Chemical Reactions and Equations

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1 1 Chemical Reactions and Equations SYLLABUS Chemical reactions: Chemical equation, balanced chemical equation, implications of a balanced chemical equation. Types of chemical reactions: combination, decomposition, displacement, double displacement, precipitation, neutralization, oxidation and reduction. BASIC CNCEPTS 1. Chemical reactions taking place are represented by fully balanced chemical equations, i.e., the number of atoms of each type involved in a reaction are the same on the reactant and product sides of the chemical equation. 2. A simple equation is balanced by hit and trial method so that same total number of charges and atoms appear on both sides of the equation. For example when iron combines with steam to form Fe 3 4 and, it is balanced in the following steps (i) Fe Fe 3 4 (Skeleton equation) (ii) Fe + 4 Fe 3 4 ( is made equal on both sides) (iii) Fe + 4 Fe (H is made equal on both sides) (iv) 3Fe + 4 Fe (Fe is made equal on both sides) This is balanced equation Right Left Fe H 8 8 If the reaction is complicated, i.e., it involves large number of reactants and products, it is written in steps which are balanced as above and then added up to get the final reactants and products. This is called partial equation method. 3. A complete chemical equation gives the formulae of the reactants and products, their physical states and brief conditions under which a reaction occurs. 1

2 2 GLDEN SCIENCE X 4. There are several types of reactions. These involve breaking of bonds and formation of new ones by the effect of heat, light or mechanical shaking. (i) A decomposition reaction occurs when a compound is broken into smaller parts. It may be either thermal decomposition or electrolytic decomposition. (ii) In combination reaction, two or more substances combine to form a new substance. (iii) The reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms is called displacement reaction, when two reacting molecules exchange their partner ions in solution, it will be double displacement reaction. (iv) It is said to be a dissociation reaction, when a substance breaks up into its ions. (v) In precipitation reactions, insoluble salts are formed. (vi) In redox reactions, a substance gains oxygen or loses hydrogen, i.e., gets oxidised and simultaneously, another substance loses oxygen or gains hydrogen, i.e., gets reduced. 5. If the surface of a metal is attacked by air, water, it is said to corrode and the process is called corrosion. Rancidity is the process in which fats and oils (in food) get oxidised and the food gets spoiled. QUESTINS ANSWERS 1.1. CHEMICAL EQUATINS Q. 1. What is a chemical equation? Illustrate with an example. Ans. The mode of communicating facts in chemical language about a chemical reaction is called chemical equation. Thus, a chemical equation is an expression for given chemical change in terms of symbols or formulae of the reactants and products. For example, the reaction of zinc with dilute sulphuric acid to produce zinc sulphate and hydrogen is given by the following chemical equation: Zn Zn. Q. 2. How will you know whether a chemical reaction has taken place? Ans. When a chemical reaction occurs, one or more of the following changes take place : (i) Change in state (ii) Change in colour (iii) Evolution of a gas (iv) Evolution or absorption of heat (change in temperature). Q. 3. How would you justify that a chemical reaction has taken place in the following cases: (i) Burning of magnesium ribbon in air. (ii) Addition of lead nitrate solution to potassium iodide solution. (iii) Addition of dilute hydrochloric acid to zinc granules. Ans. (i) Burning of magnesium ribbon in air gives a powder of Mg. So there is a change of state.

3 CHEMICAL REACTINS AND EQUATINS 3 (ii) Addition of colourless lead nitrate solution to potassium iodide solution gives yellow coloured precipitate (lead iodide). So there is a change of colour. (iii) Addition of dilute hydrochloric acid to zinc granules gives a gas ( ) with effervescence. Q. 4. How do you show that respiration is a chemical change? Ans. During respiration, energy is released. Q. 5. Why is it necessary to obtain a balanced chemical equation? Ans. Law of conservation of mass is applied here. Accordingly matter can neither be created nor destroyed. Thus during a chemical reaction the total mass of the reactants and products remains the same. Therefore, for a complete chemical equation, the number of atoms of various elements on both sides are made equal, i.e., the equation is balanced. Q. 6. How is a chemical equation written? Illustrate with an example. Ans. Reaction writing should be done in a systematic manner, using the conventions followed. These are illustrated with the help of an example. Let it be required to write a chemical equation that describes the reaction between magnesium and sulphuric acid to produce magnesium sulphate and hydrogen. (i) Write the symbols of all the substances involved in the reaction first the reactants and then the products one after the other N THE SAME LINE Reactants Products Mg Mg (ii) Separate the reacting substances from the products by a symbol meaning produce. The sign (an arrow) or sometimes sign (equal) = is used for this purpose Mg Mg (iii) Then put + sign between the reactants indicating that these react and a (plus) + sign between the products which signifies and, Mg Mg Q. 7. What do you understand by skeletal and balanced chemical equation? Ans. If the number of atoms of any element in a chemical equation is not equal on both sides, then it is a skeleton equation. For example, Mg + HCl MgCl 2 Here, the number of chlorine and hydrogen atoms are not equal on both sides. In a balanced equation, the number of atoms of different elements on both sides of a chemical equation are equal. For example, Mg + 2HCl MgCl 2 Q. 8. Describe the method of balancing a chemical equation taking an example. Ans. Example I: Let us consider the formation of water from the combination of oxygen and hydrogen. The following sequential steps be taken to obtain a balanced chemical equation. (i) First write the skeleton equation. For example, + 2 Caution: Do not change formula of any constituent while balancing the equation.

4 4 GLDEN SCIENCE X (ii) The same total of charges should appear on the left and right side of the equation. (iii) Make one of the atoms equal on both sides by multiplying a molecule or compound with an integral number so that the desired element is balanced. It is convenient to start with the molecule or compound that contains the maximum number of atoms. Here, contains maximum number of atoms. It contains one short of oxygen atom and so it is multiplied by 2 as shown (iv) Next examine the effect of this multiplication of the molecule on the balance of other atoms. As is seen, the oxygen balances but now hydrogen on left is 2 less. So, multiply on the left by (v) Further count the number of atoms of each type on both sides. In the above equation Right Left H So, the equation is balanced. If the number of atoms on both sides do not agree, continue with the above steps till Balanced Chemical Equation is obtained. This is called hit and trial method. Example II: Let us take a little more difficult equation, when iron is combined with steam ( ). (i) The skeleton equation for the above reaction is Fe Fe 3 4 (ii) Then, Fe 3 4 is selected which contains the maximum of atoms. It contains 4 oxygen atoms whereas there is only one oxygen atom on the other side, i.e., L.H.S, in. So is multiplied by 4. Fe + 4 Fe 3 4 (iii) Again examine the effect of step (ii). xygen is balanced but Fe and H are not yet balanced. So to balance H, multiply by 4. Fe + 4 Fe (iv) When counting the number of atoms on both sides, it is seen Fe is one on L.H.S. and is 3 on R.H.S. To equalise Fe on both sides, multiply Fe on L.H.S. by 3. 3Fe + 4 Fe Now this is a balanced chemical equation. Example III: Let us consider a reaction in which sodium metal reacts with water to form sodium hydroxide and hydrogen. (i) The skeleton equation for the above reaction is: Na NaH (ii) When considering number of each type of atoms on both sides, it is seen hydrogen atoms on both sides are not equal, it is 2 atoms on left side and 3 atoms on right side. So, we multiply by 2. Na + 2 NaH (iii) This step (ii) makes oxygen atoms deficient on right side and hydrogen atoms becomes more on left side. So, we multiply NaH by 2.

5 CHEMICAL REACTINS AND EQUATINS 5 Na + 2 2NaH (iv) Now oxygen and hydrogen atoms are equal on both sides but sodium atom is one less on left side. So, we multiply Na by 2. 2Na + 2 2NaH Now this is a balanced chemical equation. Example IV: If the reaction is complicated i.e., it involves large number of reactants and products, it is preferred to write the equation in steps (the actual reaction may or may not be taking in these steps). Each step should be a balanced chemical equation. For example, when copper reacts with conc. nitric acid, products are cupric nitrate, nitrogen dioxide and water. Cu + HN 3 Cu(N 3 + N 2 We can write this reaction in three steps and balance each step separately. (i) First, HN 3 is decomposed to give nitrogen dioxide (N 2 ) and water ( ) and atomic oxygen. 2HN 3 2N (i) (ii) In the next step, copper is oxidised to copper (II) oxide (Cu) Cu + Cu...(ii) (iii) Copper oxide so formed then reacts with nitric acid to form copper nitrate [Cu(N 3 ] and water. Cu + 2HN 3 Cu(N 3...(iii) Next multiply Eq. (i), (ii), (iii) by an integer so that on adding (i), (ii) and (iii), intermediate products (which do not appear in the final reaction) cancel out. Now in this case, we find that the integer is one. So add (i), (ii) and (iii) and write the final balanced equation as shown Cu + 4HN 3 Cu(N 3 + 2N This method is known as Partial Equation Method. Q. 9. Balance the following skeletal equations: (i) KCl 3 KCl + 2 (ii) Na 2 C 3 + HCl NaCl + C 2 (iii) CH C 2 (iv) NaCl + HCl (v) Fe Fe 3 4 (vi) S S (vii) + N 2 NH 3. Ans. (i) (akcl 3 KCl (bkcl 3 2KCl (ii) (a) Na 2 C 3 + HCl 2NaCl + C 2 (b) Na 2 C 3 + 2HCl 2NaCl + C 2 (iii) (a) CH C (b) CH C (iv) (anacl + HCl

6 6 GLDEN SCIENCE X (bnacl + 2HCl (v) (a) 3Fe Fe 3 4 (b) 3Fe + 4 Fe 3 4 (c) 3Fe + 4 Fe (vi) (a) S S (b S S (vii) (a) N 2 2NH 3 (b) N NH 3 Q. 10. Balance the following chemical equations: (i) NaH (ii) HN 3 + Ca(H Ca(N 3 (iii) FeCl 2 S HCl + FeS (A.I. CBSE 2008) Ans. (inah 2NaH + 2 (iihn 3 + Ca(H Ca(N 3 2HN 3 + Ca(H Ca(N (iii) FeCl 2 S 2HCl + FeS. Q. 11. What does a balanced chemical equation convey to a chemist? Ans. The following information is conveyed by a balanced chemical equation: (i) Formulae of substances taking part in chemical reaction i.e., reactants. (ii) Formulae of substances produced in the reaction i.e., products. (iii) The relative number of molecules of reactants and products. (iv) The relative masses of reactants and products. (v) The relative volumes of gaseous substances involved in the reaction. Q. 12. Enlist the limitations of chemical equation. Ans. Limitations or defects of a chemical equation: A chemical equation does not indicate about (i) the physical states of the reactants and products. (ii) the concentrations of the reactants and products. (iii) the heat evolved (exothermic reaction) or absorbed (endothermic reaction) during the chemical reaction. (iv) the condition of temperature, pressure, catalyst etc. which are needed for a reaction. (v) the rate of reaction, whether slow or fast. (vi) the reversibility of a reaction. (vii) whether any precipitate is formed or gas is evolved in the reaction. (viii) the extent upto which reaction takes place. (ix) time taken by the reaction for completion. (x) mechanism of the reaction.

7 CHEMICAL REACTINS AND EQUATINS 7 Q. 13. What are the essentials of a chemical equation? Ans. (i) It must refer to an actual chemical reaction. (ii) It should be molecular. For example, H,, N should be written as, 2, N 2. (iii) It should be balanced and conform to the law of conservation of matter. Q. 14. What information is available from the following equation: CaC 3 + 2HCl CaCl 2 + C 2 Ans. The following information can be derived from the given equation: (i) Calcium carbonate and hydrochloric acid are reactants and calcium chloride, water and carbon dioxide are products. (ii) 1 molecule of calcium carbonate reacts with 2 molecules of hydrochloric acid to produce 1 molecule each of calcium chloride, water and carbon dioxide. (iii) 1 g molecule, i.e., 100 g CaC 3 reacts with 2 g molecules, i.e., 73 g HCl and produces, 1 g molecule, i.e., 111 g CaCl 2, 1 g molecule, i.e., 18 g and 1 g molecule, i.e., 44 g C 2. (iv2.4 litres of C 2 is formed at S.T.P. Q. 15. What is a thermochemical equation? Give two examples. Ans. Most chemical reactions are accompanied by either evolution or absorption of heat. These reactions are known as exothermic and endothermic reactions respectively. Such an equation in which information about heat change is included is called a thermochemical equation. It is very important in such a case to indicate the physical state of the various species involved. Examples are: (i) C(s) + 2 ( g) C 2 ( g); H = kj (Exothermic reaction) (ii) C(graphite) + 2 ( g) CH 4 ( g); H = kj (Endothermic reaction) Q. 16. Write and balance the following equations presented to you as written statements: (i) Magnesium carbonate plus hydrochloric acid produces magnesium chloride plus water plus carbon dioxide gas. (ii) Aluminium plus chlorine gas produces aluminium trichloride. (iii) Nitrogen plus hydrogen produces ammonia. (iv) Calcium plus sulphur produces calcium sulphide. (v) Magnesium burns in C 2 to form magnesium oxide and carbon. Ans. (i) MgC 3 + HCl MgCl 2 + C 2 MgC 3 + 2HCl MgCl 2 + C 2 (ii) Al + Cl 2 ( g) AlCl 3 Al + 3Cl 2 ( g) 2AlCl 3 2Al + 3Cl 2 ( g) 2AlCl 3 (iii) N 2 NH 3

8 8 GLDEN SCIENCE X N 2 2NH 3 N NH 3 (iv) Ca + S CaS (v) Mg + C 2 Mg + C Mg + C 2 2Mg + C 2Mg + C 2 2Mg + C. Q is a balanced chemical equation. It is considered to be incorrect. Why? Ans. xygen does not occur as oxygen atoms () but as oxygen molecules ( 2 ) and so the above reaction should be written with oxygen molecule and then balanced as Q. 18. Complete and balance the following equations: (i) NaH +... (ii) Ca(H +... CaC 3 (iii)... + HCl(aq) MgCl 2 (aq) +... (iv)... + Na 2 (aq) Ba (s) + NaCl(aq) Ans. (inah + 2 (ii) Ca(H + C 2 CaC 3 (iii) Mg + 2HCl(aq) MgCl 2 (aq) ( g) (iv) BaCl 2 (aq) + Na 2 (aq) Ba (s) + 2NaCl(aq) Q. 19. What are the drawbacks of a chemical equation? Illustrate with an example. Ans. Let us consider the following reaction: 2 S + S 2 3S + 2 This is a balanced chemical equation but it does not give the following information: (i) The physical state of the reactants and products whether these are solids, liquids or gaseous etc. (ii) Concentration of reactants and products. (iii) Condition of reaction such as temperature, pressure, catalyst, light. (iv) The rate of reaction whether it is slow or fast. (v) Time taken for completion. (vi) Heat changes whether heat is evolved or absorbed. (vii) Whether reaction is reversible or irreversible. Q. 20. While writing the chemical reaction for the oxidation of calcium in oxygen, a student wrote the following balanced equation: Ca + Ca Is it correct? If not, explain and write the correct balanced equation. Ans. The given equation is not molecular equation. The oxygen should be written in the molecular form. The correct equation is:

9 CHEMICAL REACTINS AND EQUATINS 9 Ca + 2 Ca Ca + 2 2Ca and balanced equation is 2Ca + 2 2Ca Q. 21. How can a chemical reaction be made more informative? Ans. (i) The physical condition of the various species may be indicated by sign (s), (l) and ( g) for solid, liquid and gas respectively after the formula of a substance, e.g., 2P(s) + 3Cl 2 ( g) 2PCl 3 (l) (ii) The precipitate formed in the reaction is shown by an arrow pointing downward ( ) e.g., BaCl 2 Ba + 2HCl (iii) ( ) Sign is used for gas evolved. Zn + 2HCl ZnCl 2 (iv) If a substance dissolved in water is used or produced, sign (aq) is used. NaH(aq) + HCl(aq) NaCl(aq) (v) Heat changes occurring during a chemical reaction may also be indicated such as 2 ( g) + 2 ( g) 2 (l ); H = kj (Endothermic reaction) ( g) + Cl 2 ( g) 2HCl( g); H = kj (Exothermic reaction) Q. 22. Which of the following statement is true for a balanced chemical equation? (i) The number of atoms of each kind on each side should be equal. (ii) The sum of coefficients on both sides is the same. (iii) Atoms and ions are conserved. (iv) Atoms and mass are conserved. Ans. (i) The number of atoms of each kind on each side should be the same. (ii) Atoms and mass are conserved. Q. 23. (i) Rewrite the following equation in a balanced form: Fe + 2 Fe 2 3 (ii) Rewrite the following equation indicating that a solution of lime in water has been used and that a precipitate is formed in the reaction. Ca(H + C 2 CaC 3 Ans. (i) 4Fe Fe 2 3 (ii) Ca(H (aq) + C 2 CaC 3 Q. 24. Calcium carbonate on heating forms calcium oxide and carbon dioxide. Write the equation for the reaction and indicate the evolution of gas. Ans. CaC 3 Ca + C 2 Q. 25. What happens when silver chloride is placed in sunlight in a china dish. Write the balanced chemical equation. Ans. Silver chloride (white) on exposure to sunlight for sometime becomes grey. This is due to decomposition of silver chloride into silver and chlorine by sunlight.

10 10 GLDEN SCIENCE X 2AgCl(s) 2Ag(s) + Cl 2 ( g) White Grey Q. 26. A student strongly heated green ferrous sulphate (Fe.7 ) solid in a dry test tube. He found that after sometime reddish brown residue was left and some pungent smelling gas evolved. Explain the chemical reaction and write balanced chemical equation. Ans. n heating ferrous sulphate, S 2 and S 3 are produced leaving behind reddish brown ferric oxide. 2Fe.7 (s) Fe 2 3 (s) + S 2 ( g) + S 3 ( g) + 14 (l) Green Reddish brown Q. 27. Convey the following information in the form of a balanced chemical equation: n adding aqueous solution of sodium hydroxide to an aqueous solution of copper sulphate, copper hydroxide is precipitated and sodium sulphate remains in solution. Ans. 2NaH(aq) + Cu (aq) Cu(H + Na 2 (aq) Q. 28. Rewrite the following equation in a balanced form and show that in it, Al(H)3 is an insoluble product Al 2 ( ) 3 + NaH Al(H) 3 + Na 2 Ans. Al 2 ( ) 3 + 6NaH 2Al(H) 3 + 3Na 2 Q. 29. How are the following information indicated in a chemical equation? (i) Formation of a precipitate (ii) Evolution of a gas (iii) A solution made in water (iv) Evolution of heat (v) Absorption of heat. Ans. (i) or (s) (ii) or ( g) (iii) (aq) (iv) + heat ( H = ve) (v) heat ( H = +ve). Q. 30. Write a balanced equation with state symbols for the following reactions: (i) Solutions of sulphuric acid and sodium hydroxide in water react to give sodium sulphate solution and water. (ii) When solid mercury oxide is heated, liquid mercury and oxygen gas are produced. (iii) When aluminium metal is dipped in a solution of hydrochloric acid in water, hydrogen gas is evolved with the formation of aluminium chloride solution. (iv) Solutions of silver nitrate and sodium chloride react to give insoluble silver chloride and the solution of sodium nitrate. Ans. (i) (aq) + 2NaH(aq) (aq) + 2 (l) (iihg(s) 2Hg(l) + 2 ( g) (iiial(s) + 6HCl(aq) 2AlCl 3 (aq) + 3 ( g) (iv) AgN 3 (aq) + NaCl(aq) AgCl(s) + NaN 3 (aq) Q. 31. Write balanced chemical equations to represent the following statements: (i) Carbon (coke) burns in air to form carbon dioxide gas. (ii) A piece of sodium metal on putting in water forms caustic soda and hydrogen gas. Ans. (i) C(s) + 2 C 2 (iina(s) + 2 2NaH(aq).

CHEMICAL REACTIONS & EQUATIONS

CHEMICAL REACTIONS & EQUATIONS PHYSICAL AND CHEMICAL CHANGE In our daily life many processes occur around us. Some of them do not lead to formation of any new substance, while others may lead to formation