[100]Enthalpy and entropy problems with solutions(pdf)

Thermochemistry – Explanation

Thermochemistry is a branch of chemistry that deals with the study of heat energy changes during chemical reactions and physical transformations. It focuses on how energy is absorbed or released when substances undergo changes. These energy changes are typically measured in terms of enthalpy (H), which represents the total heat content of a system. Reactions that release heat are called exothermic, while those that absorb heat are called endothermic.

A key concept in thermochemistry is the enthalpy change (ΔH), which indicates whether a reaction absorbs or releases heat. If ΔH is negative, the reaction is exothermic; if positive, it is endothermic. Another important principle is Hess’s Law, which states that the total enthalpy change of a reaction is the same regardless of the pathway taken. This allows chemists to calculate unknown enthalpy changes using known values from other reactions.

Thermochemistry also involves calorimetry, the measurement of heat changes using a device called a calorimeter. The equation $q = mc\Delta T$q=mcΔT is commonly used, where q is heat, m is mass, c is specific heat capacity, and ΔT is temperature change. Understanding thermochemistry is essential in fields like energy production, environmental science, and industrial chemistry, where controlling heat flow is crucial.

100 Thermochemistry Numerical Problems with Answers

Basic Heat Calculations (q = mcΔT)

1. Calculate heat when 2 g water heated by 5°C (c = 4.18 J/g°C)
   Ans: 41.8 J
2. 10 g substance, c = 2 J/g°C, ΔT = 3°C
   Ans: 60 J
3. 50 g water, ΔT = 2°C
   Ans: 418 J
4. 100 g, c = 1 J/g°C, ΔT = 10°C
   Ans: 1000 J
5. 25 g, c = 4 J/g°C, ΔT = 1°C
   Ans: 100 J
6. 5 g water, ΔT = 20°C
   Ans: 418 J
7. 200 g, c = 0.5 J/g°C, ΔT = 4°C
   Ans: 400 J
8. 80 g, c = 2 J/g°C, ΔT = 5°C
   Ans: 800 J
9. 60 g water, ΔT = 3°C
   Ans: 752.4 J
10. 40 g, c = 1.5 J/g°C, ΔT = 2°C
    Ans: 120 J

Enthalpy Change (ΔH)

11. Reaction releases 200 J
    Ans: ΔH = -200 J
12. Absorbs 150 J
    Ans: +150 J
13. Releases 500 kJ
    Ans: -500 kJ
14. Absorbs 75 kJ
    Ans: +75 kJ
15. Heat lost = 300 J
    Ans: -300 J
16. Heat gained = 100 J
    Ans: +100 J
17. 250 J released
    Ans: -250 J
18. 400 J absorbed
    Ans: +400 J
19. 600 kJ released
    Ans: -600 kJ
20. 50 kJ absorbed
    Ans: +50 kJ

Calorimetry Problems

21. 100 g water, ΔT = 5°C
    Ans: 2090 J
22. 200 g water, ΔT = 2°C
    Ans: 1672 J
23. 50 g water, ΔT = 10°C
    Ans: 2090 J
24. 25 g water, ΔT = 4°C
    Ans: 418 J
25. 150 g water, ΔT = 3°C
    Ans: 1881 J

Hess’s Law (Concept-based numericals)

26. ΔH1 = -100, ΔH2 = -50 → total
    Ans: -150 kJ
27. -200 + 100
    Ans: -100 kJ
28. -300 + (-200)
    Ans: -500 kJ
29. 150 – 50
    Ans: 100 kJ
30. -400 + 250
    Ans: -150 kJ

Specific Heat Capacity

31. q = 200 J, m = 10 g, ΔT = 5°C
    Ans: 4 J/g°C
32. q = 100 J, m = 5 g, ΔT = 2°C
    Ans: 10 J/g°C
33. q = 500 J, m = 25 g, ΔT = 5°C
    Ans: 4 J/g°C
34. q = 300 J, m = 10 g, ΔT = 3°C
    Ans: 10 J/g°C
35. q = 600 J, m = 20 g, ΔT = 3°C
    Ans: 10 J/g°C

Mixed Practice

36. 100 g, c = 4.18, ΔT = 1
    Ans: 418 J
37. 20 g, c = 2, ΔT = 5
    Ans: 200 J
38. 10 g, c = 1, ΔT = 10
    Ans: 100 J
39. 50 g, c = 4, ΔT = 2
    Ans: 400 J
40. 80 g, c = 1, ΔT = 5
    Ans: 400 J

Continue (41–100 Short Practice)

41. 10g water, ΔT=1 → 41.8 J
42. 20g water, ΔT=2 → 167.2 J
43. 30g water, ΔT=3 → 376.2 J
44. 40g water, ΔT=1 → 167.2 J
45. 50g water, ΔT=2 → 418 J
46. 60g, c=2, ΔT=2 → 240 J
47. 70g, c=1, ΔT=3 → 210 J
48. 80g, c=2, ΔT=1 → 160 J
49. 90g, c=1, ΔT=2 → 180 J
50. 100g, c=1, ΔT=1 → 100 J
51. Heat released 700 J → -700 J
52. Heat absorbed 800 J → +800 J
53. Heat released 900 J → -900 J
54. Heat absorbed 1000 J → +1000 J
55. Heat released 1100 J → -1100 J
56. ΔH: -50 + (-50) → -100 kJ
57. ΔH: 100 – 200 → -100 kJ
58. ΔH: -300 + 100 → -200 kJ
59. ΔH: 200 + 200 → 400 kJ
60. ΔH: -150 + 50 → -100 kJ
61. q=420, m=10, ΔT=10 → 4.2 J/g°C
62. q=210, m=5, ΔT=10 → 4.2 J/g°C
63. q=84, m=2, ΔT=10 → 4.2 J/g°C
64. q=42, m=1, ΔT=10 → 4.2 J/g°C
65. q=4200, m=100, ΔT=10 → 4.2 J/g°C
66. 10g, c=2, ΔT=2 → 40 J
67. 20g, c=3, ΔT=2 → 120 J
68. 30g, c=1, ΔT=3 → 90 J
69. 40g, c=2, ΔT=4 → 320 J
70. 50g, c=1, ΔT=5 → 250 J
71. 5g water ΔT=2 → 41.8 J
72. 15g water ΔT=2 → 125.4 J
73. 25g water ΔT=2 → 209 J
74. 35g water ΔT=2 → 292.6 J
75. 45g water ΔT=2 → 376.2 J
76. ΔH: -10 + -20 → -30 kJ
77. ΔH: -30 + 10 → -20 kJ
78. ΔH: 40 – 10 → 30 kJ
79. ΔH: -100 + 50 → -50 kJ
80. ΔH: 60 + 40 → 100 kJ
81. q=100, m=10, ΔT=1 → 10 J/g°C
82. q=200, m=20, ΔT=1 → 10 J/g°C
83. q=300, m=30, ΔT=1 → 10 J/g°C
84. q=400, m=40, ΔT=1 → 10 J/g°C
85. q=500, m=50, ΔT=1 → 10 J/g°C
86. 60g, c=1, ΔT=6 → 360 J
87. 70g, c=2, ΔT=5 → 700 J
88. 80g, c=1, ΔT=4 → 320 J
89. 90g, c=2, ΔT=3 → 540 J
90. 100g, c=1, ΔT=2 → 200 J
91. Heat absorbed 2000 J → +2000 J
92. Heat released 1500 J → -1500 J
93. Heat absorbed 1200 J → +1200 J
94. Heat released 800 J → -800 J
95. Heat absorbed 600 J → +600 J
96. ΔH: -200 + -300 → -500 kJ
97. ΔH: 300 – 100 → 200 kJ
98. ΔH: -400 + 200 → -200 kJ
99. ΔH: 500 – 200 → 300 kJ
100. ΔH: -600 + 100 → -500 kJ