Question

Explain the six tenets of Dalton’s atomic theory, and clarify their relation to modern chemistry, understanding as we do now the results of experiments in nuclear chemistry which have resulted in the development of the notions of atomic mass (isotopes) and the failure of the Law of Conservation of Matter. Explain how the theory accounts for the three laws of matter – conservation of matter and energy, definite proportions, and multiple proportions.

a) Write the equation for the reaction which occurs by combining manganese (III) chlorate with ammonium sulfide, if any occurs. b) Classify the reaction by type and what atoms do. c) What evidence of a reaction would one see if these two solutions were combined?

Explain the difference between chemical and physical properties, and how one is able to use physical changes to determine whether a chemical change occurred.

Answer 1

Tenets of Dalton’s atomic theory:

1. All matter is made of atoms. Atoms are indivisible and indistinguishable.

2. All the atoms of a given element are identical in mass and properties.

3. A compound is the combination of two or more different types of atoms.

4. A chemical reaction is the rearrangement of atoms.

In case of nuclear reactions, the law of conservation of mass is failed.

Reason: Existence of different isotopes (elements having the same no. of protons but different different no. of neutrons as well as different mass no. [A = z (no. of protons) + n (no. of neutrons)]

Law of conservation of mass and energy: In a reaction, the total mass and energy of reactants is exactly the same as that of products.

Law of definite proportions: A given chemical compound always contains its component elements in a fixed ratio (or proportions) irrespective of its source and method of preparation.

Law of multiple proportions: When two elements combine with each other to form more than one compound, the ratio of unknown mass of one element that combines with fixed mass of another will be a small whole number.

a) The balanced equation for the reaction of manganese(III) chlorate with ammonium sulfide can be written as 2Mn(ClO₃)₃(aq) + 3(NH₄)₂S(aq) Mn₂S₃(s) + 6NH₄ClO₃(aq).

b) The above reaction is an atom-economical reaction, i.e. all the atoms on the left hand side are exactly matched with those on the right hand side.

Here, (i) The manganese (Mn) atoms of Mn(ClO₃)₃and the sulfur (S) atoms of (NH₄)₂S are responsible for the formation of manganese(III) sulfide (Mn₂S₃); (ii) the nitrogen (N) atoms of (NH₄)₂S and the chlorine (Cl) atoms of Mn(ClO₃)₃are responsible for the formation of ammonium chlorate (NH₄ClO₃).

c) If the the solutions of Mn(ClO₃)₃ and (NH₄)₂S are mixed, then there will be a formation of a precipitate (i.e. Mn₂S₃), which is the evidence for the reaction occured there.

The main difference between the physical and chemical properties of a substance is its outward appearance (physical property) and its inner nature (chemical property).

To explain a chemical change in terms of physical changes, let's consider the case of given reaction.

(i) In terms of states of matter: a) Before chemical change, Mn(ClO₃)₃and (NH₄)₂S were in solution, i.e. in liquid phase. b) After the chemical change, there appears a precipitate, i.e. Mn₂S₃, a solid, in the reaction mixture.

(ii) In terms of color: a) Before chemical change, the color of the solution of Mn(ClO₃)₃ is purple-red. b) After the chemical change, there appears a precipitate, i.e. solid Mn₂S₃ (white or different from purple-red in color).

The above two physical changes clearly illustrates the chemical change occured.