1. The Brønsted-Lowry Theory of Acids Two important theories have been developed to explain the properties of acids, bases, and salts. These are the Arrhenius Theory and the Bronsted-Lowry Theory. Here, we’ll look at the Bronsted-Lowry theory.

2. This Theory of Acids was proposed independently in 1923 by This Theory of Acids was proposed independently in 1923 by two people

3. This Theory of Acids was proposed independently in 1923 by Johannes Nicolaus Brønsted, a Danish chemist, and Johannes Nicolaus Brønsted, a Danish chemist, and

4. This Theory of Acids was proposed independently in 1923 by Johannes Nicolaus Brønsted, a Danish chemist, and Thomas Martin Lowry, an English chemist. Thomas Martin Lowry, an English chemist. We’ll introduce the main points of this theory.

5. A hydrogen atom “H” has 1 proton and 1 electron. Before we do that, just a quick word here about a hydrogen atom. It’s atomic number is one, so it has 1 proton.

6. A hydrogen atom “H” has 1 proton and 1 electron. A neutral hydrogen atom also has one electron.

7. A hydrogen atom “H” has 1 proton and 1 electron. It is known that over 99.98% of the hydrogen atoms on Earth contain no neutrons. Over 99.98% of the hydrogen atoms on Earth contain no neutrons.

8. A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. So we’ll state here that a hydrogen atom has no neutrons, which is nearly always the case. Over 99.98% of the hydrogen atoms on Earth contain no neutrons.

9. A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. If 1 electron is removed from an H atom, it forms an H + ion. If one electron is removed from a hydrogen atom, it forms an H + ion.

10. A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. If 1 electron is removed from an H atom, it forms an H + ion. Here’s the equation showing that.

11. A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. If 1 electron is removed from an H atom, it forms an H + ion. A neutral H atom has 1 proton and 1 electron, H+H+ 1 p 1 e –

12. A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. If 1 electron is removed from an H atom, it forms an H + ion. So removing an electron H+H+ 1 p 1 e – Removing 1 electron,

13. will leave a charge of +1, 1 proton, and zero electrons. H+H+ 1 p 1 e – + A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. If 1 electron is removed from an H atom, it forms an H + ion. Removing 1 electron, leaves 1 proton and “0” electrons. Removing 1 electron,

14. A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. If 1 electron is removed from an H atom, it forms an H + ion. Removing 1 electron, leaves 1 proton and “0” electrons. So an H+ ion contains no electrons, no neutrons, and one proton H 1 p +

15. A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. If 1 electron is removed from an H atom, it forms an H + ion. Removing 1 electron, leaves 1 proton and “0” electrons. Therefore, an H+ ion is the same thing as one proton. Therefore, an H + ion is the same as 1 proton. H 1 p +

16. A hydrogen atom “H” has 1 proton and 1 electron, and no neutrons. If 1 electron is removed from an H atom, it forms an H + ion. Removing 1 electron, leaves 1 proton and “0” electrons. Or we can say that H+ equals 1 proton. We use the terms “H+ ion” and “proton” interchangeably in Chemistry 12. H + = 1 proton Therefore, an H + ion is the same as 1 proton.

17. According to the Brønsted-Lowry Theory:

18. An acid is any species that donates a proton (H + ) to another species. An acid is any species that donates a proton, or H + ion to another species. Let‘s look at an example of this.

19. Let’s start with a molecule of Hydrogen chloride, HCl, and a molecule of water, H2O. Here are Lewis structures for these molecules. ClH O H H HCl H2OH2O According to the Brønsted-Lowry Theory: An acid is any species that donates a proton (H + ) to another species.

20. HCl is a polar molecule. There is a partial negative charge on the chlorine atom and a partial positive charge on the hydrogen atom, shown by the delta minus and delta plus. ClH O H H HCl H2OH2O ++ ––

21. The water molecule is also polar: the oxygen atom has a partial negative charge and each hydrogen atom has a partial positive charge. ClH O H H HCl H2OH2O ++ –– –– ++ ++

22. The negative charge on the oxygen pulls the partially positive hydrogen atom (click) away from the chlorine. The hydrogen leaves its shared electron with the chlorine atom. ClH O H H HCl H2OH2O ++ –– –– ++ ++

23. The chlorine atom has gained an electron, so it acquires a negative charge and becomes a Cl minus ion. Cl O H H Cl – H2OH2O –– ++ ++ H

24. The hydrogen atom lost an electron, so it acquires a positive charge, forming an H+ ion. Cl O H H Cl – H2OH2O –– ++ ++ H + proton

25. Which is also called a proton. Cl O H H Cl – H2OH2O –– ++ ++ H + proton

26. The proton moves to the water molecule and (click) attaches to one of its lone pairs. Cl O H H Cl – H2OH2O –– ++ ++ H + proton

27. Instead of staying with the hydrogen atom, the positive charge is considered as the charge of the whole ion. So we’ll (click) move it over here. Cl O H H Cl – H +

28. There are now 3 H atoms attached to one O, so the formula is H3O instead of H2O O H H H + H3O+H3O+ Cl Cl –

29. We’ll draw square brackets around the H3O because it is an ion. H3O+H3O+ + O H H H Cl Cl –

30. And write the positive charge here in the formula. H3O+H3O+ + O H H H Cl Cl –

31. Chemists call the H3O + ion, the hydronium ion. H3O+H3O+ + Hydronium ion O H H H Cl Cl –

32. So we can summarize the whole process here. We started with a molecule of HCl

33. And we added a molecule of water. +

34. Which gave us +

35. An H3O +, or hydronium ion +

36. plus + +

37. a Cl minus, or chloride ion. + +

38. We can now write an equation to show this process. We write HCl gas +

39. Plus H2O liquid. + +

40. forms + +

41. H3O + aqueous + +

42. Plus Cl minus aqueous + +

43. We can also represent the process like this.

44. This shows that an H+ ion, or proton is being transferred from the HCl molecule to the water molecule. H+H+

45. So this can also be called a proton transfer. proton transfer

46. H 2 O has gained 1 proton, so it forms an H 3 O + ion proton transfer H 2 O has gained 1 proton, so it forms an H 3 O + ion

47. HCl has lost 1 proton, or H+ ion, so it forms a Cl – ion proton transfer HCl has lost 1 proton, so it forms a Cl – ion

48. According to the Brønsted-Lowry Theory: An acid is any species that donates a proton (H + ) to another species. Because the HCl is losing, or donating a proton, according to the Bronsted-Lowry theory, it is called an acid. To indicate this, we often call it a Bronsted-Lowry acid, or Bronsted acid for short. proton transfer Brønsted-Lowry Acid

49. According to the Brønsted-Lowry Theory: An acid is any species that donates a proton (H + ) to another species. A base is any species that accepts a proton (H + ) from another species. According the Bronsted-Lowry theory, a base is defined as any species that gains or accepts a proton, or H+ ion, from another species. Brønsted- Lowry definition of a BASE

50. According to the Brønsted-Lowry Theory: An acid is any species that donates a proton (H + ) to another species. A base is any species that accepts a proton (H + ) from another species. Because the H2O is gaining, or accepting a proton, it can be defined as a Bronsted-Lowry Base, or Bronsted base for short. proton transfer Brønsted-Lowry Base

51. According to the Brønsted-Lowry Theory: An acid is any species that donates a proton (H + ) to another species. So in this reaction, HCl is an acid, proton transfer Base Acid

52. According to the Brønsted-Lowry Theory: An acid is any species that donates a proton (H + ) to another species. A base is any species that accepts a proton (H + ) from another species. and water is a base. proton transfer Base Acid

53. Because the reactants HCl and H2O are not ions. proton transfer Not ions

54. But the products H3O+ and Cl minus ARE ions. proton transfer Not ions Ions

55. We can call this process the ionization of HCl.

56. Here’s a question. We’re asked to write the equations for the ionization of the following acids when they are added to water, and to identify the acids and the bases on the reactant side: Write the equations for the ionization of the following acids when they are added to water. Identify the acids and the bases on the reactant side:

57. We’ll start the first reaction by adding liquid water as the other reactant.

58. The formula for this compound starts with an H, so we assume it acts as an acid.

59. And we’ll label it as an acid. acid

60. So the other reactant, water, must act as a base in this case. acidbase

61. Now an acid is a proton donor acidbase proton donor

62. And a base is a proton acceptor. acidbase proton donor proton acceptor

63. So that means a proton, or H+ ion will be transferred (click) from the acid, HNO3, to the base, H2O. acidbase proton donor proton acceptor H+H+

64. So this means that water will gain an H+, which is one H and one + charge acidbase H+H+ Will gain an H +, which is one H and one + charge

65. Adding an H and one + charge to water, gives us H3O+, the hydronium ion. acidbase H+H+ The hydronium ion

66. The acid HNO3, has lost one H+, which means it has lost one H and one + charge acidbase H+H+ Has lost one H +, which is one H and one + charge

67. We remove one H atom from HNO3 giving us NO3, and taking away one + charge is the same as adding one negative charge, so we have NO3 minus, the nitrate ion. Both of the new ions formed are aqueous. acidbase H+H+ So it will form NO 3 –, the nitrate ion

68. So this is the equation for the ionization of HNO3. HNO3 is the acid on the left side and water is the base on the left side. acidbase

69. Now, we’ll see what we get if we add HCN to water. acidbase

70. Because HCN starts with H, we’ll treat it as an acid. acidbase acid

71. And H2O as a base acidbase acidbase

72. So there will be a (click) proton transfer from HCN to H2O. acidbase acidbase H+H+

73. The H2O will gain a proton, or H+ and form H3O +, or hydronium acidbase acidbase H+H+

74. And the HCN will lose a proton, or H+, and form CN minus acidbase acidbase H+H+

75. So this is the equation for the ionization of HCN. HCN is the acid on the left and water is the base. A double arrow is used here because the ionization of HCN does not go to completion. acidbase acidbase

76. In a solution of HCN, only a few molecules are ionized. You’ll be shown how you can tell whether to use a single arrow or double arrow later in the course. acidbase acidbase

78. Now, lets consider this reaction. Again, the double arrow here just tells us that this reaction does not go to completion. Instead, an equilibrium exists here. Consider the reaction:

79. We see that the NH3 has been converted to NH4 +.

80. In doing so, it gains one H and one + charge, therefore it gains one H+, or one proton.

81. Because it gains a proton in this case, the NH3 is classified as a base. base

82. Looking at the water, we see that is has been converted to OH minus. base

83. OH minus has one less H and one less + than H2O, therefore it has one less H+ than H2O. base One less H + than H 2 O

84. therefore when H2O converts to OH minus, it loses an H+, or proton. base

85. For that reason, we identify H2O as an acid in this case. baseacid

86. Now, we’ll look at a previous reaction we had in which HNO3 reacts with water. baseacid base

87. In this reaction, water acted as a base. baseacid base

88. So we can see that, depending on what it’s reacting with, water can play the role of an acid or the role of a base. baseacid base

89. Such a species is said to be amphiprotic. baseacid base Amphiprotic

90. An amphiprotic species is one that can act either as an acid or as a base, depending on what it is with. Water is one amphiprotic substance. There are many more, as we shall see later in this unit. baseacid base Amphiprotic can act either as an acid or as a base

92. Consider these two acids, HCl, and HNO3. baseacid base

93. Both of these are able to lose one proton only. baseacid base

94. An acid that has one proton available to donate is called a monoprotic acid. So both HCl and HNO3 are monoprotic acids. baseacid base Monoprotic

95. Now, consider this acid, H2SO4. It’s called

96. sulphuric acid

97. Notice it has 2 H atoms at the beginning of the formula. sulphuric acid

98. Acids that have two protons they can donate, are said to be diprotic. So H2SO4 is a diprotic acid. sulphuric acid Diprotic acid

99. However, when diprotic acids like H2SO4, are added to water, they do not lose both of their protons at once. They do it in steps, losing one proton at a time. sulphuric acid Diprotic acid

100. In the first step of the reaction of H2SO4 in water, the H2SO4 loses one proton to water. sulphuric acid H+H+

101. So the water is converted to H3O +, or hydronium. (end of statement) sulphuric acid H+H+

102. Because the H2SO4 is losing an H+, it means it’s losing one H atom and one + charge. sulphuric acid H+H+ Will lose one H and one + charge

103. This gives us HSO4 with a minus charge. sulphuric acid H+H+ Will lose one H and one + charge

104. HSO4 minus is called the hydrogen sulphate, or bisulphate ion. sulphuric acid H+H+ Called the hydrogen sulphate or bisulphate ion

105. Notice that HSO4 minus has one hydrogen it can lose. sulphuric acid H+H+ Has one H that it can lose

106. So in the second step of the ionization of sulphuric acid, the HSO4 minus will react with sulphuric acid H+H+

107. Water. sulphuric acid H+H+

108. And donate its proton to water sulphuric acid H+H+ H+H+

109. Which would produce another hydronium ion. (end of statement) sulphuric acid H+H+ H+H+

110. The HSO4 minus ion loses a proton, so it loses one hydrogen and one positive charge. sulphuric acid H+H+ H+H+ Loses one H and one + charge

111. So it will be left as SO4. sulphuric acid H+H+ H+H+ Loses one H and one + charge

112. And losing one positive charge will cause its charge to go down one, from negative 1 to negative 2, or 2 minus. sulphuric acid H+H+ hydrogen sulphate ion

113. The product SO4 2minus, or sulphate ion, does not have any hydrogens to donate, so this is the last step in the ionization of sulphuric acid sulphuric acid H+H+ hydrogen sulphate ion

114. The double arrow here implies there is an equilibrium. This reaction does not go to completion. sulphuric acid H+H+ hydrogen sulphate ion

115. Whereas in the first step of the ionization of sulphuric acid, there is a single arrow, which means this step goes to completion. 100% of the sulphuric acid is converted to hydronium and hydrogen sulphate ions. sulphuric acid hydrogen sulphate ion

116. Now we’ll look at phosphoric acid. phosphoric acid

117. We see that it has 3 H’s at the front of its formula, which means it has three protons it can lose. phosphoric acid

118. For that reason, phosphoric acid is called a triprotic acid. phosphoric acid Triprotic acid

119. When it’s combined with water, it doesn’t lose all three protons at once. Just one at a time. phosphoric acid Triprotic acid

120. In the first step, one proton is transferred to a water molecule, so it produces one hydronium ion. (end of statement) phosphoric acid H+H+

121. because it loses one H and one positive charge, H+H+ Loses one H and one + charge

122. the other product would have 2 H’s and a charge of negative 1. H+H+ Loses one H and one + charge

123. H2PO4 minus is called the dihydrogen phosphate ion. We see that it still has two protons it can donate. H+H+ called the dihydrogen phosphate ion

124. In the second step, the H2PO4 minus ion loses one of its protons to water H+H+ H+H+

125. so it produces another hydronium ion H+H+ H+H+

126. And since this loses one H and one positive charge, H+H+ H+H+ Loses one H and one + charge

127. The other product would be HPO4 2 minus. H+H+ H+H+ Loses one H and one + charge

128. this product is called the monohydrogen phosphate ion. We can see that it has one proton available to donate. H+H+ H+H+ Called the monohydrogen phosphate ion

129. So in the last step, the HPO4 2minus donates its single proton to water, producing another hydronium ion. (end of statement) H+H+ H+H+ H+H+

130. And because it loses one H and one positive charge, H+H+ H+H+ H+H+ Loses one H and one + charge

131. the final ion that it forms is just PO4 3 minus. H+H+ H+H+ H+H+ Loses one H and one + charge

132. This is simply called the phosphate ion H+H+ H+H+ H+H+ The phosphate ion

133. Polyprotic acid Polyprotic Acid

134. is just a general term for any acid that can donate more than one proton. Polyprotic Acid —just a general term for any acid that can donate more than one proton.

135. Examples could be carbonic acid, which is diprotic Polyprotic Acid —just a general term for any acid that can donate more than one proton. e.g.) H 2 CO 3 (carbonic acid-diprotic)

136. phosphoric acid, which is triprotic Polyprotic Acid —just a general term for any acid that can donate more than one proton. e.g.) H 2 CO 3 (carbonic acid-diprotic) H 3 PO 4 (phosphoric acid-triprotic)

137. and pyrophosphoric acid, which has 4 protons it can donate. Polyprotic Acid —just a general term for any acid that can donate more than one proton. e.g.) H 2 CO 3 (carbonic acid-diprotic) H 3 PO 4 (phosphoric acid-triprotic) H 4 P 2 O 7 (pyrophosphoric acid)