compound, let us start with oxygen as the number of oxygen atoms is highest in the compound.<\/p>\n
There are four oxygen atoms in iron oxide on the RHS. However, there is only one oxygen atom on the LHS. To balance this, we shall put 4 as the coefficient for water (H2<\/sub>O).<\/p>\nNow let us balance the iron atoms. This can be done by putting 3 as the coefficient for Fe on the LHS. Now let us balance the hydrogen atoms. This is done by putting 4 as the coefficient for H2<\/sub> on the RHS. Now the equation becomes 3 Fe + 4 H2<\/sub>O \u2192 Fe3<\/sub>O4<\/sub> + 4 H2<\/sub><\/p>\n<\/p>\n
Step 6:
\nLet us now check for the correctness of the balance by comparing the number of atoms on either side of the equation.
\n<\/p>\n
Step 7:
\nSince each type of atom is balanced on both sides, the equation is balanced. Let us now mention the physical state of each of the reactants and products.<\/p>\n
The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively. The word aqueous (aq) is written if the reactant or product is present as a solution in water.
\n3 Fe (s) + 4 H2<\/sub>O (g) \u2192 Fe3<\/sub>O4<\/sub> (s) + 4 H2<\/sub> (g)
\nNote that the symbol (g) is used with H2<\/sub>O to indicate that in this reaction water is used in the form of steam.<\/p>\n<\/p>\n
Question 18.
\nWrite the balanced equation for the following chemical reactions:<\/p>\n
\n- Hydrogen + Chlorine \u2192 Hydrogen chloride<\/li>\n
- Barium chloride + Aluminium sulphate \u2192 Barium sulphate + Aluminium chloride<\/li>\n
- Sodium + Water \u2192 Sodium hydroxide + Hydrogen.<\/li>\n<\/ol>\n
Answer:<\/p>\n
\n- H2<\/sub> (g) + Cl2<\/sub> (g) \u2192 2 HCl (g)<\/li>\n
- 3 BaCl2<\/sub> (aq) + Al2<\/sub>(SO4<\/sub>)3<\/sub> (aq) \u2192 3 BaSO4<\/sub> (s) + 2 AlCl3<\/sub> (aq)<\/li>\n
- 2 Na (s) + 2 H2<\/sub>O (l) \u2192 2 NaOH (aq) + H2<\/sub> (g).<\/li>\n<\/ol>\n
Question 19.
\nWrite balanced chemical equations with symbols for the following reactions:<\/p>\n
\n- Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.<\/li>\n
- Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.<\/li>\n<\/ol>\n
Answer:
\n1. Barium chloride + Sodium sulphate \u2192 Barium sulphate + Sodium chloride
\nBaCl2<\/sub> (aq) + Na2<\/sub>SO4<\/sub> (aq) \u2192 BaSO4<\/sub> (s) + 2 NaCl (aq)<\/p>\n2. Sodium hydroxide + Hydrochloric acid \u2192 Sodium chloride + Water
\nNaOH (aq) + HCl (aq) \u2192 NaCl (aq) + H2<\/sub>O (l)<\/p>\n<\/p>\n
Question 20.
\nTranslate the following statements into chemical equations and then balance them:<\/p>\n
\n- Hydrogen gas combines with nitrogen to form ammonia.<\/li>\n
- Hydrogen sulphide gas burns in air to give water and sulphur dioxide.<\/li>\n
- Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.<\/li>\n
- Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.<\/li>\n<\/ol>\n
Answer:
\n1. Hydrogen + Nitrogen \u2192 Ammonia
\n3 H2<\/sub> (g) + N2<\/sub> (g) \u2192 2 NH3<\/sub> (g)<\/p>\n2. Hydrogen sulphide + Oxygen \u2192 Water + Sulphur dioxide
\n2 H2<\/sub>S (g) + 3 O2<\/sub> (g) \u2192 2 H2<\/sub>O (l) + 2 SO2<\/sub> (g)<\/p>\n3. Barium chloride + Aluminium sulphate \u2192 Barium sulphate + Aluminium chloride
\n3 BaCl2<\/sub>(s) + Al2<\/sub>(SO4<\/sub>)3<\/sub> (s) \u2192 3 BaSO4<\/sub> (s) + 2 AlCl3<\/sub> (s)<\/p>\n4. Potassium + Water \u2192 Potassium hydroxide + Hydrogen
\n2 K (s) + 2 H2<\/sub>O (l) \u2192 2 KOH (aq) + H2<\/sub> (g)<\/p>\nQuestion 21.
\nBalance the following chemical equations:<\/p>\n
\n- HNO3<\/sub> + Ca(OH)2<\/sub> \u2192 Ca(NO3<\/sub>)2<\/sub> + H2<\/sub>O<\/li>\n
- NaOH + H2<\/sub>SO4<\/sub> \u2192 Na2<\/sub>SO4<\/sub> + H2<\/sub>O<\/li>\n
- NaCl + AgNO3<\/sub> \u2192 AgCl + NaNO3<\/sub><\/li>\n
- BaCl2<\/sub> + H2<\/sub>SO4<\/sub> \u2192 BaSO4<\/sub> + HCl.<\/li>\n<\/ol>\n
Answer:<\/p>\n
\n- 2 HNO3<\/sub> + Ca(OH)2<\/sub> \u2192 Ca(NO3<\/sub>)2<\/sub> + 2 H2<\/sub>O<\/li>\n
- 2 NaOH + H2<\/sub>SO4<\/sub> \u2192 Na2<\/sub>SO4<\/sub> + 2 H2<\/sub>O<\/li>\n
- NaCl + AgNO3<\/sub> \u2192 AgCl + NaNO3<\/sub><\/li>\n
- BaCl2<\/sub> + H2<\/sub>SO4<\/sub> \u2192 BaSO4<\/sub> + 2 HCl.<\/li>\n<\/ol>\n
<\/p>\n
Question 22.
\nWrite the balanced chemical equations for the following reactions:<\/p>\n
\n- Calcium hydroxide + Carbon dioxide \u2192 Calcium carbonate + Water<\/li>\n
- Zinc + Silver nitrate \u2192 Zinc nitrate + Silver<\/li>\n
- Aluminium + Copper chloride \u2192 Aluminium chloride + Copper<\/li>\n
- Barium chloride + Potassium sulphate \u2192 Barium sulphate + Potassium chloride.<\/li>\n<\/ol>\n
Answer:<\/p>\n
\n- Ca(OH)2<\/sub> (aq) + CO2<\/sub> (g) \u2192 CaCO3<\/sub> (s) + H2<\/sub>O (l)<\/li>\n
- Zn (s) + 2 AgNO3<\/sub> (aq) \u2192 Zn(NO3<\/sub>)2<\/sub> (aq) + 2 Ag (s)<\/li>\n
- 2Al (s) + 3 CuCl2<\/sub> (aq) \u2192 2 AlCl3<\/sub> (aq) + 3 Cu (s)<\/li>\n
- BaCl2<\/sub> (aq) + K2<\/sub>SO4<\/sub> (aq) \u2192 BaSO4<\/sub> (aq) + 2 KCl (aq)<\/li>\n<\/ol>\n
Question 23.
\nA solution of a substance \u2018X\u2019 is used for white washing.<\/p>\n
\n- Name the substance \u2018X\u2019 and write its formula.<\/li>\n
- Write the reaction of the substance \u2018X\u2019 named in (i) above with water.<\/li>\n<\/ol>\n
Answer:<\/p>\n
\n- The substance \u2018X\u2019 is known as calcium oxide (quick lime). Its formula is CaO.<\/li>\n
- Calcium oxide reacts with water forming calcium hydroxide.<\/li>\n<\/ol>\n
Calcium oxide + Water \u2192 Calcium hydroxide
\nCaO (s) + H2<\/sub>O(l) \u2192 Ca(OH)2<\/sub> (aq)<\/p>\nQuestion 24.
\nList the various types of chemical reactions.
\nAnswer:
\nThe different types of chemical reactions are:<\/p>\n
\n- Chemical combination<\/li>\n
- Chemical decomposition<\/li>\n
- Chemical displacement<\/li>\n
- Double displacement.<\/li>\n<\/ol>\n
<\/p>\n
Question 25.
\nWhat are exothermic and endothermic chemical reactions? Give an example each.
\nAnswer:
\n1. Exothermic reactions:
\nThose chemical reactions during which heat is released are known as exothermic chemical reactions.
\nE.g.: Burning of fuels and respiration are examples of exothermic reactions.<\/p>\n
2. Endothermic reactions:
\nThose chemical reactions during which heat is absorbed are known as endothermic reactions.
\nE.g.: Reaction between nitrogen and oxygen that results in nitric oxide is an example of an endothermic reaction.<\/p>\n
Question 26.
\nWhat is meant by chemical combination or combination reaction? Give an example.
\nAnswer:
\nA chemical reaction in which two or more substances (reactants) chemically combine to form a product is called chemical combination. It is also called combination reaction.
\nE.g.: Magnesium on heating bums in air forming magnesium oxide. This is a combination reaction.
\n2Mg + O2<\/sub> \u2192 2MgO.<\/p>\nQuestion 27.
\nExplain the reaction that occurs when calcium oxide is made to react with water. Write the balanced chemical equation for the reaction. What type of chemical reaction is this? Why is it called an exothermic reaction?
\nAnswer:
\nTake a small amount of calcium oxide (quick lime) in a beaker. Slowly add water to this. Touch the beaker. The beaker will have become warmer. During this reaction, calcium oxide reacts with water forming calcium hydroxide. This reaction also releases heat. This reaction can be represented by the following equation:
\nCaO (s) + H2<\/sub>O(l) (Quick lime) \u2192 Ca(OH)2<\/sub> (aq) (Slaked lime) + Heat.
\nIn this reaction two substances are combining chemically forming a single compound. This is an example of chemical combination. The reaction above produces heat. Therefore, it is an exothermic reaction.<\/p>\n<\/p>\n
Question 28.
\nDistinguish between quick lime and slaked lime.
\nAnswer:
\nQuick lime is the common name of a compound called calcium oxide. Its molecular formula is CaO. Slaked lime is the common name of a chemical compound called calcium hydroxide. Its molecular formula is Ca(OH)2<\/sub>.<\/p>\nQuestion 29.
\nWhy do walls appear shiny two to three days after they are white washed with calcium hydroxide? Write the chemical equation for the reaction.
\nAnswer:
\nWalls are white washed with calcium hydroxide. Calcium hydroxide slowly reacts with carbon dioxide present in air forming calcium carbonate. Calcium carbonate gives shiny finish to the walls. The reaction that occurs here can be represented by the following equation:
\nCa(OH)2<\/sub> (aq) + CO2<\/sub> (g) \u2192 CaCO3<\/sub> (s) + H2<\/sub>O (l).<\/p>\nQuestion 30.
\nRepresent the reaction involving burning of coal by a balanced chemical equation. What type of chemical reaction is this? Why? Why is this reaction called an exothermic reaction?
\nAnswer:
\nCoal, on heating, combines with oxygen present in air, forming carbon dioxide. This reaction also releases plenty of heat.
\nC (s) + O2<\/sub> (g) \u2192 CO2<\/sub> (g) + Heat
\nIn this reaction two chemical substances combine to form a single compound. Therefore, this is an instance of chemical combination reaction. This reaction releases heat. Therefore, it is an exothermic reaction.<\/p>\n<\/p>\n
Question 31.
\nWhy is respiration considered an exothermic reaction? Explain.
\nAnswer:
\nDuring respiration, glucose reacts with oxygen forming carbon dioxide and water.
\nC6<\/sub>H12<\/sub>O6<\/sub> (aq) + 6O2<\/sub> (aq) \u2192 6CO2<\/sub> (aq) + 6H2<\/sub>O (l) + energy
\nThis reaction also releases heat. This is why respiration is considered an exothermic reaction.<\/p>\nQuestion 32.
\nHydrogen combines with oxygen to form water. What type of reaction is this? Why? Write a balanced chemical equation to represent this reaction.
\nAnswer:
\nIn the reaction above, two substances namely hydrogen and oxygen chemically combine to form a single compound namely water. Therefore, this is a case of chemical combination. The reaction can be represented by the following equation:
\n2H2<\/sub> (g) + O2<\/sub> (g) \u2192 2H2<\/sub>O (l)<\/p>\nQuestion 33.
\nWhat is meant by decomposition reaction? Explain with an example.
\nAnswer:
\nA chemical reaction in which a single compound breaks up into two or more products is called a decomposition reaction. It is also called chemical decomposition.<\/p>\n
For example, ferrous sulphate decomposes on heating forming anhydrous ferrous sulphate and water. Anhydrous ferrous sulphate on further heating decomposes to form ferric oxide, sulphur dioxide and sulphur trioxide. In this reaction, there are two decomposition reactions. The first one is:
\nFeSO4<\/sub>.7H2<\/sub>O (Ferrous sulphate crystals) (s) \u2192 FeSO4 <\/sub>\u00a0(Anhydrous ferrous sulphate) (s) + 7H2<\/sub>O
\nAnhydrous ferrous sulphate on further heating decomposes to give the following reaction:
\n<\/p>\n<\/p>\n
Question 34.
\nDescribe an experiment to show the decomposition of ferrous sulphate crystals. What are the products forrtied during the reaction?
\nAnswer:
\nTake about 2 g of ferrous sulphate crystals in a dry boiling tube. Note the green colour of the ferrous sulphate crystals. Heat the boiling tube over a flame using a spirit lamp. Observe the changes.<\/p>\n
Now the green ferrous sulphate changes its colour on losing water present in its crystal and forms anhydrous ferrous sulphate. On further heating, ferrous sulphate decomposes to form a reddish brown residue. This is ferric oxide. Gases with a characteristic smell are also produced during the reaction. The products formed in this reaction are ferric oxide, sulphur dioxide and sulphur trioxide.<\/p>\n
Question 35.
\nSuma took a pale green substance A in a test tube and heated it over the flame of a burner. A brown coloured residue B was formed along with the evolution of two gases with burning smell of sulphur. Identify A and B. Write the chemical reaction involved.
\nAnswer:
\nSubstance A is ferrous sulphate (FeSO4<\/sub>). Two gases, sulphur dioxide and sulphur trioxide evolved.
\nSubstance B is ferric oxide (Fe2<\/sub>O3<\/sub>).
\nThe reaction involve is
\n<\/p>\nQuestion 36.
\nExplain the thermal decomposition of lead nitrate. Write a balanced chemical equation for the reaction. Why is this reaction called decomposition reaction?<\/p>\n
OR<\/p>\n
Which coloured fumes are obtained when lead nitrate is heated? Write the balanced chemical equation for this reaction. Name the type of this chemical reaction.<\/p>\n
OR<\/p>\n
Name the brown fumes liberated when lead nitrate is heated. Write the balanced chemical equation for this reaction.
\nAnswer:
\nTake about 2 g of lead nitrate powder in a boiling tube. Hold the boiling tube with a pair of tongs and heat it over a flame using a spirit lamp. Now, brown fumes of nitrogen dioxide are produced. A yellow residue remains in the boiling tube. This is lead oxide. This reaction can be represented by the following equation:
\n<\/p>\n
In this reaction, a single compound breaks down chemically forming three new substances (products). Hence, it is called decomposition reaction.<\/p>\n
<\/p>\n
Question 37.
\nLimestone on heating decomposes to form quick lime and carbon dioxide. Represent this reaction by a balanced chemical equation.
\nAnswer:
\nThe chemical name of limestone is calcium carbonate. This compound on heating decomposes to give calcium oxide (quick lime) and carbon dioxide.
\n<\/p>\n
Question 38.
\nDraw a neat, labelled diagram to show the electrolytic decomposition of water.<\/p>\n
OR<\/p>\n
Draw the diagram of the apparatus used to show that water is a compound of hydrogen and oxygen. Label the following parts:<\/p>\n
\n- The part where oxygen is collected.<\/li>\n
- The part where hydrogen is collected.<\/li>\n<\/ol>\n
Answer:
\n<\/p>\n
Question 39.
\nDescribe an experiment to show the electrolytic decomposition of water. Write a suitable balanced equation for the reaction.
\nAnswer:
\nTake a plastic mug and drill two holes at its base. Fit a rubber stopper in each of these holes. Insert carbon (graphite) electrodes in these rubber stoppers. Connect the electrodes to a 6-volt battery. Fill the mug with water such that the electrodes get fully immersed. Add a few drops of dilute sulphuric acid to the water to make it conducting.<\/p>\n
Invert a test tube filled with water over each of the two carbon electrodes. Switch on the current and leave the apparatus undisturbed for some time. Now, water decomposes to form hydrogen and oxygen.<\/p>\n
Hydrogen gets collected in the test tube inverted over the cathode while the oxygen gets collected in the test tube inverted over the anode. It is also found that the volume of hydrogen is twice the volume of oxygen. This reaction is represented by the following equation:
\n2 H2<\/sub>O (l) \u2192 2H2<\/sub>(g) + O2<\/sub>(g).<\/p>\n<\/p>\n
Question 40.
\nDuring the electrolytic decomposition of water, why is the amount of sas collected in one of the test tubes double of the amount collected in the other? Name this eas.
\nAnswer:
\nIn water, hydrogen and oxygen are present in the ratio 2:1 by volume. During decomposition, hydrogen and oxygen are produced in the same ratio. Therefore, the amount of gas in one of the test tubes is double the amount collected in the other. The gas present in higher volume is hydrogen and the other gas is oxygen.<\/p>\n
Question 41.
\nWhat is photo-decomposition? Give an example.
\nAnswer:
\nThe decomposition of a chemical compound in presence of light is known as photo-decomposition. For example, silver chloride decomposes in presence of sunlight into silver and chlorine.<\/p>\n
Question 42.
\nSilver chloride decomposes in the presence of sunlight. How do you show this? Write a balanced chemical equation for the reaction.
\nAnswer:
\nTake about 2 g of silver chloride in a china dish. Place this china dish in sunlight for some time. Observe the changes. The white crystals of silver chloride turn grey in sunlight. In the presence of sunlight, silver chloride decomposes into silver and chlorine.
\n2AgCl(s) \u2192 2Ag (s) + Cl2<\/sub> (g)<\/p>\n<\/p>\n
Question 43.
\nSilver chloride is usually stored in dark coloured bottles. Give reason.
\nAnswer:
\nSilver chloride decomposes into silver and chlorine when exposed to light. In order to prevent photo-decomposition, silver chloride is stored in dark coloured bottles.<\/p>\n
Question 44.
\nWrite one equation each for decomposition reactions where energy is supplied in the form of<\/p>\n
\n- heat<\/li>\n
- light and<\/li>\n
- electricity.<\/li>\n<\/ol>\n
Answer:
\n1. CaCO3<\/sub> (s) \u2192 CaO (s) + CO2<\/sub> (g)
\nIn the reaction above, energy for decomposition is supplied in the form of heat.<\/p>\n2. 2AgCl (s) \u2192 2Ag (s) + Cl2<\/sub> (g)
\nIn the reaction above, energy for decomposition is supplied in the form of sunlight.<\/p>\n3. 2H2<\/sub>O (l) \u2192 2H2<\/sub> (g)+ O2<\/sub> (g)
\nIn the reaction above, energy for decomposition is supplied in the form of electricity.<\/p>\n<\/p>\n
Question 45.
\nIdentify the reactant which is oxidized and that which is reduced in the following chemical
\nreaction: CuO + H2<\/sub> \u2192 Cu + H2<\/sub>O.
\nAnswer:
\nIn this chemical reaction, the reactant that is reduced is CuO and the reactant that is oxidised is H2<\/sub>.<\/p>\nQuestion 46.
\nWhy are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
\nAnswer:
\nIn decomposition reaction a single substance decomposes to form two or more substances. This is the exact opposite of combination reaction in which two or more reactants combine to form a single product. This is why composition reactions are called the opposite of combination reactions.
\nGeneral equation for a combination reaction: A + B \u2192 AB.
\nGeneral equation for a decomposition reaction: AB \u2192 A + B.<\/p>\n
Question 47.
\nWhat is meant by displacement reaction? Give an example.
\nAnswer:
\nA chemical reaction in which a more reactive element displaces a less reactive element from its compound is known as a displacement reaction.<\/p>\n
For example, iron is more reactive than copper. Therefore, iron displaces copper from copper sulphate solution.
\n<\/p>\n
Question 48.
\nWhy does the colour of copper sulphate solution change when an iron nail is dipped in it?
\nAnswer:
\nCopper sulphate solution is blue in colour. When an iron nail is dipped in a solution of copper
\nsulphate, iron displaces copper from copper sulphate and forms iron sulphate. Iron sulphate is greenish. Therefore, the colour of copper solution changes to green when an iron nail is dipped in it.<\/p>\n
<\/p>\n
Question 49.
\nDescribe a simple experiment to show that a more active element displaces a less active element from the solution of its compound. Represent this reaction by a suitable equation.
\nAnswer:
\nTake two iron nails and clean them by rubbing with sand paper. Take about 10 mL of copper sulphate solution in a test tube. Tie one of the nails with a thread and carefully immerse it inside the copper sulphate solution. Keep the other nail for comparison. After about 15-20 minutes, take out the nail from the copper sulphate solution.<\/p>\n
A brownish substance is formed on the iron nail and the solution will have turned greenish. In this reaction, iron displaces copper from copper sulphate. The brownish colour on the iron nail is due to the deposition of displaced copper. The greenish colour of the solution is due to the formation of iron sulphate.
\nIron + Copper sulphate \u2192 Iron sulphate + Copper
\nFe (s) + CuSO4<\/sub> (aq) \u2192 FeSO4<\/sub> (aq) + Cu (s)
\nThis experiment shows that iron is more reactive than copper.<\/p>\nQuestion 50.
\nGive an example of a double displacement reaction.
\nAnswer:
\nZinc is a more reactive element than copper. Zinc displaces copper from copper sulphate solution. The reaction is represented by the following equation:
\nZinc + Copper sulphate \u2192 Zinc sulphate + Copper
\nZn(s) + CuSO4<\/sub> (aq) \u2192 ZnSO4<\/sub> (aq) + Cu (s)<\/p>\nQuestion 51.
\nDescribe the reaction between lead and copper chloride solution. What type of reaction is this? Between lead and copper, which is more reactive? Write a balanced chemical equation for this reaction.
\nAnswer:
\nWhen a lead rod is dipped in a solution of copper chloride, lead displaces copper from copper chloride. During this reaction lead chloride and copper are formed as products. This is a displacement reaction.<\/p>\n
Between lead and copper, lead is more reactive. This is why lead displaces copper from copper chloride solution.
\nThe reaction above is represented by the following equation:
\nLead + Copper chloride \u2192 Lead chloride + Copper
\nPb (s) + CuCl2<\/sub> (aq) \u2192 PbCl2<\/sub> (aq) + Cu(s)<\/p>\n<\/p>\n
Question 52.
\nZinc liberates hydrogen gas when it reacts with dilute hydrochloric acid whereas copper does not. Explain why.
\nAnswer:
\nZinc is chemically more reactive than hydrogen. Therefore, zinc can displace hydrogen from dilute hydrochloric acid. However, copper is less reactive than hydrogen. Therefore, copper cannot displace hydrogen from dilute hydrochloric acid.<\/p>\n
Question 53.
\nWhat is meant by double displacement reaction? Give an example.
\nAnswer:
\nA chemical reaction in which the two reactant compounds exchange their positive ions to form two new compounds is known as a double displacement reaction.<\/p>\n
For example, silver nitrate reacts with sodium chloride forming silver chloride and sodium nitrate. Silver nitrate + Sodium chloride \u2192 Silver chloride + Sodium nitrate.
\nAgNO3<\/sub> (aq) + NaCl (aq) \u2192 AgCl (s) + NaNO3<\/sub> (aq).<\/p>\nQuestion 54.
\nDescribe an experiment to show double displacement reaction.
\nAnswer:
\nTake about 3 mL of sodium sulphate solution in a test tube. Add about 3 mL of barium chloride solution to it. Shake well to ensure proper mixing of the two solutions. Soon we observe the formation of a white substance, which is insoluble in water.<\/p>\n
This insoluble substance formed is known as a precipitate. This is barium sulphate. Another product formed during this reaction is sodium chloride. In this reaction, the chloride ion present in barium chloride and sulphate ion in sodium sulphate are exchanged to form two new compounds. This is an instance of double displacement reaction.
\nSodium sulphate + Barium chloride \u2192 Barium sulphate + Sodium chloride
\nNa2<\/sub>SO4<\/sub> (aq) + BaCl2<\/sub> (aq) \u2192 BaSO4<\/sub> (s) + 2NaCl (aq)<\/p>\n<\/p>\n
Question 55.
\nIn the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal Write down the reaction involved.
\nAnswer:
\nSilver nitrate + Copper \u2192 Copper nitrate + Silver
\n2AgNO3<\/sub> (aq) + Cu (s) \u2192 Cu(NO3<\/sub>)2<\/sub> (aq) + 2Ag (s)<\/p>\nQuestion 56.
\nWhat is a precipitation reaction? Give an example.
\nAnswer:
\nAn insoluble substance formed during a chemical reaction is called a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction. It is represented by a downward arrow (\u2193).<\/p>\n
For example, sodium sulphate reacts with barium chloride to form barium sulphate and sodium chloride. Here barium chloride is a precipitate. Therefore, this reaction is a precipitation reaction.<\/p>\n
Question 57.
\nWhen the solutions of lead (II) nitrate and potassium iodide are mixed,<\/p>\n
\n- What is the colour of the precipitate formed? Name the compound precipitated,<\/li>\n
- Write the balanced chemical equation for this reaction,<\/li>\n
- Is this also a double displacement reaction?<\/li>\n<\/ol>\n
Answer:
\n1. When lead nitrate and potassium iodide are mixed, a yellow precipitate is formed.’ The compound formed in the form of a precipitate is lead iodide (PbI2<\/sub>).<\/p>\n2. Pb(NO3<\/sub>)2<\/sub> (aq) + 2KI (aq) \u2192 PbI2<\/sub> (s) + 2KNO3<\/sub> (aq).<\/p>\n3. Yes, this reaction is a double displacement reaction.<\/p>\n
<\/p>\n
Question 58.
\nWhat is the difference between displacement and double displacement reactions? Write equations for these reactions.
\nAnswer:
\nWhen a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. When there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction.<\/p>\n
In a displacement reaction one displacement takes place, while in a double displacement reaction two displacements take place.<\/p>\n
Example for a displacement reaction:
\nMg (s) + 2HCl (aq) \u2192 MgCl2<\/sub> + H2<\/sub> (g).<\/p>\nExample for a double displacement reaction:
\n2KBr (aq) + BaI2<\/sub> (aq) \u2192 2KI (aq) + BaBr2<\/sub> (aq).<\/p>\nQuestion 59.
\nWrite the balanced chemical equations for the following and identify the type of reaction in each case:<\/p>\n
\n- Potassium bromide (aa) + Barium iodide (aa) \u2192 Potassium iodide (aq) + Barium bromide(s).<\/li>\n
- Zinc carbonate (s) \u2192 Zinc oxide (s) + Carbon dioxide (g)<\/li>\n
- Hydroeen fg) + Chlorine (fg) \u2192 Hydrogen chloride (s)<\/li>\n
- Magnesium (s) + Hydrochloric acid (aq) \u2192 Magnesium chloride (aq) + Hydrogen (s).<\/li>\n<\/ol>\n
OR<\/p>\n
Write the balanced chemical equations for the following chemical reactions:<\/p>\n