# 平衡常数

## 平衡常數的種類

### 濃度平衡常數

${\displaystyle K_{\mathrm {c} }={\frac {c^{c}(\mathrm {C} )c^{d}(\mathrm {D} )}{c^{a}(\mathrm {A} )c^{b}(\mathrm {B} )}}}$

### 壓強平衡常數

${\displaystyle K_{\mathrm {p} }={\frac {p^{c}\mathrm {(C)} p^{d}\mathrm {(D)} }{p^{a}\mathrm {(A)} p^{b}\mathrm {(B)} }}}$

### 濃度平衡常數和壓強平衡常數的關係

${\displaystyle K_{\mathrm {p} }=K_{\mathrm {c} }(RT)^{\Delta n}}$

#### 證明過程

${\displaystyle pV=nRT}$

${\displaystyle p={\frac {n}{V}}RT}$

${\displaystyle p(\mathrm {A} )=c(\mathrm {A} )RT}$
...

### 單位

• ${\displaystyle K_{\mathrm {c} }}$${\displaystyle \mathrm {mol} ^{\Delta n}}$，單位常常省略不寫。
• ${\displaystyle K_{\mathrm {p} }}$${\displaystyle \mathrm {atm} ^{\Delta n}}$或是${\displaystyle \mathrm {mmHg} ^{\Delta n}}$或是${\displaystyle \mathrm {Pa} ^{\Delta n}}$

## 意義

• ${\displaystyle Q>K}$　生成物濃度太高，未達到平衡，反應將向左移動，直到 ${\displaystyle Q=K}$，才達平衡。
• ${\displaystyle Q　反應物濃度太高，未達到平衡，反應將向右移動，直到 ${\displaystyle Q=K}$，才達平衡。
• ${\displaystyle Q=K}$　反應已達到平衡狀態。

## 寫法

${\displaystyle {\ce {CaCO3(s) <=> CaO(s) + CO2(g)}}}$

${\displaystyle K_{\mathrm {c} }=c(\mathrm {CO_{2}} )}$
${\displaystyle K_{\mathrm {p} }=p(\mathrm {CO_{2}} )}$

${\displaystyle {\ce {Zn(s) + 2Ag+ (aq) <=> Zn^2+ (aq) + 2Ag(s)}}}$

${\displaystyle K_{\mathrm {c} }={\frac {[\mathrm {Zn} ^{2+}]}{[\mathrm {Ag} ^{+}]^{2}}}}$

${\displaystyle {\ce {I2(s) <=> I_{2}(CCl_4)}}}$

${\displaystyle K_{\mathrm {c} }=\mathrm {[I_{2}](CCl_{4})} }$

${\displaystyle {\ce {C(s) + CO2(g) <=> 2CO(g)}}}$

${\displaystyle K_{\mathrm {c} }={\frac {c^{2}(\mathrm {CO} )}{c^{2}(\mathrm {CO} _{2})}}}$

${\displaystyle K_{\mathrm {p} }={\frac {p^{2}(\mathrm {CO} )}{p^{2}(\mathrm {CO} _{2})}}}$