Question

Explain why the percent of molecules that dissociate into atoms in reactions of the type \(\mathrm{I}_{2}(\mathrm{g}) \rightleftharpoons 2 \mathrm{I}(\mathrm{g})\) always increases with an increase in temperature.

Short answer

The percentage of molecules that dissociate into atoms in reactions like \(I_{2}(g) \rightleftharpoons 2I(g)\) increases with an increase in temperature due to an increase in kinetic energy, leading to more frequently and vigorously colliding particles. According to Le Chatelier's Principle, the system favors the endothermic process (dissociation in this case) to counteract the increased temperature, hence more molecules dissociate to absorb the excess heat.

Step-by-step solution

Understanding the Reaction

The reaction given is \(I_{2}(g) \rightleftharpoons 2I(g)\). This is a dissociation reaction where a single iodine molecule (\(I_{2}\)) is breaking down into two iodine atoms (2\(I\)). It's important to note that this is a reversible reaction, as indicated by the double arrow (\(\rightleftharpoons\)). This means that the reaction can proceed in both the forward (dissociation) and the reverse (association) directions.

Le Chatelier's Principle

Per Le Chatelier's Principle, if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium will shift to counteract the change. In this case, an increase in temperature adds more kinetic energy to the system, favoring the endothermic (heat-absorbing) process.

Effect of Temperature on Dissociation

Increasing the temperature increases the kinetic energy of the molecules, causing them to move faster. This increased motion leads to more frequent and energetic collisions between particles, which can provide the energy required for bonds in the iodine molecule to break, causing dissociation. As a result, a higher percentage of molecules dissociate into atoms at higher temperatures, in order to absorb the excess energy. This reaction being endothermic is supported by the nature of the process, as breaking bonds requires energy absorption. Consequently, the position of equilibrium shifts to the right, favoring more dissociation.

Key concepts

Le Chatelier's Principle

When we explore the factors affecting chemical reactions, Le Chatelier's Principle is fundamental. This principle describes how a dynamic equilibrium, such as the one present in the reversible iodine dissociation reaction, responds to external changes to maintain balance. If a system at equilibrium is disturbed, it will adjust to minimize that disturbance.

Imagine you're at a dance and the floor is evenly crowded. Suddenly, one side of the floor becomes overly cramped. Dancers will naturally shift to the less crowded side to regain their space. Similarly, in our iodine reaction, increasing temperature is like adding more dancers to one side. The reaction 'shifts' by creating more iodine atoms, thus balancing the overall 'crowd'. This shifting is the reaction's dance to the tune of Le Chatelier's Principle.

Endothermic Reactions

To understand temperature effects on chemical equilibrium, it's essential to grasp the heat exchange in endothermic reactions. An endothermic reaction is like a heat sponge, soaking up energy from its surroundings. These reactions require energy to proceed, essentially absorbing heat to break bonds.

In the case of the dissociative iodine reaction, when we input more thermal energy (heat) by increasing the temperature, the system uses this energy to favor the endothermic reaction direction, breaking iodine molecules into more reactive iodine atoms. This thermally driven process can be visualized as a hungry bear (the reaction) that eats up heat (food) to fuel the breaking apart of molecules (exercise).

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in no net change in the amounts of reactants and products. It's a state of balance, but not necessarily equality in terms of concentration. Think of it as two kids on a seesaw who are moving up and down at the same rate. They are balanced but may not be the same weight.

With the iodine reaction, an increase in temperature upsets this seesaw by increasing the rate of the forward (endothermic) reaction. As the process consumes more heat, the equilibrium shifts to favor the formation of iodine atoms, until a new balance is found where the forward and reverse reactions are now equal at this higher temperature.

Dissociation of Molecules

Dissociation of molecules occurs when bonds within a compound break, often resulting in the formation of smaller particles. Think of it like breaking a chocolate bar into smaller pieces. The iodine reaction (\(I_{2}(g) \rightleftharpoons 2I(g)\)) is a classic example, where the iodine molecule (\(I_{2}\) in the form of a gas) splits into two iodine atoms.

The amount of energy needed to break these molecular bonds is significant, which is why an increase in temperature, supplying more energy, enhances the dissociation process. It's like applying more force to snap the chocolate bar into more pieces. In the case of iodine, at higher temperatures, you witness a greater fraction of molecules breaking apart to form iodine atoms, which is the essence of this chemical equilibrium being temperature-sensitive.