6-1 化學平衡常數的計算
化學(二) 熱力學平衡應用篇
6. 平衡商數Q與常數K的應用
國立陽明交通大學應用化學系 李大偉
What you can learn
• How to solve the chemical equilibrium problems in general
• How to solve the chemical equilibrium problems usually
when 𝐾 > 103 or 𝐾 < 10−3
𝐾 𝑎𝑛𝑑 𝑎𝑖
• 𝑎𝐴 + 𝑏𝐵 → 𝑐𝐶 + 𝑑𝐷
• ∆𝐺𝑟° = −𝑅𝑇 ∙ 𝑙𝑛 𝐾
•𝐾 =
𝑐 𝑑
𝑎𝐶
𝑎𝐷
𝑎 𝑎𝑏
𝑎𝐴
𝐵
𝑃𝑖
• 𝑎𝑖 = ° 𝑓𝑜𝑟 𝑔𝑎𝑠 𝑖
𝑃
𝑖
• 𝑎𝑖 = °
𝑐
𝑓𝑜𝑟 𝑠𝑜𝑙𝑢𝑡𝑒 𝑖
• 𝑎𝑖 = 1 𝑓𝑜𝑟 𝑝𝑢𝑟𝑒 𝑠𝑜𝑙𝑖𝑑𝑠 𝑜𝑟 𝑙𝑖𝑞𝑢𝑖𝑑𝑠
Calculate the moles of the final state
𝐻2 𝑔 + 𝐼2 𝑔 ⇌ 2𝐻𝐼 𝑔
Species
1
Initial
Change
during rxn
Final
Species
2
Species
3
……
𝐸𝑥𝑎𝑚𝑝𝑙𝑒 1
• A 10 L reactor contains P 𝐻2 = 0.36 bar, P 𝐼2 = 0.24 bar and P 𝐻𝐼 = 1.31 bar at 430℃. The
°
standard free energy change of the following reaction is ∆𝐺𝑟𝑥𝑛
= −23.347 𝑘𝐽 at 430℃ ,
𝐻2 𝑔 + 𝐼2 (𝑔) ⇌ 2𝐻𝐼 𝑔
• Is this system at equilibrium?
• If not, determine the direction of the reaction and calculate the amounts of 𝐻2 , 𝐼2 and 𝐻𝐼 present at equilibrium.
𝐴𝑛𝑠𝑤𝑒𝑟 1
𝐻2 𝑔 + 𝐼2 (𝑔) ⇌ 2𝐻𝐼 𝑔
A 10 L reactor contains P 𝐻2 = 0.36 bar, P 𝐼2 = 0.24 bar and P 𝐻𝐼 = 1.31 bar at 430℃.
°
• Is this system at equilibrium? Here, ∆𝐺𝑟𝑥𝑛
= −23.347 𝑘𝐽 at 430℃
• If not, determine the direction of the reaction and calculate the amounts of 𝐻2 , 𝐼2 and 𝐻𝐼
present at equilibrium.
計算化學平衡問題，通常的假設：
• 𝐾 > 103 ，通常會假設「幾乎完全反應」
• 反應平衡常數越大，代表反應程度越偏向產物
• 𝐾 < 10−3 ，通常會假設「幾乎不完全反應」
• 反應平衡常數越小，代表反應程度越偏向反應物
• 103 > 𝐾 > 10−3
𝐸𝑥𝑎𝑚𝑝𝑙𝑒 2
°
The standard free energy change of the following reaction is ∆𝐺𝑟𝑥𝑛
= −160.01 𝑘𝐽 at 25℃
3𝐶𝑙𝑂− 𝑎𝑞 → 2𝐶𝑙 − 𝑎𝑞 + 𝐶𝑙𝑂3− (𝑎𝑞)
• What is 𝐾 at 25℃ ?
• Initially, [𝐶𝑙𝑂− ]=1M, [𝐶𝑙 − ]=0M and [𝐶𝑙𝑂3− ]=0M. What is the equilibrium concentration of
[𝐶𝑙𝑂− ], [𝐶𝑙 − ], and [𝐶𝑙𝑂3− ]?
𝐴𝑛𝑠𝑤𝑒𝑟 2
3𝐶𝑙𝑂− 𝑎𝑞 → 2𝐶𝑙 − 𝑎𝑞 + 𝐶𝑙𝑂3− (𝑎𝑞)
°
• What is 𝐾 at 25℃ ? Here, ∆𝐺𝑟𝑥𝑛
= −160.01 𝑘𝐽 at 25℃
• Initially, [𝐶𝑙𝑂− ]=1M, [𝐶𝑙 − ]=0M and [𝐶𝑙𝑂3− ]=0M. What is the equilibrium
concentration of [𝐶𝑙𝑂− ], [𝐶𝑙 − ], and [𝐶𝑙𝑂3− ]?
學習重點
• 回顧 ∆𝐺𝑟° , 平衡常數 𝐾與活度Activity
• 熟悉利用𝐾解平衡時的濃度或分壓