The Equilibrium Constant: Videos & Practice Problems

The equilibrium constant, denoted as $K$, represents the ratio of products to reactants at a specific temperature. A higher $K$ (greater than 1) indicates that products are favored, while a lower $K$ (less than 1) suggests reactants are favored. When $K$ equals 1, products and reactants are in equal amounts. Note that solids and liquids are not included in this ratio.

The Equilibrium Constant Concept 1

The equilibrium constant, denoted as \( K \), is a crucial concept in chemical equilibrium, representing the ratio of the concentrations of products to reactants at a specific temperature. This relationship can be expressed mathematically as:

\[ K = \frac{[\text{Products}]}{[\text{Reactants}]} \]

Temperature plays a significant role in determining the value of \( K \); an increase in temperature generally leads to an increase in \( K \), while a decrease in temperature results in a lower \( K \). The magnitude of \( K \) provides insight into the position of equilibrium for a given reaction. When \( K > 1 \), it indicates that products are favored, suggesting that the reaction proceeds predominantly in the forward direction. For example, if the concentration of products is 10 and that of reactants is 1, then:

\[ K = \frac{10}{1} = 10 \]

This value, being greater than 1, confirms that the formation of products is favored. Conversely, when \( K < 1 \), reactants are favored, indicating that the reaction is more likely to proceed in the reverse direction. For instance, if the products are 1 and the reactants are 10, then:

\[ K = \frac{1}{10} = 0.1 \]

In this case, the reaction favors the reactants. When \( K = 1 \), it signifies that the concentrations of products and reactants are equal, indicating a state of balance in the reaction. For example, if both products and reactants are present in equal amounts, such as 10 each, then:

\[ K = \frac{10}{10} = 1 \]

It is important to note that the equilibrium constant \( K \) considers all states of matter except for solids and liquids, which are not included in the expression due to their constant concentrations in a given reaction. Understanding these principles is essential for predicting the behavior of chemical reactions under varying conditions.

The Equilibrium Constant Example 1

In chemical equilibrium, the focus is primarily on gases and aqueous solutions, where "aqueous" indicates that a compound is dissolved in water. When writing the equilibrium expression for a reaction, it is essential to consider only the species in the gas or aqueous phase, while ignoring solids and liquids. For example, in the reaction:

2N₂O₅ (aq) ⇌ 4 NO₂ (aq) + O₂ (aq),

the equilibrium constant (K) is expressed as:

K = \frac{[NO₂]^{4} \cdot [O₂]}{[N₂O₅]^{2}}.

Here, the coefficients from the balanced equation become the exponents in the expression. It is crucial to note that solids and liquids do not appear in the equilibrium expression because their concentrations remain constant and are effectively treated as 1.

In another scenario, if a reaction involves solids, such as:

solid A ⇌ solid B + O₂ (g),

the equilibrium expression would be:

K = \frac{1}{[O₂]}.

This indicates that the concentration of solids is considered constant, represented by 1. Thus, the equilibrium constant can also be expressed as the inverse of the concentration of the gaseous product.

For a reaction like:

solid C + solid D ⇌ E (g),

the equilibrium expression simplifies to:

K = [E].

Again, solids are ignored, and only the gaseous products are included. Understanding these principles allows for the determination of whether products or reactants are favored at equilibrium based on the value of K. If K > 1, products are favored; if K < 1, reactants are favored; and if K = 1, neither side is favored, indicating a balance between reactants and products.

By practicing these concepts, students can enhance their understanding of chemical equilibria and the factors influencing them.

State which is greater in amount: reactants or products, based on the given equilibrium constant, K.
a) N₂ (g) + O₂ (g) ⇌ 2 NO (g) K = 1.0 x 10²⁰
b) 2 CO (g) + O₂ (g) ⇌ 2 CO₂ (g) K = 2.2 x 10^-22
c) 2 BrCl (g) ⇌ Br₂ (g) + Cl₂ (g) K = 1

• A. a) products, b) products, c) reactants
• B. a) reactants, b) products, c) reactants
• C. a) products, b) reactants, c) equal
• D. a) reactants, b) products, c) equal

Answer: (C)

The decomposition of nitrogen monoxide can be achieved under high temperatures to create the products of nitrogen and oxygen gas. 
6 NO(aq) ⇌ 3 N₂(aq) + 3 O₂(aq) 
a) What is the equilibrium equation for the reaction above?
b) What is the equilibrium expression for the reverse reaction. 

• A.
• B.
• C.
• D.

Answer: (D)

The equilibrium constant, K, for 2 NO (g) + O₂ (g) ⇌ 2 NO₂ (g) is 6.9 x 10². 
What is the [NO] in an equilibrium mixture of gaseous NO, O₂, and NO₂ at 500 K that contains 1.5 x 10 ^–2 M O₂ and 4.3 x 10 ^–3 M NO₂?

• A. 4.15 x 10^-4 M
• B. 1.34 x 10^-3 M
• C. 2.04 x 10^-2 M
• D. 1.80 x 10^-6 M

Answer: (B)

Here’s what students ask on this topic:

What is the equilibrium constant and how is it calculated?

The equilibrium constant, denoted as K, is a number that represents the ratio of the concentrations of products to reactants at equilibrium for a given chemical reaction at a specific temperature. It is calculated using the formula:

$K=\frac{\mathrm{[products]}}{\mathrm{[reactants]}}$

Here, the concentrations of the products and reactants are raised to the power of their respective coefficients in the balanced chemical equation. Note that solids and liquids are not included in this ratio.

How does temperature affect the equilibrium constant?

Temperature has a direct effect on the equilibrium constant (K). If the temperature increases, the value of K will also increase for endothermic reactions, meaning the reaction favors the formation of products. Conversely, for exothermic reactions, an increase in temperature will decrease the value of K, favoring the formation of reactants. Therefore, the equilibrium constant is temperature-dependent, and any change in temperature can shift the position of equilibrium.

What does it mean if the equilibrium constant (K) is greater than 1?

If the equilibrium constant (K) is greater than 1, it indicates that the concentration of products is greater than the concentration of reactants at equilibrium. This means that the reaction favors the formation of products, and the forward reaction is predominant. For example, if K = 10, the products are ten times more concentrated than the reactants.

Why are solids and liquids not included in the equilibrium constant expression?

Solids and liquids are not included in the equilibrium constant expression because their concentrations do not change during the reaction. The concentration of a pure solid or liquid is constant and does not affect the equilibrium position. Therefore, only the concentrations of gases and aqueous solutions are considered when calculating the equilibrium constant.

What happens when the equilibrium constant (K) is equal to 1?

When the equilibrium constant (K) is equal to 1, it means that the concentrations of products and reactants are equal at equilibrium. This indicates that neither the forward nor the reverse reaction is favored, and the system is in a state of dynamic balance. For example, if both the products and reactants have a concentration of 10, then K = 10/10 = 1.