Chemical Equilibrium

For the reaction...
N_{2 (g)} +  3 H_2 (g) <=> 2 NH_{3 (g)}

If the pressure in the system is increased,  which substance(s) will increase in concentration?

• N₂  (as 2 mol gas  → 4 mol gas)

• H₂ (as 2 mol gas  → 4 mol gas)

• N₂ and H₂ (as 2 mol gas  → 4 mol gas)

• NH₃ (as 4 mol gas  → 2 mol gas)

Answer explanation

Increasing pressure shifts the equilibrium towards the side with fewer moles of gas. Here, 2 moles of NH3 are produced from 4 moles of N2 and H2. Thus, NH3 concentration increases.

What is the K_c expression for this reaction?
   2 NO_(g)  +  O_2(g) ⇌  2 NO_2(g)

• K_c = [NO₂]² / [NO]² [O₂]

• K_c =  [NO]² [O₂] / [NO₂]²

• K_c = [NO]² [O₂] [NO₂]²

• K_c = [NO₂]² / [NO]² +  [O₂]

Answer explanation

The equilibrium constant Kc is expressed as the ratio of the concentration of products to reactants. For the reaction 2 NO(g) + O2(g) ⇌ 2 NO2(g), Kc = [NO2]^2 / ([NO]^2[O2]), making the first choice correct.

An equilibrium constant with a large value, e.g. K_c = 1000, indicates…

• A very fast reaction

• Mostly products at equilibrium

• Mostly reactants at equilibrium

• Nothing useful at all

Answer explanation

A large equilibrium constant (Kc = 1000) indicates that at equilibrium, the concentration of products is much greater than that of reactants, meaning the reaction favors the formation of products.

Le Chatelier's Principle states that.... 
If a (dynamic) equilibrium is disturbed by changing the conditions, the position of equilibrium................

• moves to increase the change

• moves to counteract the change

• does not change

Answer explanation

Le Chatelier's Principle indicates that if a dynamic equilibrium is disturbed, the system will adjust to counteract the change. Therefore, the correct answer is that the position of equilibrium moves to counteract the change.

This symbol indicates that a reaction is ____________

• reversible

• irreversible

Answer explanation

This symbol indicates that a reaction can proceed in both forward and reverse directions, meaning it is reversible. In contrast, an irreversible reaction proceeds only in one direction.

Define chemical equilibrium.

• A reaction is reversible.

• The concentration of the reactants is equal to the concentration of the products.

• The rate of a forward reaction is equal to the rate of the reverse reaction.

• The reaction stops and no further change in concentration occurs.

Answer explanation

Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction, leading to constant concentrations of reactants and products, but not necessarily equal amounts.

Tags

NGSS.HS-PS1-6

If a reaction is reversible and has reached equilibrium, what can be said about the amounts of reactants and products?

• some reactant, some product

• no reactant, all product

• all reactant, no product

Answer explanation

In a reversible reaction at equilibrium, both reactants and products are present in some amounts. This means that the reaction does not go to completion, allowing for the coexistence of some reactant and some product.

Tags

NGSS.HS-PS1-6

If the Kc of a reaction is very small this means there is a higher concentration of ____ in the equilibrium mixture.

• reactants

• products

Answer explanation

A very small Kc value indicates that the equilibrium favors the reactants, meaning there is a higher concentration of reactants compared to products in the equilibrium mixture.

Le Chatelier's Principle states that.... 
If a (dynamic) equilibrium is disturbed by changing the conditions, the position of equilibrium................

• moves to increase the change

• moves to counteract the change

• does not change

Answer explanation

Le Chatelier's Principle indicates that if a dynamic equilibrium is disturbed, the system will adjust to counteract the change. Therefore, the correct answer is that the position of equilibrium moves to counteract the change.

For the reaction...
heat  +  N₂  +  O₂  <−>  2NO
If  O₂ is removed, the  concentration of N₂ will _______.

• increase

• decrease

• remain the same

• double

Answer explanation

Removing O₂ shifts the equilibrium to the left to produce more O₂, which consumes N₂ and increases its concentration. Thus, the concentration of N₂ will increase.

Tags

NGSS.HS-PS1-5