CHAPTER 1

Chemical Reactions & Equations

1.1    Chemical Reaction: Chemical reaction is a process in which one or more substances, the reactants, are converted to one or more different substances, the products.

Example:- H₂ + O₂   = 2H2O

    Substances are either ‘chemical elements‘ or ‘compounds‘. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products. The chemical and physical properties of the product so formed is totally different from that of reactants.

1.2    Reactant: The elements or compounds take part in a chemical reaction are called Reactants.

             In a chemical reaction, reactants are written in left hand side.

Example: – H + O2   = H2O.

1.3    Product: During a chemical reaction, reactants are transformed into products.     In other words it can be said that the result of a chemical reaction is product. In a chemical reaction, product is written in right hand side.

1.4    Element: An element is a substance which does not produce any other substance on any further subdivision.

Example:- Hydrogen (O) and Oxygen (O) etc.

1.5    Compound:- A Compound is a substance which is formed by the chemical combination/reaction of two or more elements and which produces two or more elements by their further subdivision generally by chemical method.

Example:- H2O (Water) is a compound.

The physical and chemical properties of a compound is totally different from that of reactants whose result it is.

1.6    Atom:- An atom is the smallest particle of an element which does not exist in free state but which takes part in a chemical reaction.

Or

An atom is the ultimate particle of an element which possesses all physical & chemical properties of that element and can take part in every physical & chemical reaction.

2.1    Chemical Equation:- A chemical reaction is the symbolic representation of a chemical reaction wherein the reactants are placed on the left hand side (LHS) and the products are on right hand side (RHS) with ‘+’ sign between them.

    Example:- 2H₂ + O₂ = 2 H₂O

    A chemical equation represents a chemical reaction. The very first chemical equation was diagrammed by Jean Beguin in 1615.

2.2    Writing a chemical equation: It is a process of writing chemical formulae instead of words with ‘+’ sign between reactants in symbolic form and result (product) on right hand side.

    Example:- Mg + O₂ = MgO

2.3    Balanced chemical equations:
As per the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of elements (reactants) present in the product(s) of a chemical reaction has to be equal to the total mass of the elements present in reactants.

The number of atoms of each element remain the same before and after the chemical reaction. This is the reason, balancing of a chemical reaction is necessary.

2.4    Process of Balancing a Chemical Equation:- It is convenient to begin with the compound having maximum number of atoms. It may be reactant or product. By observing the number of atoms present in either side required to be identical by way of multiply with suitable numbers to make either side balance.

On observation of above chemical reaction reflected in table, it is clear that No. of Iron (Fe) & Oxygen (O) are differ in either side. Therefore, the equation may be balanced by respective multiplying factors to enable either sides equal.

2.5    Writing Symbols of Physical States: A chemical reaction should be so informative that it can even show the physical state of reactants/products. It has gaseous, liquid, aqueous and solid states.

            For this purpose, it is written as:

2.6    Process to know Chemical Reaction has taken place: Following change takes place to know a chemical reaction is undertaken.

(a) Gas Bubbles Appear. Gas bubbles appear after a chemical reaction has occurred and the mixture becomes saturated with gas. …

(b) Formation of a Precipitate. …

(c) Color Change. …

(d) Temperature Change. …

(e) Production of Light. …

(f) Volume Change. …

(g) Change in Smell or Taste.

(h) Change in state.

Questions (Page – 6)

Q-1: Why should a magnesium ribbon be cleaned before burning in air?

Ans: Magnesium (Mg) rubber should be cleaned before burning in air because Magnesium (Mg) metal reacts with the atmospheric oxygen (O) and forms Magnesium Oxide (MgO) layer which is a very stable compound. In order to prevent further reactions with Oxygen, it is therefore necessary to clean the ribbon by to remove the layer of MgO.

Q-2: Write a balanced equations for the following chemical reactions.

(i) Hydrogen + Chloride → Hydrogen chloride

(ii) Barium chloride + Aluminium Sulphate → Barium sulphate + Aluminium chloride

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Ans: Balanced equations are given below:

(i) H₂+Cl₂→2HCl

(ii) 3BaCl2+Al2(SO4)3→2AlCl3+3BaSO4

(iii) 2Na+2H2O→2NaOH+H2

Q-3: Write a balanced chemical equation with state symbols for the following reactions

(i) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and solution of Sodium chloride.

(ii) Sodium hydroxide solution in water reacts with hydrochloric acid solution to produce Sodium chloride solution and water.

Ans: Balanced chemical equations with state symbols are written as follows:

(i) BaCl₂+Na₂SO₄→BaSO₄+2NaCl

(ii) NaOH+HCl→NaCl+H₂O

3.1    Types of Chemical Reactions: There are different types of chemical reaction and the same are discussed below:

(a) Combination:- A combination reaction is a reaction where two or more elements or compounds combine to form a single compound.

            Such reactions may be represented by equations of the following form:

X + Y → XY.

The combination of two or more elements and form one compound is called combination reaction.

Example:- 2H + O2 = H2O

Here, Hydrogen & Oxygen combine to form Water.

(b) Decomposition:- It is s a type of chemical reaction in which a single compound breaks down into two or more elements or new compounds. These reactions often involve an energy source such as heat, light, or electricity that breaks apart the bonds of compounds.

Example- 2FeSO₄ Heat  =  Fe2O3(S) + SO2(g) + SO3(g)

CaCO3 Heat =  CaO(s) + CO2 (g)

(c) Single displacement reaction:-
A Single-Displacement reaction also known as a Single-replacement reaction is a type of chemical reaction where an element reacts with a compound and takes the place of another element in that compound.

Ex- A + B – C = A – C + B

Fe (s) + CuSO4 (aq) = FeSO4 (aq)  +  Cu (s)

(d) Double displacement:- Double displacement reaction also known as double replacement reaction or Metathesis is a type of chemical reaction one component each of both the reacting molecules are exchanged to form the product.

Also, two compounds react and the positive ions (Cation) and the negative ions (anion) of the two reactants which places forming new compounds or products.

            Example:- Na2SO4(aq) + BaCl2(aq) = BaSO4 (s) + 2NaCl (aq).

(e)Combustion:- A Combustion reaction (Commonly known as burning) is an exothermic reaction in which something reacts with oxygen.

             Example: – Buring of wood etc.

(f) Exothermic Reaction:- It is a chemical reaction that releases energy through light or heat.

            Example:- Combustion of wood etc.

(g) Endothermic Reaction:- It is a kind of chemical reaction in which absorption of energy (heat) takes place. The absorbed energy provides the activation energy for the reaction to occur.

            Example:- Evaporation of perspiration.

(h) Oxydation & Reduction (Redox):- Oxidation & Reduction reactions are called Redox reactions.

(i) Oxidation Reaction:- Oxidation is a chemical process which involves, (a) combination with Oxygen or other electro-negative element, (b) removal of Hydrogen or any other electro-positive element from a compound, (c) increase in valency of metal atom in a compound, (d) increase in the proportion of an electro-negative element or group and decrease in the proportion of electro-negative element in a compound, (e) increase in oxidation number of an element, (f) loss of electron.

Example:- (a) Addition of oxygen an electro-negative element to an element or compound is oxidation:

1. S + O2 = SO2
   
2. Cu + O2 = 2CuO
   

             Example: (b) Removal of Hydrogen or an electro-positive element from a compound is called                                          Oxidation:

1. 4HCl + MnO2 –Heat = 2H2O + MnCl2 + Cl2
   

                                    (b) NaCl + H2SO4 + MnO2 –Heat = NaHSO4 + MnSO4 + H2O + Cl2

Here, H has been removed from HCl to obtain Cl2 and electro-positive element Na has been removed from NaCl to obtain Cl2. Hence, HCl & NaCl have undergone oxidation and MnO2 is the oxidising agent.

Example: (c) Increase in the valency of a metal atom in a compound is oxidation:

1. FeCl2 + Cl2 = FeCl₃
   
2. Hg2Cl2 + Cl2 = HgCl2
   

In the above reactions the valency of Fe in FeCl2 is 2 and that of Hg in Hg2Cl2 is 1, whereas the valencies of Fe and HG in Fecl3 & HgCl2 are 3 and 2 respectively. Hence, FeCl2 and Hg2Cl2 have undergone oxidation and Cl2 is the Oxidising agent.

Example: (d) Increase in the proportion of an electro-negative element or group and decrease in the proportion of an electro-positive element in a compound is called Oxidation.

1. FeCl2 + Cl2 à FeCl3 
   

            Ratio of atoms of Fe and Cl in FeCl₂ = 1:2

Ratio of atoms of Fe and Cl in FeCl₃ = 1:3

            (ii) Reduction reaction:  Reduction is a chemical process which involves – (a) combination of an electron with Hydrogen (H) or with any other electro-positive element, (b) removal of Hydrogen (H) or any other electro-negative element from a compound, (c) decrease in valency of a metal atom in a compound, (d) increase in the proportion of an electro-positive element and decrease in the proportion of an electro-negative element or group in a compound, (e) decrease in oxidation number of an element (f) gain of electron by a substance.

Example: (a) Combination of an element with Hydrogen (H) or with any other electro-positive element :

1. Cl₂ + H₂ = 2HCl
   
2. S + 2Na = Na₂S
   

In above reactions, Cl₂ and S combine with electro-positive elements H & Na respectively. Hence, Cl₂ and S have undergone reduction.

1. 1. CuO + C = Cu + CO
      
   2. 2AgBr + H₂ = 2Ag + 2HBr.
      

In the above reaction O-atom and Br-atom have been removed from CuO and AgBr respectively. Hence, CuO and AgBr have undergone reduction to Cu and Ag and C and H₂ here acts as reducing agents respectively.

1. 2CuCl₂ + SO₂ + H₂O = Cu₂Cl₂ + H₂SO₄ + 2HCl
   
2. 2HgCl₂ (aq) + SnCl₂ = Hg₂Cl₂ + SnCl₄
   

In the above reactions, valency of Cu and Hg in CuCl2 and HgCl2 are 2 and 2 respectively and as a result fo reduction their valency decreased to 1 and 1 respectively in Cu₂Cl₂ and Hg₂Cl₂ respectively

1. 2FeCl₃ + H₂S = 2FeCl₂ + HCl + S
   

   Here, the ratio of – Fe : Cl in FeCl₃ = 1:3
   

   The ratio of – Fe : Cl in FeCl₂ = 1:2
   

In the above reactions, proportion of electro-negative atom Cl has decreased from 3 to 2 and the proportion of electro-positive element Fe is increased. Hence, FeCl₃ has undergone reduction to form FeCl₂ and H₂S is the reducing agent.

Example: (e) Decrease in oxidation number of an element:

1. gain of electron by a substance: 2Hg + 2e^– = Hg₂²⁺. In this reaction Hg gains 2 electrons to form Hg₂²⁺.
   

Question (Page : 10)

Q-1: A solution of a substance ‘X’ is used for whitewashing.

1. Name the substance ‘X’ and write its formula.
   
2. Write the reaction of the substance ‘X’ named in (i) above with water.
   

Ans: (i) The substance ‘X’ which is used in whitewashing is quick lime or Calcium Oxide
and its formula is CaO.

(ii) The reaction is: CaO+ H₂O → Ca(OH)₂

Q-2: Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas

Ans: In activity 1.7, gas collected in one of the test tubes is double of the amount collected in he other because water gets hydrolysed to release H₂ and O₂ gas. After electrolysis two molecules of Hydrogen and one molecule of Oxygen gas is released, hence the amount of Hydrogen collected would be double than that of Oxygen.

4.1    Corrosion: It is a natural process that converts a refined metal into a more chemically stable form.

    Example: Oxides or Hydro-oxides or Sulfides of metals.

    Corrosion is the gradual destruction of metals by chemical and/or electro chemical reactions when exposed in air.

5.1    Rancidity:- It is generally the complete or incomplete Oxidation or Hydrolysis of fats and oils when exposed to air, light, or moisture or by bacterial action resulting in unpleasant taste and ordor.

    Question (Page : 13)

Q-1: Why does the colour of Copper sulphate solution change when an Iron nail is dipped in it?

Ans: When an Iron (Fe) nail dipped in the Copper(Cu) sulphate solution, Iron(Fe) displaces Copper (Cu) from the copper sulphate because Iron(Fe) is more reactive than Copper(Cu). Therefore, the colour of the Copper sulphate solution changes. The reaction takes place is :

Fe+CuSO₄ → FeSO₄ +Cu

Q-2: Give an example of a double displacement reaction other than the one given in Activity 1.10.

Ans: The reaction between Silver nitrate ( AgNO₃) and Sodium chloride (NaCl) is an example of double displacement reaction. Here,  Silver(Ag) donates its ion to Sodium chloride ion and pick up nitare ion to form Silver chloride and Sodium Nitrate. The following reaction takes place:

AgNO₃ + NaCl → AgCl + NaNO₃

Q-3: Identify the substances that are oxidized and that are reduced in the following equation.

(i) 4Na(s)+O2(g) → 2Na2O(s)

(ii) CuO(s)+H2(g) → Cu(s)+H2O(l)

Ans: Sodium (Na) in the first equation is getting oxidized with the addition of Oxygen (O₂) and Copper (Cu) in the second equation is reduced due to the addition of Hydrogen (H₂)

Exercise (Page – 14 to 16)

Q-1: Which of the statements about the reaction below are incorrect?

2PbO(s)+C(s)→2Pb(s)+CO₂(g)

(a) Lead is getting reduced

(b) Carbon Dioxide is getting oxidised

(c) Carbon is getting oxidised

(d) Lead oxide is getting reduced

       (i) (a) and (b)

(ii) (a) and (c)

      (iii) (a), (b) and (c)

       (iv) all

Ans: (i) (a) and (b)

Explanation: (a) because Oxygen(O) is being removed and (b) because the removed Oxygen(O) from Lead (Pb) is added to the elemental Carbon (C).

Q-2:  Fe₂O₃+2Al→Al₂O3+2Fe

The above reaction is an example of a

1. Combination reaction.
   
2. Double displacement reaction.
   
3. Decomposition reaction.
   
4. Displacement reaction.
   

Ans: The above reaction is an example of Displacement reaction. Hence, the correct answer is 4.

Explanation: The Oxygen (O) from the Ferrous oxide is getting displaced to the Aluminium (Al) metal to form Aluminium Oxide. In this reaction, Aluminum (Al) is more reactive metal than Fe. Therefore, Al will displace Fe from its oxide. This type of chemical reactions in which one of the elements displace another is called displacement reaction. Here, less reactive metal is displaced by more reactive metal. Since one-time displacement is occurring, therefore, it is called a single displacement reaction.

Q-3: What happens when dilute Hydrochloric acid is added to Iron fillings? Tick the correct answer.

1. Hydrogen gas and Iron chloride are produced.
   
2. Chlorine gas and Iron hydroxide are produced.
   
3. No reaction takes place.
   
4. Iron salt and water are produced.
   

Ans: (1)
Hydrogen (H) gas and Iron chloride are produced.

Explanation: The Chlorine from Hydrogen chloride is displaced by the Iron (Fe) fillings to undergo the following reaction.

2HCl+Fe→FeCl₂+H₂

Q-4: What is a balanced chemical equation? Why should a chemical equation be balanced?

Ans: A balanced chemical equation is the chemical reaction in which number of different atoms on both sides the reactant and product sides are equal.

Balancing of chemical equation is necessary to obey The Law of conservation of energy. Balancing of a chemical equation has no defined method and is purely a trial and error attempt.

Q-5: Translate the following statements into chemical equations and balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(c) Barium chloride reacts with aluminium sulphate to give Aluminium chloride and a precipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide and Hydrogen gas.

Ans: The chemical equations are:

(a) Unbalanced: H₂+N₂→NH₃

Balanced: 3H₂+N₂→2NH₃

(b) Unbalanced: H₂S+O²→H₂O+SO₂

Balanced: 2H2S+3O₂→2H₂O+2SO₂

(c)  Unbalanced: BaCl₂+Al₂(SO4)₃→AlCl₃+BaSO₄

Balanced: 3BaCl₂+Al₂(SO4)₃→2AlCl₃+3BaSO₄

(d) Unbalanced:K+H₂O→KOH+H₂

Balanced:  2K+2H₂O→2KOH+H₂

Q-6: Balance the following chemical equations.

(a) HNO₃ +Ca(OH)₂ → Ca(NO₃)₂ + H₂O

(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O

(c) NaCl + AgNO₃ → AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Ans:

(a)2HNO₃+2Ca(OH)2→2Ca(NO3)₂+2H2O

(b)6NaOH+3H₂SO4→H₂SO4+6H₂O

(c)NaCl+AgNO₃→AgCl+NaNO₃

(d)BaCl₂+H2SO₄→BaSO₄+2HCl

Q-7: Write the balanced chemical equation for the following reactions.

Calcium hydroxide + Carbon dioxide —-> Calcium carbonate + Water

Zinc + Silver nitrate —-> Zinc nitrate + Silver

Aluminium + Copper chloride —-> Aluminium chloride + Copper

Barium chloride + Potassium sulphate —-> Barium sulphate + Potassium chloride

Ans: The balanced chemical equations are as follows:
2Ca(OH)₂+2CO₂→2CaCO₃+2H₂O

Zn+2AgNO₃→Zn(NO₃)₂+2Ag

2Al+3CuCl₃→2AlCl₃+3Cu

BaCl₂+K₂SO₄→BaSO₄+2KCl

Q-8: Write a balanced chemical equation for the following and identify the type of reaction of each case

KBr+BaI₂→KI+BaBr₂

ZnCO₃→ZnO+CO₂

H₂+Cl→HCl

Mg+HCl→MgCl₂+H₂

Ans: The equations are balanced and their types are written as follows:

2KBr+BaI₂→2KI+BaBr₂ (Double Displacement Reaction)

ZnCO₃→ZnO+CO₂ (Decomposition Reaction)

H₂+Cl→2HCl  (Combination Reaction)

Mg+2HCl→MgCl₂+H₂ (Displacement Reaction)

Q-9: What does one mean by exothermic and endothermic reactions? Give examples.

Ans: An endothermic reaction occurs when energy is absorbed from the surroundings in the form of heat. Example: Photosynthesis, melting of ice, evaporation. Conversely, an exothermic reaction is one in which energy is released from the system into the surroundings. Example: Explosions, concrete setting, nuclear fission and fusion.

Q-10: Why is respiration considered an exothermic reaction? Explain.

Ans: For the survival of life (plant and animal), energy is required. Animals obtain this energy from the food they eat. The food molecules, through the process of digestion, is broken down into a simpler molecule like glucose. These substances come in contact with the Oxygen (O) present in our body cells to form Carbon dioxide (CO2) and water along with a certain amount of energy (Respiration process). Since, the energy is in the form of heat (that maintains our body temperature) the respiration is considered to be an exothermic reaction. The reaction taking place is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Q-11: Why are decomposition reactions called the opposite of Combination reactions? Write equations for these reactions.

Ans: Combination reaction is a reaction between two or more molecules to form a larger molecule; whereas the decomposition reaction is defined as the splitting of larger molecules into two or more smaller molecules. This essentially explains that the decomposition reaction is the opposite of the combination reaction.

In most of the cases the decomposition reaction is endothermic since heat from the surrounding or induced heat is used to break the bonds of the larger molecule. Examples of decomposition reactions are:

ZnCO₃ → ZnO + CO₂

CaCO₃ + Energy → CaO + CO₂

2HgO → 2Hg + O₂

Q-12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Ans: The reactions are as follows:

(a) Thermal decomposition reaction (Thermolysis)

Decomposition of Potassium chlorate: When heated strongly, Potassium chlorate decomposes into Potassium chloride and Oxygen. This reaction is used for the preparation of oxygen.

2KClO₃ + Heat → 2KCl + 3O₂

(b) Electrolytic decomposition reaction (Electrolysis)

Decomposition of Sodium chloride: On passing electricity through molten Sodium chloride, it decomposes into Sodium and chlorine.

2NaCl → electricity 2Na + Cl₂

(c) Photodecomposition reaction (Photolysis)

Decomposition of hydrogen peroxide: In the presence of light, Hydrogen peroxide decomposes into water and Oxygen

2H₂O₂ (light) → 2H₂O + O₂

Q-13:  What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Ans: (a) A displacement reaction is the one when a more reactive substance displaces a less reactive one from its salt solution whereas a double displacement reaction is the one where a mutual exchange of ions happens between two compounds.

(b) In a displacement reaction, only a single displacement takes place whereas in the double displacement reaction, as the name suggests two displacement takes place between the molecules.

Example: Displacement reaction

Mg + 2HCl → MgCl₂ + H₂

Double displacement reaction

2KBr + BaI₂ → 2KI + BaBr₂

Q-14: In the refining of Silver, the recovery of Silver from Silver nitrate solution involved displacement reaction by Copper metal. Write down the reaction involved.

Ans: The reaction so involved is written as:

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

Q-15: What do you mean by a precipitation reaction? Explain by giving examples.

Ans: When two solutions containing soluble salts are combined, a double displacement reaction takes place in which the ions are exchanged between the compounds. When one of such compounds formed is in solid form (that is insoluble in aqua) then it settles down at the bottom of the container. This solid is known as the precipitate and the respective reaction is termed as the precipitation reaction. Examples of precipitation reactions are:

CdSO₄(aq) + K₂S(aq) →CdS(s) + K₂SO₄(aq)

2NaOH(aq) + MgCl₂(aq) →2NaCl(aq) + Mg(OH)₂(s)

Q-16: Explain the following in terms of gain of oxygen with two examples each.

(a) Oxidation

(b) Reduction

Ans: (a) In a chemical reaction, when the oxygen is added to the element to form its respective oxide it is the element being oxidised. Example:

4Na(s) + O₂(g) → 2Na₂O(s)H₂S + O₂→ H₂O+SO₂

(b) In a chemical reaction, when the oxygen is being removed from the compound then it is said to be reduced. Example:

CuO(s) + H₂(g) → Cu(s) + H₂O(l) 2HgO → 2Hg+O₂

Q-17: A shiny brown coloured element ‘X’ on heating in the air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Ans: The shiny brown coloured element is the Copper metal(Cu). When the metal is heated in air, it reacts with atmospheric oxygen to form copper oxide. Hence the black coloured compound is the Copper oxide.

2Cu(s) + O₂(g) → 2CuO(s)

Q-18: Why does apply paint on iron articles?

Ans: Iron articles are painted to prevent them from rusting (corrosion). When Iron is left unpainted, the metal surface comes in contact with the atmospheric Oxygen and in the presence of moisture it from Iron(III) oxide. But if the surface of Iron is painted, the surface does not come in contact with moisture and air thus no chemical reaction takes place and hence Rusting (corrosion) is prevented.

Q-19: Oil and Fat containing food items are flushed with Nitrogen. Why?

Ans: The main purpose of flushing Nitrogen(N) into food packets that contain oil and fat items is to prevent Rancidity which occurs when the oil or fat reacts with the oxygen letting out an unpleasant smell and taste. Therefore, by flushing Nitrogen, an unreactive surrounding is created thus preventing rancidity.

Q-21: Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Ans: (a) Corrosion is a process where a refined metal is oxidised by atmospheric oxygen to form a more stable compound such as oxides. The metal gradually degrades during the corrosion process. Rusting of iron is a good example of corrosion where the iron is converted to Iron oxide. Millions of dollars are spent annually in preventing rusting from bridges and other monuments.

(b) The condition produced by the aerial oxidation of the oil and fat present in the food material that produces an unpleasant taste and smell. The rancidity is retarded when the food is kept inside the refrigerator since the low temperature does not promote the oxide.

********