Question

The position of the equilibrium for the formation of ammonia is product- favored at \(25^{\circ} \mathrm{C}\), but reactant-favored at \(500^{\circ} \mathrm{C}\). Explain why the position of the equilibrium changes.

Short answer

The shift in the equilibrium position of the formation of Ammonia from being product-favored at 25 degrees Celsius to reactant-favored at 500 degrees Celsius is due to Le Chatelier's principle. The formation of Ammonia is an exothermic reaction that releases heat. Therefore when the temperature is increased, the system tries to offset it by shifting the equilibrium to the reactants side, hence it is reactant favored at 500 degrees Celsius. Conversely, at 25 degrees Celsius, it's product-favored because the system tries to produce more heat by moving towards the products side.

Step-by-step solution

Understanding the principle of Le Chatelier

According to Le Chatelier's principle, if a change occurs in the conditions to which a system in equilibrium is exposed, the system adjusts itself to offset the change and restore the equilibrium. In chemical reactions, such changes can be in concentration, temperature, or pressure.

Analysing Reaction

In this specific case, we are dealing with the formation of ammonia (NH3), which is an exothermic reaction (it releases heat). This means that the reaction can be written as: \( N_{2(g)} + 3H_{2(g)} ⇌ 2NH_{3(g)} + heat \)

Explaining the Shift in Equilibrium

At 25°C, the lower temperature, the equilibrium shifts to the right, or towards the products as it is trying to counteract the 'lack' of heat and generate more. Thus, it is product-favored. However, at the higher temperature of 500°C, the equilibrium shifts to the left to offset the 'excess' heat in the system. In trying to absorb some of the heat, it favors the reactants and hence, it becomes reactant-favored.

Key concepts

Equilibrium Shift

Imagine a seesaw in perfect balance. If someone adds more weight to one side, the seesaw will tip in that direction until someone adjusts the weights to reach a balance again. This is similar to an equilibrium shift in chemistry.

When a chemical reaction reaches equilibrium, there's a stable ratio of reactants to products; neither is being produced or consumed rapidly. If something disturbs the system, like a change in temperature, pressure, or concentration, the system will adjust to compensate.

For example, if we increase the temperature, the equilibrium position of an exothermic reaction will shift in the direction that absorbs heat (toward the reactants), in order to counteract the change. This shift allows the system to regain balance, adhering to Le Chatelier's principle which states that a system at equilibrium reacts to stresses in a way that seeks to counteract or buffer those changes.

Exothermic Reaction

An exothermic reaction is like a warm hug on a cold day—it releases heat. In chemical terms, these reactions give off energy to their surroundings, often in the form of heat, making the environment warmer.

An example of this can be seen during the combustion of wood in a fireplace. When the wood burns, it releases heat into the room. This release of energy is what characterizes exothermic reactions. In the context of ammonia formation, the reaction of nitrogen and hydrogen to produce ammonia is exothermic because heat is a product of the reaction.

Ammonia Formation

The creation of ammonia from nitrogen and hydrogen gases is an essential process, famously known as the Haber process. This reaction is critical in producing fertilizers and various chemicals.

The balanced equation for ammonia formation looks like this:
\[ N_{2(g)} + 3H_{2(g)} \rightarrow 2NH_{3(g)} \]
Here, nitrogen (N_2) and hydrogen (H_2) gases react to form ammonia (NH_3), and in the process, release heat. This is why this reaction is not just important for industrial applications but also an excellent illustration of how exothermic reactions work.

Chemical Equilibrium

The term chemical equilibrium refers to a state of balance in a chemical reaction where the rates of the forward reaction (reactants turning into products) and the reverse reaction (products reverting to reactants) are equal.

In this balanced state, the amounts of reactants and products remain constant over time, not because the reactions have stopped, but because they continue at identical rates. Imagine a room where two doors open at the same rate people enter through one door and leave through another. The number of people in the room stays the same if the in-and-out flow rate is equal.

So, when we discuss the equilibrium of ammonia formation at different temperatures, what we’re really talking about is the rate at which nitrogen and hydrogen combine to form ammonia, compared to the rate at which ammonia breaks down into nitrogen and hydrogen.