chemical
Reaction and Equations q&a
Q.1: Which of the statements about the
reaction below are incorrect?
2PbO + C ---> 2Pb + CO2
(a) Lead is getting reduced.
(b) Carbon dioxide is getting
oxidized.
(c) Carbon is getting oxidized.
(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.
Q.2: Fe2O3 + 2Al ---> Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction (b) double displacement reaction
(c) decomposition reaction (d)
displacement reaction.Ans: (d)
Q.3: What happens when dilute HCl is
added to iron filings ? Tick the correct answer.
(a) Hydrogen gas and iron chloride are
produced.
(b) Chlorine gas and iron hydroxide
are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Ans: (a)
Q.4: What is a balanced chemical
equation ? Why should chemical equation be balanced ?
Ans: An equation having an equal
number of atoms of each element on both the sides is called a balanced chemical
equation.
According to this law, mass can
neither be created nor destroyed. In other words, the total mass of reactants
should be equal to the total mass of the products in a chemical equation.
Therefore, the total number atoms of each element participating in the reaction
must be equal on both sides of the chemical equation i.e. before and after
reaction. Hence, it is for the above reason that a chemical equation should be
balanced.
Q.5: Translate the following
statements into balanced chemical equations.
(a) Hydrogen gas combines with
nitrogen to form ammonia.
(b) Hydrogen sulphide 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 andhydrogen gas.
Ans:
(a) N2 + 3H2 ---> 2NH3
(b) 2H2S + 3O2 ---> 2H2O + 2SO2
(c) 3BaCl2 + Al2(SO4)3 ---> 3BaSO4
+ 2AlCl3
(d) 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 + Ca(OH)2 ---> Ca(NO3)2 +
2H2O
(b) 2NaOH + H2SO4 ---> Na2SO4 +
2H2O
(c) NaCl + AgNO3 ---> AgCl + NaNO3
(d) BaCl2 + H2SO4 ---> BaSO4 + 2HCl
Q.7: Write balanced chemical equations
for the following reactions.
(a) Calcium hydroxide + Carbon dioxide
---> Calcium carbonate + Water
(b) Zinc + Silver Nitrate ---> Zinc
nitrate + Silver
(c) Aluminium + Copper chloride
---> Aluminium chloride + Copper
(d) Barium chloride + Potassium
sulphate --->. Barium sulphate + Potassium chloride
Ans:
(a) Ca(OH)2 + CO2 ---> CaCO3 + H2O
(b) Zn + 2AgNO3 ---> Zn(NO3)2 + 2Ag
(c) 2Al + 3CuCl2 ---> 2AlCl3 + 3Cu
(d) BaCl2 + K2SO4 ---> BaSO4 + 2KCl
Q.8: Write the balanced chemical
equations for the following and identify the type of reaction in each case.
(a) Potassium bromide + Barium iodide
---> Potassium iodide + Barium bromide
(b) Zinc carbonate ---> Zinc oxide
+ Carbon dioxide
(c) Hydrogen + Chlorine --->
Hydrogen chloride
(d) Magnesium + Hydrochloric acid
---> Magnesium chloride + Hydrogen
Ans:
(a) 2KBr + BaI2 ---> 2KI + BaBr2
(b) ZnCO3 ---> ZnO + CO2
(c) H2 + Cl2 ---> 2HCl
(d) Mg + 2HCl ---> MgCl2 + H2
Q.9: What does one mean by exothermic
and endothermic reactions? Give examples.
Ans:Exothermic reactions: Those
reactions in which energy is released in the form of heat are called exothermic
reactions. Examples -
(1) All combustion reactions e.g. CH4 + 2O2 ---> CO2 + 2H2O + Heat
(2) Thermite reactions e.g. 2Al + Fe2O3 ---> 2Fe + Al2O3 +
Heat
Combinations are generally exothermic
in nature. The decomposition of organic matters into compost is an example of
exothermic reaction.
Endothermic reactions: Those reactions
in which energy is absorbed are called endothermic reactions. Examples CaCO3
------> Cao + O2
also, the reaction of photosynthesis -
6CO2 + 6H20 (SUNLIGHT AND CHLOROPHYL)--->C6H12O6+6O2
Q.10: Why is respiration considered an
exothermic reaction? Explain.
Ans: For the sustenance of life we
require energy. We obtain this energy from our food. In the process of
digestion, food molecules get converted into simpler substances such as,
glucose. Then glucose combine with oxygen and provide energy to our body. The
whole process is known as respiration. As because, energy is released during
the process of respiration, so it is considered an exothermic reaction. For
example,
C6H12O6 + 6O2 ---> 6CO2 + 6H2O +
energy
Q.11: Why are decomposition reactions
called the opposite of combination reactions? Write equations for these
reactions.
Ans: Decomposition reactions are those
reactions in which a compound is broken into two or more new compounds.
Heat
CaCO3 -----------------> Cao + O2
Combination reactions are those
reactions in which two substances are combined to form a new substance.
CaO + CO2 ---> CaCO3 + energy
In above examples, both the reactions
are same but show opposite directions, so decomposition reaction is called the
opposite of combination reaction.
Q.12: Write one equation each for
decomposition reactions where energy is supplied in the form of heat, light or
electricity.
Ans:
(a) Decomposition by heat or thermal
energy
Heat
CaCO3 -----------------> Cao + O2
(b) Decomposition by light or
photolysis
2AgCl ---> 2Ag + Cl2 (in presence
of light)
(c) Decomposition by electricity or
electrolysis
electricity
2H2O------------------>2H2 +O2
Q.13: What is the difference between
the displacement and double displacement reactions? Write equations for these
reactions.
Ans: In displacement reaction, one
element from its salt is displaced by a more reactive element e.g. in following
example, Cu is displaced by Zn from CuSO4 because Zn is more reactive.
CuSO4 + Zn ---> ZnSO4 + Cu
In double displacement reaction,
exchange of ions takes place between two reactants to form new products.
Na2SO4 + BaCl2 ---> BaSO4 + 2NaCl
Q.14: In the refining of silver, the
recovery of silver from silver nitrate solution involved displacement by copper
metal. Write down the reaction involved.
Ans: When copper is mixed in silver
nitrate solution, it displaces the silver because copper is more reactive than
silver.
2AgNO3 + Cu ---> Cu(NO3)2 + 2Ag
Q.15: What do you mean by a
precipitation reaction? Explain by giving examples.
Ans: The reactions in which a
precipitate is formed are called precipitation reactions. Examples:
1. Na2SO4 + BaCl2 ---> BaSO4 +
2NaCl
In this reaction, BaSO4 is obtained as
precipitate.
2. Na2CO3 + CaCl2 ---> CaCO3 +
2NaCl
Here calcium carbonate is obtained as
precipitate.
Q.16: Explain the following in terms
of gain or loss of oxygen with two examples each.
(a).Oxidation
(b).Reduction
Ans:
(a) Oxidation: The reactions in which
gain of oxygen takes place are called oxidation. Examples -
1. 2Cu + O2 ---> 2CuO
2. 2H2 + O2 ---> 2H2O
(b) Reduction: The reactions in which
loss of oxygen takes place are called reduction. Examples -
1. ZnO + C ---> Zn + CO
2. CuO + H2 ---> Cu + H2O
Q.17: A shiny brown coloured element
‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the
black coloured compound formed.
Ans: The shinny brown colored element
‘X’ is copper. When it is heated in air, it becomes black due to the deposit of
copper oxide. 2Cu + O2 ----------------heat--> 2CuO
Q.18: Why do we apply paint on iron
articles ?
Ans: We apply paint on iron articles
to prevent them from corrosion. Paint prevents iron from coming in contact with
air and moisture. Thus, painting prevents rusting of iron articles.
Q.19: Oil and fat containing food
items are flushed with nitrogen. Why ?
Ans: Nitrogen being an inert gas does
not react easily with the oil and fat present in the food substances. But
oxygen reacts easily with the food substances and makes them rancid. It is because of this reason that food items
containing oil and fat are flushed with nitrogen gas before their packingto
remove oxygen present in the pack.
Q.20: Explain the following terms with
an example each:
(a) Corrosion (b) Rancidity
Ans:
(a) Corrosion: Corrosion is a process
in which materials deteriorate as a result of a chemical reaction with moisture
and other chemicals present in its surroundings. Generally, metals get corroded more easily.
For example, iron in the presence of moisture, reacts with oxygen (air) to form hydrated iron oxide also called
Rust.
4Fe + 3O2 +nH2O ---> 2Fe2O3.nH2O
(b) Rancidity: It is a process of
oxidation in which fats and oils present in food items are oxidized that can be
easily noticed by the change in their taste and smell.
Rancidity can be prevented by:
1. Storing food in refrigerators or in
air-tight containers.
2. Flushing oil and fat containing
food items with nitrogen before their packing.
3. By adding anti-oxidant
Acids, Bases and Salts q&A
Q.2: A solution reacts with crushed egg-cells to give a gas that turns lime water milky. Thesolution contains -
Acids, Bases and Salts q&A
Q.1: A solution turns red litmus blue; its pH is likely to
be a) 1 b) 4 c) 5 d) 10
Answer: (d)
Q.2: A solution reacts with crushed egg-cells to give a gas that turns lime water milky. Thesolution contains -
a) NaCl b) HCl c) LiCl d) KCl
Answer: (b)
Q.3: 10 ml of a solution of NaOH is found to be completely neutralized by 8 ml of a givensolution of HCl. If we take 20 ml of the same solution of NaOH, the amount of HCL (the samesolution as before) required to neutralize it will be - a) 4 ml b) 8 ml c) 12 ml d) 16 ml
Answer: (b)
Q.3: 10 ml of a solution of NaOH is found to be completely neutralized by 8 ml of a givensolution of HCl. If we take 20 ml of the same solution of NaOH, the amount of HCL (the samesolution as before) required to neutralize it will be - a) 4 ml b) 8 ml c) 12 ml d) 16 ml
Answer: (d)
Q.4: Which one of the following types of medicoines is used fr treating indigestion?
a) Antibiotic b) Analgesic c) Antacid d) Antiseptic
Q.4: Which one of the following types of medicoines is used fr treating indigestion?
a) Antibiotic b) Analgesic c) Antacid d) Antiseptic
Answer: (c)
Q.6: Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Answer: The nails are fitted in a cork and are kept in a 100 ml beaker. Then these are connected to the two terminals of a 6 volt battery along with a bulb and a switch. Now some dilute HCl is poured in the beaker and current is allowed to pass through the circuit. The same experiment is carried out using glucose and alcohol solutions separately in place of HCl.
Q.6: Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Answer: The nails are fitted in a cork and are kept in a 100 ml beaker. Then these are connected to the two terminals of a 6 volt battery along with a bulb and a switch. Now some dilute HCl is poured in the beaker and current is allowed to pass through the circuit. The same experiment is carried out using glucose and alcohol solutions separately in place of HCl.
Observations
The bulb glows in HCl solution only. The reason is
that HCl solution dissociates into H+ and Cl– ions which help to
conduct electricity while glucose and alcohol solutions do not dissociate into
ions and so, do not conduct electricity. That is why we find the bulb
glowing in HCl solutiononly.
Conclusions
The above activity proves that all acids contain hydrogen but not all compounds containing hydrogen are acids. Due to this reason compounds such as alcohols and glucose although contain hydrogen, but they are not categorized as acids.
Q.7: Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water which is a pure form of water, is neither acidic nor basic in nature. So distilled water does not dissociate into ions. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.
The above activity proves that all acids contain hydrogen but not all compounds containing hydrogen are acids. Due to this reason compounds such as alcohols and glucose although contain hydrogen, but they are not categorized as acids.
Q.7: Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water which is a pure form of water, is neither acidic nor basic in nature. So distilled water does not dissociate into ions. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.
Rain
water being an impure form of water, contains many ionic species. These ions
are responsible for electrical conductivity of rain water.
Q.8: Why do acids not show acidic behavior in the absence of water?
Answer: In absence of water acids do not dissociate into ions and hence, they do not show acidic behaviour.
Q.9: Five solutions A, B, C, D, and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is:
a) Neutral? b) Strongly alkaline? c) Strongly acidic? d) Weakly acidic? e) Weakly alkaline?
Arrange the pH in increasing order of hydrogen ion concentration.
Ans: (a) D (b) C (c) B (d) A (e) E. pH in increasing order of H+ ions concentration:
C (11) < E (9) < D (7) < A (4) < B (1).
Q.10: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why?
Ans: In test tube A, fizzing occur more vigorously. It is because HCl is a stronger acid than acetic acid and therefore, produces H2 gas at a faster rate due to which fizzing occurs.
Q.11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans: Curd is acidic in nature which means that the pH of fresh milk will start decreasing from 6.
Q.12: A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Answer:
(a) By adding small amount of baking soda, the milkman shifts the pH of the fresh milk from 6 to slightly alkaline so that he can keep it for longer time as the milk in alkaline condition, does not set curd easily.
(b) Due to the addition of baking soda this milk becomes slightly basic because of which acidsproduced in this milk due to bacterial actions are neutralized. Hence, this milk takes longer time to set as curd.
Q.13: Plaster of Paris should be stored ion a moisture-proof container. Explain why.
Answer: Plaster of Paris easily absorbs water and forms hard gypsum. Thus, if Plaster of Paris is not kept in a moisture-proof container, then all Plaster of Paris will get converted into gypsum.
CaSO4.½H2O + 1½H2O --> CaSO4.2H2O
Q.14: What is a neutralization reaction? Give two examples.
Ans: A reaction in which an acid and a base react to give a salt and water is known as a ‘neutralization reaction’. For example,
NaOH (aq) + HCl (aq) --> NaCl (aq) + H2O (liq)
Ca(OH)2 (aq) + H2SO4 (aq) --> CaSO4 (s) + H2O
Q.15: Give two important uses of washing soda and baking soda.
Ans:
Uses of Washing Soda:
1. Sodium carbonate (washing soda) is used in glass, soap, and paper industries.
2. It is used in the manufacture of sodium compounds such as Borax.
Uses of Baking Soda:
1. Sodium hydrogen carbonate is also ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.
2. It is also used in soda-acid fire extinguishers.
3. It is used as baking powder in confectionary items like cake, pastry etc.
Q.8: Why do acids not show acidic behavior in the absence of water?
Answer: In absence of water acids do not dissociate into ions and hence, they do not show acidic behaviour.
Q.9: Five solutions A, B, C, D, and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is:
a) Neutral? b) Strongly alkaline? c) Strongly acidic? d) Weakly acidic? e) Weakly alkaline?
Arrange the pH in increasing order of hydrogen ion concentration.
Ans: (a) D (b) C (c) B (d) A (e) E. pH in increasing order of H+ ions concentration:
C (11) < E (9) < D (7) < A (4) < B (1).
Q.10: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why?
Ans: In test tube A, fizzing occur more vigorously. It is because HCl is a stronger acid than acetic acid and therefore, produces H2 gas at a faster rate due to which fizzing occurs.
Q.11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans: Curd is acidic in nature which means that the pH of fresh milk will start decreasing from 6.
Q.12: A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Answer:
(a) By adding small amount of baking soda, the milkman shifts the pH of the fresh milk from 6 to slightly alkaline so that he can keep it for longer time as the milk in alkaline condition, does not set curd easily.
(b) Due to the addition of baking soda this milk becomes slightly basic because of which acidsproduced in this milk due to bacterial actions are neutralized. Hence, this milk takes longer time to set as curd.
Q.13: Plaster of Paris should be stored ion a moisture-proof container. Explain why.
Answer: Plaster of Paris easily absorbs water and forms hard gypsum. Thus, if Plaster of Paris is not kept in a moisture-proof container, then all Plaster of Paris will get converted into gypsum.
CaSO4.½H2O + 1½H2O --> CaSO4.2H2O
Q.14: What is a neutralization reaction? Give two examples.
Ans: A reaction in which an acid and a base react to give a salt and water is known as a ‘neutralization reaction’. For example,
NaOH (aq) + HCl (aq) --> NaCl (aq) + H2O (liq)
Ca(OH)2 (aq) + H2SO4 (aq) --> CaSO4 (s) + H2O
Q.15: Give two important uses of washing soda and baking soda.
Ans:
Uses of Washing Soda:
1. Sodium carbonate (washing soda) is used in glass, soap, and paper industries.
2. It is used in the manufacture of sodium compounds such as Borax.
Uses of Baking Soda:
1. Sodium hydrogen carbonate is also ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.
2. It is also used in soda-acid fire extinguishers.
3. It is used as baking powder in confectionary items like cake, pastry etc.
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