Chemical Equilibrium flash cards

1. 1.What is a reversible reaction? Give an example.

   A reaction that proceeds in both forward and backward directions under the same conditions, so reactants are never fully consumed. Example: $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$.

   Hint: Both arrows, never goes to completion.

2. 2.Define dynamic equilibrium.

   The state in a reversible reaction where the forward and backward reaction rates become equal, so concentrations of all species stay constant with time even though both reactions continue. It is 'dynamic' because reactions never stop.

   Hint: Rates equal, concentrations constant, but activity continues.

3. 3.List the characteristics of chemical equilibrium.

   Attained only in a closed system; dynamic in nature (rates equal, not reactions stopped); reachable from either direction; concentrations of reactants and products stay constant; a catalyst does not shift it; $\Delta G = 0$ at equilibrium.

   Hint: Closed system, dynamic, both directions, $\Delta G=0$.

4. 4.State the Law of Mass Action.

   At constant temperature, the rate of a reaction is proportional to the product of the active masses (molar concentrations) of the reactants, each raised to the power of its stoichiometric coefficient.

   Hint: Rate $\propto$ product of active masses raised to coefficients.

5. 5.For $a\text{A} + b\text{B} \rightleftharpoons c\text{C} + d\text{D}$, write the equilibrium constant $K_c$.

   $K_c = \dfrac{[\text{C}]^c[\text{D}]^d}{[\text{A}]^a[\text{B}]^b}$, where the brackets denote equilibrium molar concentrations.

   Hint: Products over reactants, powers = coefficients.

6. 6.Distinguish $K_c$ and $K_p$.

   $K_c$ is the equilibrium constant expressed using molar concentrations, while $K_p$ is expressed using partial pressures of gaseous species. $K_p$ is defined only when gases are involved.

   Hint: Concentration vs partial pressure.

7. 7.Derive/state the relation between $K_p$ and $K_c$.

   $K_p = K_c(RT)^{\Delta n}$, where $\Delta n = (\text{moles of gaseous products}) - (\text{moles of gaseous reactants})$ and $R$ is in units consistent with pressure and concentration.

   Hint: Use $PV=nRT$, so $P=(n/V)RT=CRT$.

8. 8.For which reactions is $K_p = K_c$?

   When $\Delta n_g = 0$, i.e. equal moles of gas on both sides, since $(RT)^0 = 1$. Example: $\text{H}_2 + \text{I}_2 \rightleftharpoons 2\text{HI}$.

   Hint: $\Delta n_g = 0 \Rightarrow (RT)^0$.

9. 9.Are $K_c$ and $K_p$ dimensionless? Explain.

   Thermodynamically, $K$ is defined using activities (or ratios to standard states), making it dimensionless. In JEE numericals, $K_c$ and $K_p$ carry apparent units of $(\text{mol L}^{-1})^{\Delta n}$ and $(\text{atm})^{\Delta n}$ respectively unless $\Delta n = 0$.

   Hint: Rigorously dimensionless; apparent units from $\Delta n$.

10. 10.What is the equilibrium constant in terms of mole fraction, $K_x$, and how does it relate to $K_p$?

    $K_x = \dfrac{\prod x_i^{\nu_i}(\text{products})}{\prod x_i^{\nu_i}(\text{reactants})}$ and $K_p = K_x \cdot P^{\Delta n}$, where $P$ is total pressure. Thus $K_x$ depends on pressure unless $\Delta n = 0$.

    Hint: $K_p = K_x P^{\Delta n}$.

11. 11.How does the equilibrium constant change when a reaction is reversed?

    The equilibrium constant of the reversed reaction is the reciprocal: $K' = 1/K$.

    Hint: Reverse $\Rightarrow$ invert.

12. 12.How does $K$ change if all coefficients of a reaction are multiplied by $n$?

    The new equilibrium constant becomes $K^n$. For example, doubling coefficients gives $K^2$.

    Hint: Multiply coefficients by $n$ $\Rightarrow$ raise $K$ to power $n$.

13. 13.If reaction 1 ($K_1$) and reaction 2 ($K_2$) are added to give reaction 3, what is $K_3$?

    For added reactions, equilibrium constants multiply: $K_3 = K_1 \times K_2$.

    Hint: Add reactions $\Rightarrow$ multiply $K$'s.

14. 14.Define the reaction quotient $Q$ and how it differs from $K$.

    $Q$ has the same algebraic form as $K$ but uses concentrations (or pressures) at any arbitrary instant, not necessarily equilibrium. At equilibrium $Q = K$.

    Hint: Same form as $K$, any-instant values.

15. 15.How is the direction of a reaction predicted using $Q$ and $K$?

    If $Q < K$, forward reaction dominates (more products form); if $Q > K$, backward reaction dominates; if $Q = K$, the system is at equilibrium.

    Hint: $Q<K$ forward, $Q>K$ backward.

16. 16.For a reaction, $Q = 25$ and $K = 100$. Which direction does it proceed?

    Since $Q < K$, the reaction proceeds forward (to the right) to form more products until $Q$ rises to $100$.

    Hint: Compare $Q$ with $K$; $Q<K$.

17. 17.What does the magnitude of $K$ tell you about the extent of reaction?

    $K \gg 1$ means the equilibrium lies far to the right (products favoured); $K \ll 1$ means it lies to the left (reactants favoured); $K \approx 1$ means comparable amounts of both.

    Hint: Large $K$ = product-favoured.

18. 18.Define degree of dissociation $\alpha$.

    The fraction of one mole of a substance that has dissociated at equilibrium: $\alpha = \dfrac{\text{moles dissociated}}{\text{initial moles}}$. It lies between 0 and 1 (or expressed as a percentage).

    Hint: Fraction dissociated per mole.

19. 19.For $\text{PCl}_5 \rightleftharpoons \text{PCl}_3 + \text{Cl}_2$ starting with 1 mol at total pressure $P$, express $K_p$ in terms of $\alpha$.

    Total moles $=1+\alpha$. $K_p = \dfrac{\alpha^2}{1-\alpha^2}\,P$, since each product mole fraction is $\frac{\alpha}{1+\alpha}$ and reactant $\frac{1-\alpha}{1+\alpha}$.

    Hint: $K_p=\frac{\alpha^2}{1-\alpha^2}P$ for 1$\to$2 dissociation.

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