• when one substance is formed by the chemically combination of two or more substance.
• It is an irreversible reaction i.e once the reactant changes to product they will not come back by physical process.
e.g.• Food gets digested in our body
• Rusting of iron.
• when one substance is formed by the physically combination of two or more substance.
• It is an reversible reaction i.e once the reactant changes to product they can come back by physical process.
• e.g. Melting of ice
• A chemical reaction can be expressed symbolically by using chemical equation
• eg Magnesium is burnt into air to form Magnesium Oxide can be represented as
Mg + O2 ⇒ MgO
• The reactant always come left hand side in the chemical equation as in equation Mg and O2.
• The product always come right hand side in the chemical equation as in equation Mgo.
• When two or more reactant or product are there in any chemical equation then they combine with + sign.
⇒ How could we recognise a change is chemical or physical ?
• We can observe or recognise a chemical reaction by observing change
(i) in state
(iii) by evolution of gas
(iv) by change in temperature.
Physical state of the reactant and products are mentioned to make chemical reaction more informative.
(i) (g) for gas
(ii) (l) for liquid
(iv) (s) for solid
(v) (aq) for aqueous.
• We balance the chemical equation so that no. of atoms of each element involved in the reaction(Left side) remain same at the reactant and product side(Right side).
• eg Fe + H2O ⇒ Fe2O3 + H2
• can be written as
• 2Fe(s) + 3H2O(g) ⇒ Fe2O3(s) +3H2(g)
• So the number of Fe atom is 2 both side, number of H atom is 6 both side and the number of O atom is 3 both side.
• The reaction in which two or more substances combine to form a new single substance(product).
• eg CaO(s) + H2O(l) ⇒ Ca(OH)2 (aq)
• when Calcium and Water react to form Calcium hydroxide
Uses of Ca(OH)2 slaked lime
• For white washing walls.
• It reacts will CO2 to form CaCO3 and gives a shiny finish to the walls.
• Ca(OH)2(aq)+ CO2(g) ⇒ CaCO3(s)+ H2O (l)
• In above reaction when carbon dioxide is passed from calcium hydroxide it gives calcium carbonate and water.
• Burning of Coal
• C(s) + O2(g) ⇒ CO2(g) + heat + light
• Formation of water
• 2H2(g) + O2(g) ⇒ 2H2O(l)
• Reaction in which heat is released along with the formation of products.
• eg. CH4(g) + 2O2(g) ⇒ CO2(g) + 2H2O(g)
• Respiration is also exothermic reaction.
• De composition of vegetable matter into compost.
• The reaction in which a single substance decomposes to give two or more substances.
• De composition reactions can be of three types
(i) Thermal Decompositon
• When a decompositon reaction is carried out by heating.
• Silver bromide behaves similarly
• 2Ag Br ⇒ 2Ag(s) + Br2(g)
• The above two reactions are used in black and white photography.
(ii) Endothermic Reactions
•The reactions which require energy in the form of heat, light or electricty are called Endothermic Reactions.
• 2Ba(OH)2 + NH4Cl ⇒ 2BaCl2 + NH4OH
• The chemical Reaction in which an element displaces another element from its solution.
• When highly reactive element displaces the least react element in an chemical reaction.
• Fe(s) + CuSO4(aq) ⇒ FeSO4 + Cu(s)
• The nail becomes brownish in colour and the blue colour of Copper Sulphate solution fade.
• Other examples Zn(s) + CuSO4 ⇒ ZnSO4 + Cu(s)
• Pb(s) + CuCl2 ⇒ PbCl2 + Cu(s)
• Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
Double Displacement Reaction
• The reaction in which two different atoms or group of atoms are mutually exchanged.
• eg. Na2 SO4 + BaCl2 ⇒ BaSO4(s) + 2NaCl
• A white substance is formed due to above reaction. The insoluble substance is called precipitate.
• Any reaction that produces a precipitate is called a precipitation reaction.
• eg. Pb(NO3)2 + 2KI (⇒) PbI2 +2KNO3
• Oxidation is the gain of oxygen or loss of hydrogen
• eg. 2Cu + O2 ⇒ 2CuO
• When Copper is heated a black colour appears. If this CuO is reacted with hydrogen gas then again Cu becomes brown as reverse reaction takes place.
• CuO + H2 ⇒ Cu + H2O
• Reduction is the loss of oxygen or gain of hydrogen.
• The reaction in which one reactant gets oxidized while other gets reduced.
• eg. ZnO + C ⇒ Zn + CO
• MnO2 + 4HCl ⇒ MnCl2 + 2H2O + Cl2
• When a metal is attacked by substances around it such as moisture, acids etc.
• When the metal surface is eaten by the moisture, water etc.
• eg. Reddish brown coating on iron.
• When fats and oils are oxidized they become rancid and their smell and taste change.
• Antioxidants are added to foods containing fats and oil.
Q1) Why should a Magnesium ribbon be cleaned before burning in air?
Soln: When left unused, the outer layer of Magnesium metal reacts with the atmospheric oxygen and forms Magnesium Oxide (MgO) layer which is a very stable compound thus preventing further reactions with Oxygen. It is therefore necessary to clean the ribbon by brushing it to remove the layer of MgO so that the reaction becomes feasible with the atmospheric oxygen.
Q2) Write a balanced equations for the following chemical reactions.
i) Hydrogen + Chloride —-> Hydrogen chloride
ii) Barium chloride + Aluminium Sulphate —-> Barium sulphate + Aluminum chloride
iii) Sodium + Water —-> Sodium hydroxide + Hydrogen
Q3) Write a balanced chemical equation with state symbols for the following reactions
i) Solutions of Barium chloride and Sodium sulfate in water react to give insoluble Barium sulfate and solution of Sodium chloride.
ii) Sodium hydroxide solution in water reacts with hydrochloric acid solution to produce Sodium chloride solution and water.
Q4) A solution of a substance ‘X’ is used for whitewashing.
i) Name the substance ‘X’ and write its formula.
ii) Write the reaction of the substance ‘X’ named in (i) above with water.
i)The substance ‘X’ which is used in whitewashing is quick lime or Calcium Oxide and its formula is CaO.
Q5) Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
When an iron nail dipped in the copper sulphate solution, the iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore the colour of the copper sulphate solution changes. The reaction is :
Q6) Identify the substances that are oxidized and that are reduced in the following equation.
The Sodium (Na) in the first equation is getting oxidized with the addition of Oxygen (O2) and the Copper (Cu) in the second equation is reduced since the addition of Hydrogen (H2).
Q1) Which of the statements about the reaction below are incorrect?
(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)
(i) (a) and (b)
Explanation: (a) because Oxygen is being removed and (b) because the removed oxygen from Lead is added to the elemental Carbon.
The above reaction is an example of a
- Combination reaction.
- Double displacement reaction.
- Decomposition reaction.
- Displacement reaction.
4. Displacement reaction.
Explanation: The Oxygen from the Ferrous oxide is getting displaced to the Aluminium metal to form Aluminium Oxide.
Q3) What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
- Hydrogen gas and Iron chloride are produced.
- Chlorine gas and Iron hydroxide are produced.
- No reaction takes place.
- Iron salt and water are produced.
- Hydrogen gas and Iron chloride are produced.
Explanation: The Chlorine from the Hydrogen chloride is displaced to the Iron fillings undergoing the following reaction.
Q4) What is a balanced chemical equation? Why should a chemical equation be balanced?
A balanced chemical equation is the one in which the number of different atoms on both the sides of the chemical equation that is on the reactant side and the product side of a reaction are equal. The chemical equation needs to be balanced so that it obeys the Law Of Conservation of Mass. Balancing of chemical equation has no defined method and is purely a trial and error attempt.
Q5) 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.
(a) Unbalanced: H2+N2→NH3
(b) Unbalanced: H2S+O2→H2O+SO2
(c) Unbalanced: BaCl2+Al2(SO4)3→AlCl3+BaSO4
Q6) Balance the following chemical equations.
Q7) 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
Q8) Write a balanced chemical equation for the following and identify the type of reaction of each case
- 2KBr+BaI2→2KI+BaBr2 (Double Displacement Reaction)
- ZnCO3→ZnO+CO2 (Decomposition Reaction)
- H2+Cl→2HCl (Combination Reaction)
- Mg+2HCl→MgCl2+H2 (Displacement Reaction)
Q9) What is meant by exothermic and endothermic reactions? Give examples.
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).
Q10) Why is respiration considered to be an exothermic reaction?
For the survival of life we require energy.We obtain this energy from the food we eat. The food molecules, through the process of digestion is broken down into simpler molecule like glucose. These substances come in contact with the Oxygen present in our body cells to form Carbon dioxide 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:
Q11) Why are decomposition reactions called the opposite of Combination reactions? Write equations for decomposition reactions.
Combination reaction is said to be the reaction between two or more molecules to form a larger molecule whereas the decomposition reaction is defined as the splitting of a larger molecules into two or more smaller molecules. That 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. Few examples of decomposition reactions are:
Q12) Write one equation each for decomposition reactions in which energy is supplied in the form of heat, light or electricity.
(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.
(b) Electrolytic decomposition reaction (Electrolysis)
Decomposition of sodium chloride: On passing electricity through molten sodium chloride, it decomposes into sodium and chlorine.
(c) Photodecomposition reaction (Photolysis)
Decomposition of hydrogen peroxide: In the presence of light, hydrogen peroxide decomposes into water and oxygen
Q13) What is the difference between displacement and double displacement reactions? Write relevant equations for the above.
- 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.
- 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.
- Displacement reaction
- Double displacement reaction
Q14) In the refining of Silver, the recovery of silver from Silver nitrate solution involves displacement reaction by Copper metal. Write down the reaction involved.
Q15) What do you mean by precipitation reaction? Explain by giving examples.
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 compound 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. Few examples of precipitation reactions are:
Q16) Explain the following in terms of gain of oxygen with two examples each.
(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:
(b) In a chemical reaction, when the oxygen is being removed from the compound then it is said to be reduced. Example:
Q17) A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
The shiny brown coloured element is the Copper metal(Cu). When the metal is heated in air, it reacts with the atmospheric oxygen to form copper oxide. Hence the black coloured compound is the copper oxide.
Q18) Why do we apply paint on iron articles?
Iron articles are painted to prevent them from rusting. When 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 painted the surface does not come in contact with moisture and air thus preventing Rusting.
Q19) Oil and Fat containing food items are flushed with Nitrogen. Why?
The main purpose of flushing Nitrogen 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.
Q20) Explain the following terms with one example each.
(a) Corrosion is a process where a refined metal is oxidised by the atmospheric oxygen to form a more stable compound such as oxides. The metal gradually degrades during the corrosion process. Rusting of iron is an good example of corrosion where the iron is converted to Iron oxide. Millions of dollars are spent annually to 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 oxidation reaction.
1. What happens chemically when quick lime is added to water ?
Ans. Quick lime reacts vigorously with water to produce calcium hydroxide releasing a large amount of heat.
CaO(s) + H2O – Ca(OH)2
2. On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue-green . Predict the new compound formed which imparts a blue-green colour to the solution.
Ans. The new compound formed is cuprous chloride. It imparts a blue-green colour to the solution.
CuO(s) + 2HCL(aq) – CuCl2(aq) + H2O
3. Why is repiration considered an exothermic process ?
Ans. We need energy to stay alive. We get this energy from the food we eat. During disgestion, the food is broken down to simple substances, For example rice, patatoes etc,. contain starch which is broken down to glucose. During respiration, glucose is oxidised to CO2 and H2O with the help of oxygen inhaled during respiration. Energy is evolved in this process.
C6H12O6(aq) + 6O2(g) – 6CO2(g) +H2O(l) + Heat
4. Patato chips manufacturers fill the packet of chips with nitrogen gas. Why ?
Ans. Flushing bags of chips with nitrogen cuts off oxygen and protects the food from rancidity.
5. If copper metal is heated over a flame it develops a coating. What is the colour and composition of coating ?
Ans. We get a black coating of CuO, copper oxide.
6. Write a balanced chemical equation to represnt the following reaction :
Carbon monoxide reacts with hydrogen gas at 340 atm to form methyl alcohol
Ans. CO(g) + 2H2 – (340 atm) CH3OH(l)
7. Which one is a chemical change – fermentation of fruit juice or diluting fruit juice ?
Ans. Fermentation of fruit juice is a chemical change.
8. Is burning of a candle wax a physical change or a chemical change ?
Ans. It is a chemical change. Thy hydrocarbon of wax burns to produce CO2 and H2O.
9. State one basic difference between a physical and chemical change.
Ans. No new substance is formed in a physical change while a new substace is formed in a chemical change.
10. Name ans state the law which is kept in mind when we balance a chemical equation.
Ans. While balancing a chemical equation, we follow the law of conservation of mass which states : matter can neither be created nor destroyed in a chemical reaction.
11. What is an oxidation reaction ? Identify in the following reaction :
(i) The substance oxidised and the substance reduced
ZnO + C – Zn + CO
Ans. Oxidation reaction is one which involces gain of oxygen or loss of hydrogen.
(i) C is oxidised to CO and ZnO is reduced to Zn.
12. When potassium iodide solution is added to a solution os lead(ii) nitrate in a test tube, a precipitate is formed.
(i) What is the colour of this precipitate ?
(ii) Name the compound precipitated.
(iii) Write the balanced chemical equation for this reaction.
(iv) What type of reaction is this?
Ans. (i) The colour of the precipitation is yellow.
(ii) The compound precipitated is lead iodide(PbI2)
(iii) The balanced chemical equation for this reaction is :
Pb(NO3)2)(aq) + 2KI(aq) – PbI2(s) + 2KNO3()aq
(iv) It is a double displacement reaction.
13. What is a redox reaction ? Write down a chemical equation representing it.
Ans. A reaction in which one reactant gets oxidised while the other gets reduced is called redox reaction.
ZnO + C – Zn + CO
Zinc oxide is reduced to zinc while carbon is oxidised to carbon monoxide.
14. When a green compund is heated strongly, its colour changes to black and odour of burning sulphur is given out.
(a) Name the copound
(b) State the type of reaction.
(c) Write the chemical equation involved.
Ans. (a) The green compound is FeSO4.
(b) It is thermal decompostion reaction.
(c) 2FeSO4 (s) -(Heat) Fe2O3 (s) + SO2(g) + SO3 (g)
15. A white compound on heating decomposes to give brown fumes and a yellow residue is left behind. Name the compound. Write the chemical equation of the reaction stating its type.
Ans. The compound is lead nitrate, Pb(NO3)2
The reaction involved is
2Pb(NO3)2 – (Heat) 2 Pb(NO3)2
16. A light sensitive compound ‘X’ of silver is used in black and white photography. On exposure to sunlight its colour changes to grey.
(a) Identify ‘X’.
(b) Write a chemical equation to express the above change.
(c) Identify the type of chemical reaction.
Ans. (a) The compound X is silver bromide. AgBr.
(b) The chemical equation for the change is
2AgBr – (sunlight) 2Ag + Br2
(c) It is a photochemical decomposition reaction.
17. Solutions of lead nitrate and potassium iodide are mixed in a test tube.
(a) Write the chemical equation involved in a balanced form.
(b) What is the colour of the precipitate ? Name the precipitate.
Ans. (a) The chemical equation involved is
Pb(NO3)2 + KI – PbI2 + KNO3
(b) Colour of the precipitate is yellow. It is lead iodide, PbI2
18. Why do we store silver chloride in dark coloured bottles ? Explain in brief.
A small quantity of silver chloride is kept in the sunlight in a china dish for about half an hour.
(i) State the change you would observe in the colour of silver chloride. Suggest a reason for this change.
(ii) Write balanced chemical equation for the reaction taking place in this case.
Ans. Silver chloride on exposure to sunlight decomposes according to the following equation :
2AgCl – (sunlight) Ag + Cl2
That is why silver chloride is kept in dark coloured bottles.
19. Why do silver articles turn black and copper items turn green after sometime ?
Ans. Silver articles turn black and copper items turn green after sometime.
This is because these metals are attacked by moisture, acids etc. present in the atmosphere.
The black colour on silver is due to the formation of Ag2S and green colour on copper items is due to the formation of CuCO3.
20. Why do fire flies glow at night ?
Ans. Fire flies have a protein which in the presence of an enzyme undergoes aerial oxidation. This is a chemical reaction which involves emission of visible light. Therefore, fire flies flow at night.
21. Name the substances oxidised and redused, and also identify the oxidising agents and reducing agents in the following reactions :
(i) Fe2O3 + 3CO – 2Fe + 3CO2
(II) 3MnO2 + 4Al – 3Mn + 2Al2O3
(iii) H2S + SO2 – S + H2O
Ans. Fe2O3 + 3CO – 2Fe + 3CO2
Fe2O3 is reduced because oxygen has been reduced and CO is oxidised because O has been added to it. Fe2O3 has acted as oxidising agent and CO has acted as reducing agent.
(ii) 3MnO2 + 4Al – 3Mn + 2Al2O3
MnO2 is reduced because O has been removed from it to give Mn. Al has been oxidised because O has been added to it to obtain Al203. MnO2 is the oxidising agent while Al is reducing agent.
(iii) 2H2S + SO2 – 3S + 2H2O
H2S has lost H to change into S. Thus, it has been oxidised. SO2 has lost O to change into S. Thus, it has been reduced. H2S acted as reducing agent while SO2 has acted as oxidising agent.
21. (a) A solution of a substance ‘X’ is used for testing carbon dioxide. What will be the reaction of ‘X’ with carbon dioxide ? Write balanced equation for this reaction .
(b) How is ‘X’ obtained ? Give chemical equation.
Ans. ‘X’ is calcium hydroxide or slaked lime.
‘X’ reacts with carbon dioxide to form calcium carbonate which appears as a white precipitate. The balanced equation for this reaction is :
Ca(OH)2 + CO2 – CaCO3 + H2O
(b) ‘X’ i.e., calcium hydroxide is obtained by adding water to quick lime(CaO) as per the following equation :
CaO + H2O – Ca(OH)2
22. (a) Can we stir silver nitrate solution with a copper spoon ? Why of why not ? Support your answer with reason.
(b) Why a brown coating is formed on the iron rod when iron rod is kept dipped in copper sulphate solution for sometime ? What change will be observed in the colour of the solution ?
(c) A green coating develops on the copper vessel in the rainy season. Why ?
Ans. (a) We cannot stir silver nitrate with a copper spoon. Copper being more reactive than than silver, displaces silver from silver nitrate, that is, a reaction will take place.
(b) Iron being more reactive than copper, displaces copper from copper sulphate by the following reaction :
Fe + CuSO4 – FeSO4 +Cu
Brown coating of copper is deposited on the iron rod. Colour of the solution becomes green due to the formation of ferrous sulphate.
(c) Copper reacts with moist carbon dioxide in the air in rainy season and a green coating is formed. this is due to the formation of copper carbonate.
23. (a) Illustrate an activity along with a labelled diagram, to show that a change in the state of matter and change in temperature takes place during a chemical reaction.
(b) Write balanced chemical equations for the following equations :
(i) Natural gas burns and combines with oxygen to produce carbon dioxide and water.
(ii) Ferrous sulphate crystals on heating break up into ferric oxide, sulphur dioxide and sulphur trioxide.
Ans. (a) Take about 5g of quick lime in a beaker and add to it about 50ml of water. A brisk reaction takes place and a lot of heat is evolved.
CaO + H2O – Ca(OH)2 + Heat
Quick lime is a white solid but after the reaction, calcium hydroxide is formed which is soluble in water and a clear solution is obtained. Thus, there has been a change in the state of matter and change in temperature.
(b) (i) CH4 + 2O2 +2H2O
(ii) 2FeSO4.7H2O -(Heat) Fe2O3 + SO2 + SO3 + 14H2O