Saturday, July 28, 2012

CBSE Class 11: Chemistry - CH1 - Laws of Chemical Combinations

Antoine Lavoiser
Laws of Chemical Combinations

Q1: List the laws that are collectively called Laws of Chemical combinations.

  1. Law of Conservation of Mass
  2. Law of Constant composition
  3. Law of Multiple proportions
  4. Law of reciprocal proportions
  5. Law of Combining Volumes (Gay Lussac's law of Gaseous Volumes)
  6. Avogadro Law

Q2: State Law of conservation of mass.

Answer: The law states, "In all physical and chemical changes, matter can neither be created nor be destroyed". Experimentally, it can be proved for both changes i,e. physical changes and chemical changes.

(a) When matter undergoes physical change e.g. a piece of ice cube in closed flask is heated. Ice cube gently melts to liquid state. Here,
 mass of ice in enclosed flask (before melting) = mass of water in the closed flask (after melting)

(b) When matter undergoes chemical change,
     Mass of reactants = Mass of products  + Mass of un-reacted reactants.

For exampLe. 100 g of mercury Oxide, is heated and is decomposed to produce, 92.6 g of mercury, and 7.4 g of oxygen.

        HgO (s)     →    Hg (l) + ½O2
        100g         →    92.6g   + 7.4g

Q3:Who proposed law of conservation of mass.

Answer: Antoine Lavoiser in 1789

Q4: What mass of Silver Nitrate will react with 5.85g of sodium chloride to produce 14.35 g of Silver Chloride and 8.5g of Sodium Nitrate, if law of conservation of mass holds true?

Answer:  Let mass of Silver Nitrate (AgNO3) in this reaction = x g

Law of conservation is true,
∴     AgNO3 + NaCl     →   AgCl + NaNO3
            x g    + 5.85g      =   14.35g + 8.5 g
⇒        x = 14.35g + 8.5 g - 5.85g = 17.0 g

Q5: What is the Law of Definite Proportions?

Answer: It states, "A given chemical substance (compound) always contains the same elements combined in a fixed proportion by weight”. It is also called law of constant composition.

For example, 1.0 gram of sodium chloride always contains 0.39 gram of sodium and 0.61 gram of chlorine, irrespective if it chemically combined or naturally found. Sodium chloride has definite proportions of sodium and chlorine; that is, it has definite proportion.

Q6: Who proposed the Law of Definite Proportions?

Answer: Louis Proust

Q7: What are the limitations of Law of Definite Proportions?

  1. The law does not hold true if the different isotopes of the element are involved in making chemical compound. e.g. For making CO2 gas, C-12 combines with O in ratio C:O :: 12:32 while C-14 carbon isotope combines in ratio C:O::14:32.
  2. The law is not applicable when elements combine in the same ratio but different compounds are formed. e.g. C2H5OH (ethyl alcohol) and CH3OCH3 (methol gas) are two different compounds but having same molecular formula i.e. C2H6O. In both compounds, the ratio of C:H:O by mass is same i.e. 12:3:8.
Q8: Is there any limitation of Law of Conservation of mass?

Answer: Yes. Law of conservation of mass holds true for most of the chemical equations, because heat energy dissipated or consumed is within error limits. For nuclear reactions, where a very large amount of energy is released, this law is not applicable.

Q9: When 1.375 g of CuO was reduced by H2, 1.098 g of Cu was obtained. In another experiment, 1.178 g of Cu was dissolved in nitric acid and the resulting copper nitrate was converted into CuO by ignition The weight of CuO formed was 1.476 g. Show that in both these experiments the law of definite proportion holds true.

In the first experiment:
        Weight of  CuO  = 1.375 g
         Weight of Cu     = 1.098 g
         Weight of O       = 1.375 - 1.098 = 0.277 g
Ratio of Cu:O             = 1.098 : 0.277  = 3.96 : 1

In the second experiment,
         Weight of CuO   = 1.476 g
         Weight of Cu      = 1.178 g
          Weight of O       = 1.476 – 1.178 = 0.298 g
     Ratio of Cu:O         = 1.178 : 0.298 = 3.96 : 1
In both experiments we obtain same Cu:O ratio i.e. 3.96:1. Hence law of definite proportions is applied here.

Q10: What is meant by the Law of multiple proportions?

Answer: Law of multiple proportions states, "If two elements chemically combine to give two or more compounds, then the weights of one element, which combine, with the fixed weight of the other element in those compounds, bear a simple multiple ratios to one another."

The law was proposed by Dalton based on his atomic theory.

e.g. 1g of Carbon(C) combines with oxygen and forms two compounds i.e. CO and CO2. In CO, 1g of C combines with 1.33g of oxygen (O). And in CO2, 1g of C combines with 2.66 g of O.
Weight of oxygen that combines with carbon are in ratio 1:2.

Q11: What was the importance of  Law of multiple proportions?

Answer: The law of multiple proportions was deduced from Dalton's atomic theory.  The law hold true was able to convince chemists the validity of atomic theory.

Q12: A metal combines with oxygen to form two oxides, having the following composition:
(i) 0.398 g of first metal oxide contains 0.318 g of metal.
(i) 0.716 g of second oxide contains 0.636 g of metal.
Show that the above data agrees with the law of multiple proportions.

Answer: Wt of oxygen in first oxide = 0.398 - 0.318 = 0.080 g
Wt of oxygen in second oxide = 0.716 - 0.636 = 0.080 g

For fixed mass of oxygen, (0.080g), metal forms oxides in the ratio = 0.318:0.636 = 1:2
Since 1:2 is a simple ratio, it agrees with the law of multiple proportions.

Q13: Hydrogen combines with oxygen and forms two compounds. In the first compound, hydrogen content is 5.93% while in the other compound it is 11.2%. Verify whether the data agrees with law of multiple proportions.

Answer: In 1st compound, Hydrogen = 5.93% ⇒ Oxygen = 100 - 5.93 = 94.07%
⇒ If 5.95 part of H combines with parts of Oxygen = 94.07
   1 part of Hydrogen combines with O = 94.07/5.93 = 15.86 parts.

In 2nd compound, Hydrogen composition = 11.2%, Oxygen composition = 88.8%
⇒ If 11.2 part of H combines with parts of Oxygen = 88.8
   1 part of Hydrogen combines with O = 88.8/11.2 = 7.9 parts.

The ratio of oxygen in parts that combine with 1 part of hydrogen = 15.86:7.9 = 2:1
Since 2:1 is a simple ratio, it is in agreement with Law of multiple proportions.

Q14: State Law of reciprocal proportions.

Answer: Law of reciprocal proportions states, "When two elements combine separately with a fixed mass of a third element, then the ratio of their masses in which they do so is either same or some whole number multiple of the ratio in which they combine with each other".

Q15: Who proposed law of reciprocal proportions.

Answer: Ritcher.

Q16: What is Gay Lussac’s Law of Gaseous Volumes or Combining Volumes?

Answer: The law of combining volumes was given by Gay Lussac in 1808. It is applicable only to gaseous reaction. According to this law, gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.

For example, two volumes of hydrogen react with one volume of oxygen gas to produce water. They bear a simple ratio 2:1

       2 volumes     1 volume  → 2 volumes
Law of combining volumes can be derived from Law of definite proportions when expressed in terms of volumes. Later Avagadro interpreted this law in terms of molecules. e.g. the ratio in the above equation 2:1:2 is called molar ratio of this gas reaction.

Q17: What is Avogadro's Law? 

Answer: According to this law, equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules.

Avogadro's hypothesis reveals the following facts:
  1. It introduces the fact molecules are polyatomic. 
  2. It provided the basis for determining the relative masses of atoms.
  3. One mole of all gases contain NA number of molecules.
  4. Mathematically at standards pressure and temperature (STP),  volume (V) ∝ n( no. of moles or molecules). i.e. V1/n1 = V2/n2 
  5. Volume of 1 mole of gas at STP (P  = 1 atm and T = 273K) = 22.4 litres.
  6. Molecular mass of a gas = 2 × vapour density of gas.

Q18: A wooden log burns completely into ash. Justify conservation of mass is applied here.

Answer: Whenever a chemical change occurs, we consider mass of both reactants and products.
When a wooden log (its composition is mainly organic matter like hemi-sllulose, lignin etc.) burns and decomposes into Carbon-DiOxide gases, ash (Carbon) and water. Law of conservation of mass holds true here, the mass reactant (wood and oxygen) is equal to mass of products i.e. gases, water vapours released and ash.

Q19 (NCERT): The following data are obtained when dinitrogen and dioxygen react together to form different compounds:

   Mass of dinitrogen    Mass of dioxygen
(i)    14 g                       16 g
(ii)   14 g                       32 g
(iii)  28 g                       32 g
(iv)  28 g                       80 g

Which law of chemical combination is obeyed by the above experimental data? Give its statement.

Answer: For the fixed mass of dinitrogen at 28 g, the masses of dioxygen that combines
with the the fixed mass dinitrogen are  = 32 g, 64 g, 32 g, and 80 g.
i.e. 2:4:2:5. Since The masses of dioxygen bear a whole number ratio, hence, the given experimental data obeys the law of multiple proportions.

The law states that if two elements combine to form more than one compound, then the masses of one element that combines with the fixed mass of another element are in the ratio of small whole numbers.

Q20 (NCERT): If ten volumes of dihydrogen gas react with five volumes of dioxygen gas, how many volumes of water vapour would be produced?

Answer: Reaction of dihydrogen with dioxygen is:

Now, two volumes of dihydrogen react with one volume of dihydrogen to produce two volumes of water vapour.
Applying law of combining volumes, the two gases combine in molar ratio 2:1:2.
Therefore, ten volumes of dihydrogen will react with five volumes of dioxygen to produce
ten volumes of water vapour.

Q21: What was Dalton's atomic theory?

  1. Matter is made up of extremely small, indivisible particles called atoms.
  2. Atoms of same substance are identical in all respects, i.e., they possess same size, shape,
    mass, chemical properties, etc.
  3. Atoms of different substances are different in all respects, i.e., they possess different
    sizes, shapes, masses, chemical properties, etc.
  4. Atom is the smallest particle that takes part in chemical reactions.
  5. Atoms of different elements may combine with each other in a fixed, simple, whole
    number ratio to form compound atoms.
  6. Atoms can neither be created nor destroyed, i.e., atoms are indestructible.

Q22: What are main failures of  Dalton theory?

Answer: The main failures or limitation of Dalton’s atomic theory are:
  1. It failed to explain anything about structure of the atom.
  2. It could not explain how and why atoms of different elements combine with each other to form compound-atoms or molecules.
  3. It failed to explain the nature of forces that bind together different atoms in a molecule.
  4. It failed to explain Gay Lussac’s law of combining volumes.
  5. It failed to account for the stability of atom after the discovery of sub-atomic particles.
  6. It failed to understand the origin and nature of the characteristics of electromagnetic
    radiation absorbed or emitted by atoms.

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