Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 13.1–13.2 Kc and Kp expressions-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 13.1–13.2 Kc and Kp expressions- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 13.1–13.2 Kc and Kp expressions- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
13.1 Equilibrium Constant (\( \mathrm{K_c} \)) Expressions
The equilibrium constant \( \mathrm{K_c} \) expresses the position of equilibrium for a reversible reaction in terms of the concentrations of reactants and products at equilibrium. The form of the expression depends on the balanced equation and whether the system is homogeneous or heterogeneous.
General Expression for \( \mathrm{K_c} \)
For a general reaction:
\( \mathrm{aA + bB \rightleftharpoons cC + dD} \)
The equilibrium constant is:
\( \mathrm{K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}} \)
- Square brackets denote equilibrium concentrations (in \( \mathrm{mol\ dm^{-3}} \)).
- Coefficients from the balanced equation become powers.
Homogeneous Systems
A homogeneous system is one in which all reactants and products are in the same physical state (usually all gases or all aqueous).
Key Rule
- All species are included in the \( \mathrm{K_c} \) expression.
Example
\( \mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)} \)
\( \mathrm{K_c = \frac{[NH_3]^2}{[N_2][H_2]^3}} \)
Heterogeneous Systems
A heterogeneous system contains substances in different physical states.
Key Rule
- Pure solids and pure liquids are not included in the \( \mathrm{K_c} \) expression.
- Only aqueous and gaseous species are included.
Reason
- The concentration of pure solids and liquids is constant.
- Therefore, they do not affect the value of \( \mathrm{K_c} \).
Example
\( \mathrm{CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)} \)
\( \mathrm{K_c = [CO_2]} \)
Only \( \mathrm{CO_2} \) appears because the solids are omitted.
Key Features
- \( \mathrm{K_c} \) is written using equilibrium concentrations.
- Stoichiometric coefficients become powers.
- Homogeneous: include all species.
- Heterogeneous: exclude solids and liquids.
Example 1:
Write the expression for \( \mathrm{K_c} \) for:
\( \mathrm{2NO(g) + O_2(g) \rightleftharpoons 2NO_2(g)} \)
▶️ Answer/Explanation
All species are gases, so all are included.
\( \mathrm{K_c = \frac{[NO_2]^2}{[NO]^2[O_2]}} \)
Example 2:
Write the expression for \( \mathrm{K_c} \) for:
\( \mathrm{Fe_3O_4(s) + 4CO(g) \rightleftharpoons 3Fe(s) + 4CO_2(g)} \)
▶️ Answer/Explanation
Solids are not included in the expression.
\( \mathrm{K_c = \frac{[CO_2]^4}{[CO]^4}} \)
13.2 Equilibrium Constant (\( \mathrm{K_p} \)) in Terms of Partial Pressures
The equilibrium constant \( \mathrm{K_p} \) is used for reactions involving gases and is expressed in terms of the equilibrium partial pressures of the gaseous reactants and products. It is particularly useful when dealing with gaseous systems instead of concentrations.
General Expression for \( \mathrm{K_p} \)
For a general reaction:
\( \mathrm{aA(g) + bB(g) \rightleftharpoons cC(g) + dD(g)} \)
The equilibrium constant is:
\( \mathrm{K_p = \frac{(p_C)^c (p_D)^d}{(p_A)^a (p_B)^b}} \)
- \( \mathrm{p_A, p_B, p_C, p_D} \) are equilibrium partial pressures (in atm).
- Stoichiometric coefficients become powers.
Homogeneous Systems
A homogeneous system contains substances all in the same phase (for \( \mathrm{K_p} \), typically all gases).
Key Rule
- All gaseous species are included in the \( \mathrm{K_p} \) expression.
Example
\( \mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)} \)
\( \mathrm{K_p = \frac{(p_{NH_3})^2}{(p_{N_2})(p_{H_2})^3}} \)
Heterogeneous Systems
A heterogeneous system contains substances in different physical states.
Key Rule
- Only gaseous substances are included in \( \mathrm{K_p} \).
- Solids and liquids are omitted.
Reason
- Solids and liquids do not exert partial pressure.
- Their activity is constant and does not affect equilibrium.
Example
\( \mathrm{CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)} \)
\( \mathrm{K_p = p_{CO_2}} \)
Key Features
- \( \mathrm{K_p} \) uses partial pressures (in atm).
- Only gaseous species are included.
- Powers come from coefficients in the balanced equation.
- Solids and liquids are excluded.
Example 1:
Write the expression for \( \mathrm{K_p} \) for:
\( \mathrm{2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)} \)
▶️ Answer/Explanation
All species are gases, so all are included.
\( \mathrm{K_p = \frac{(p_{SO_3})^2}{(p_{SO_2})^2(p_{O_2})}} \)
Example 2:
Write the expression for \( \mathrm{K_p} \) for:
\( \mathrm{C(s) + H_2O(g) \rightleftharpoons CO(g) + H_2(g)} \)
▶️ Answer/Explanation
Solid carbon is not included.
\( \mathrm{K_p = \frac{p_{CO} \cdot p_{H_2}}{p_{H_2O}}} \)