1. Lecture No. 1 Laws of Chemical Combinations Chemistry

2. Session Objectives

3. Session Objectives: The law of conservation of mass The law of definite proportions The law of multiple proportions The low of combining volumes(Gay Lussac’s law of gaseous volumes) Dalton’s atomic theory Modern atomic theory Avogadro's hypothesis

4. law of conservation of mass: Lavoisier (France 1789) states that ‘Mass can neither be created nor destroyed in a chemical reaction’. This means that during a chemical reaction the sum of the masses of the reactants and products remain unchanged. Total mass of the products remains equal to the total mass of the reactants. H 2 + Cl 2 2 HCl 2g 71g 73g

5. Question

6. Illustrative Problem 8.4 g of sodium bicarbonate on reaction with 20.0 g of acetic acid (CH 3 COOH) liberated 4.4 g of carbon dioxide gas into atmosphere. What is the mass of residue left? Solution: 8.4 + 20.0 = m + 4.4 m = 24 g It proves the law of conservation of mass.

7. law of definite proportions: Joseph Proust (France 1799) states that ‘ In a chemical compound the elements are always present in a definite proportion by mass’. Eg 1: Water ( H 2 O) always contains two elements (atoms) of hydrogen and one atom oxygen combined together in the same ratio of 2:16 or 1:8 by mass. If 9 g of water is decomposed we get 1 g of hydrogen and 8 g of oxygen.

8. Ice water H 2 O 1 : 8 River water H 2 O 1 : 8 Sea water H 2 O 1 : 8 A chemical compound always contains same elements combined together in same proportion of mass.

9. Example: KCl always contains one atom of K for every one atom of Cl In KCl, potassium and chlorine always have a ratio of “39.09 to 35.45” or “1.1 to 1” by mass.

10. Law of multiple proportions (John Dalton, England) When the same two elements combine to form more than one compound: the ratios of the mass of one element in the first compound to its mass in the second compound, (as it combines with the same mass of the other element), can always be expressed as ratios of small whole numbers( ex: 1:3 or 2:5).

11. Example of Law of Multiple Proportions 14:814:1614:2414:3214:40 Ratio of oxygen combining with 14 parts of nitrogen 8:16:24:32:40 = 1:2:3:4:5 Two elements combine two or more compounds The mass of one of the elements which combines with fixed mass of the others, bear a simple whole number ratio to one another.

12. Example: Carbon combines with oxygen to form CO and CO 2. Ratio of O in CO 2 to O in CO Mass of oxygen(g) Mass of Carbon(g) 16.0012.01 CO 2:132.0012.01 CO 2

13. Question

14. Practice Problem 1 In the carbon compounds ethane (C 2 H 6 ) and ethene (C 2 H 4 ), what is the lowest whole number ratio of H atoms that react with the same number of C atoms? Answer: 3:2

15. Practice Problem 2 Carbon reacts with oxygen to form two compounds as shown: Compound A: 2.41 g C, 3.22 g O. Compound B: 6.71 g C, 17.9 g O. Find the lowest whole number ratio of C that react with an equal mass of O.

16. Solution Strategy 1. Find the grams of carbon per 1 g of oxygen for each compound. 2. Divide the highest answer by the smallest answer. Answer: 2:1

17. Home Work Practice Problem 3 Lead forms two compounds with oxygen as shown: Compound A: 2.98 g Pb, 0.461 g O. Compound B: 9.89 g Pb, 0.763 g O. For a given mass of oxygen, what is the lowest whole number mass ratio of lead in the two compounds?

18. Law of combining volumes(Gay Lussac’s law of gaseous volumes) The law of combining volumes states that, when gases react together to form other gases, and all volumes are measured at the same temperature and pressure: The ratio between the volumes of the reactant gases and the products can be expressed in simple whole numbers.

19. For example, Gay-Lussac found that nitrogen oxide 1 : 1 : 2 2H 2 O 2 2H 2 O 2 : 1 : 2 1 volume nitrogen gas 1 volume oxygen gas 2 volume nitrogen oxide N2N2 O2O2 2NO 2 volume hydrogen gas 1 volume oxygen g as 2 volume steam

20. Do you know The laws of chemical combinations are based on quantitative results of chemical reactions.

22. In the last lecture we talked about: The law of conservation of mass. The law of definite proportions. The law of multiple proportions. The low of combining volumes(Gay Lussac’s law of gaseous volumes). Now we will solve some Illustrative Problems about the laws:

23. Illustrative Problem 1: Two samples of a gas were analyzed. One contained 1.2g of carbon and 3.2 g of oxygen. The other contained 27.3 % carbon and 72.7% oxygen. The experimental data are in accordance with 1. Law of conservation of mass 2. Law of definite proportions 3. Law of multiple proportions 4. Low of combining volumes

24. Solution: % of C in the 1 st sample Which is same as in the second sample. Hence law of definite proportion is obeyed.

25. Illustrative Problem 2: Carbon is found to form two oxides, which contains 42.8% and 27.27% of carbon respectively. Find out which of the laws of chemical combination is proved correct by this data? Solution: In the first oxide, Carbon :Oxygen = 42.8 : 57.2 = 0.748 : 1 In the second oxide, Carbon :Oxygen = 27.27 : 72.73 = 0.374 : 1 = 2:1 = 0.748 : 0.374

26. Illustrative Problem 3: Zinc sulphate crystals ZnSO 4 contain 22.6% of zinc and 43.6% of water. How much Zinc should be used to produce 13.7 gm of zinc sulphate crystals and how much water they will contain? (Hint: Law of constant composition) Solution: 100 gm of ZnSO 4 will have 43.9 gm water and 22.6 gm zinc 100 gm of ZnSO 4 crystals contains= 22.6gm of Zn

27. Solution: So Zn required to produce 13.7 gm of zinc sulphate crystals = 22.6 × 13.7 = 3.0962 gm 100 100 gm of zinc sulphate crystals contains= 43.6 gm water So 13.7 gm of crystals contains water = 43.6 × 13.7 100 = 5.9732 gm

28. Dalton’s Atomic Theory: Dalton’s atomic theory from about 1803 is the basis of today’s chemistry, this theory consists of six parts: 1. Matter is made up of indivisible particles called atoms. 2. Atoms cannot be created or destroyed in a chemical reaction. 3. Atoms of the same element are similar with respect to shape, size and mass.

29. 4. Atoms of different elements have different masses and properties. 5. Atoms combine in small whole number ratios to form compounds. 6. Atoms of two elements may combine in different ratios to form more than one compound. S+O 2 SO 2 1:2 SO 3 1:3

30. Limitations of Dalton’s Atomic Theory Later researches proved that Dalton’s atomic theory was not wholly correct. 1) Atoms of the same or different types have a strong tendency to combine together to form a new ‘group of atoms’. For example, hydrogen, nitrogen, oxygen gases exist in nature as ‘group of two atoms’. This indicates that the smallest unit capable of independent existence is not an atom, but a ‘group of atoms’.

31. So it didn’t explain causes of chemical combination and law of combining volume 2) With the discovery of sub-atomic particles, e.g., electrons, neutrons and protons, the atom can no longer be considered indivisible. 3) Discovery of isotopes indicated that all atoms of the same element are not perfectly identical. At least, they differ in their masses. Atoms of the same element having different masses are called isotopes.

32. Ex: carbon (atomic mass = 12.011) Carbon (14) and carbon (12) exist in nature 4) It does not give idea about structure of atom

33. Modern atomic theory 1. Atom is divisible 2. Same atom may have different atomic masses like 1 H, 2 H and 3 H. 3. Different atoms may have same atomic mass like 40 Ca and 40 Ar. 4. Atom is the smallest particle that takes part in a chemical reaction. 5. The mass of an atom can be changed into energy.

34. Avogadro's hypothesis: The law is named after Amedeo Avogadro (Italy 1811): It is a gas law which states that, under the same condition of temperature and pressure equal volumes of all gases contain the same number of molecules. Thus, the number of molecules or atoms in a specific volume of ideal gas is independent of their size or the molar mass of the gas.

35. Examples: As an example, equal volumes of molecular hydrogen and nitrogen contain the same number of molecules when they are at the same temperature and pressure, and observe ideal gas behavior. Another example: Hydrogen + Chlorine → Hydrogen Chloride 1 molecule 1 molecule 2 molecules 2 atoms 2 atoms 4 compound-atoms

36. Class Test

37. Class Exercise - 1 Percentage of copper and oxygen in samples of CuO obtained by different methods were found to be the same. This proves the law of (a) constant proportions(b) conservation of mass (c) multiple proportions(d) none of these

38. Class Exercise - 2 A balanced chemical equation is in accordance with (a) law of definite proportions (b) Avogadro’s law (c) Gay Lussac’s law (d) law of conservation of mass

39. Class Exercise - 3 Different samples of water were found to contain hydrogen and oxygen in the approximate ratio of 1 : 8. This shows the law of (a) multiple proportion (b) constant proportion (c) reciprocal proportion (d) none of these

40. Thank you