Question

For which of the following reactions would you expect the extent of the forward reaction to increase with increasing temperatures? Explain. (a) \(\quad \mathrm{NO}(\mathrm{g}) \rightleftharpoons \frac{1}{2} \mathrm{N}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \quad \Delta H^{\circ}=-90.2 \mathrm{kJ}\) (b) \(\quad \mathrm{SO}_{3}(\mathrm{g}) \rightleftharpoons \mathrm{SO}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \quad \Delta H^{\circ}=+98.9 \mathrm{kJ}\) (c) \(\mathrm{N}_{2} \mathrm{H}_{4}(\mathrm{g}) \rightleftharpoons \mathrm{N}_{2}(\mathrm{g})+2 \mathrm{H}_{2}(\mathrm{g}) \quad \Delta H^{\circ}=-95.4 \mathrm{kJ}\) (d) \(\mathrm{COCl}_{2}(\mathrm{g}) \rightleftharpoons \mathrm{CO}(\mathrm{g})+\mathrm{Cl}_{2}(\mathrm{g}) \quad \Delta H^{\circ}=+108.3 \mathrm{kJ}\)

Short answer

Reactions (b) and (d) will have an increased extent of the forward reaction with increasing temperatures because they are endothermic reactions.

Step-by-step solution

Identify Exothermic and Endothermic Reactions

We first need to identify which reactions are endothermic and which ones are exothermic. We can do this by looking at the sign of the \(\Delta H^{\circ}\) given in the reaction. If \(\Delta H^{\circ}\) is positive, the reaction is endothermic (absorbs heat), if it is negative, the reaction is exothermic (releases heat). (a) \(\Delta H^{\circ}=-90.2 \, \mathrm{kJ}\) is exothermic. (b) \(\Delta H^{\circ}=+98.9\, \mathrm{kJ}\) is endothermic. (c) \(\Delta H^{\circ}=-95.4\, \mathrm{kJ}\) is exothermic. (d) \(\Delta H^{\circ}=+108.3\, \mathrm{kJ}\) is endothermic.

Apply Le Chatelier’s Principle

Once we know which reactions are endothermic and exothermic, we can apply Le Chatelier’s Principle. This principle states that if an external condition (in this case, temperature) changes, the system will respond in a way that partially opposes that change. This means that increasing the temperature will favor the endothermic reactions (where heat is absorbed) in order to decrease the added temperature by absorbing part of it.

Determine the reactions with increased extent

From the application of Le Chatelier’s principle, we can see that the reactions with increased extent due to rising temperatures will be the endothermic reactions because they absorb heat. So, for reactions (b) and (d), which are endothermic, the forward reaction will increase with increased temperature.

Key concepts

Endothermic Reactions

Endothermic reactions are chemical processes that absorb energy from their surroundings in the form of heat. During these reactions, the system's temperature appears to decrease as the process pulls in energy to proceed. The hallmark sign of an endothermic reaction is a positive enthalpy change \( \Delta H^\circ > 0 \), indicating that heat is taken in.For instance, when ammonium nitrate dissolves in water, it absorbs heat from the water, resulting in a temperature drop of the solution. This principle can be vividly experienced with instant cold packs, which utilize an endothermic reaction to provide cooling on demand. In the context of Le Chatelier's Principle, raising the temperature of a system undergoing an endothermic reaction will drive the equilibrium to shift in the direction of the heat-absorbing, forward process. This is because the additional heat supplied adds to the energy required for the endothermic change, thereby increasing the extent of the reaction.

• An increase in temperature induces the endothermic reaction to absorb more heat, favoring the forward reaction.
• Endothermic reactions are characterized by a positive \( \Delta H^\circ \).

Exothermic Reactions

Conversely, exothermic reactions release energy to their surroundings, typically in the form of heat, which causes the temperature of the nearby environment to go up. The signature of an exothermic process is a negative enthalpy change \( \Delta H^\circ < 0 \), which indicates heat is being expelled into the environment.A common example of an exothermic reaction is the combustion of fuels like gasoline, which produces heat and light. In a classroom setting, students might witness an exothermic reaction when combining vinegar and baking soda, where the temperature of the surrounding solution increases. According to Le Chatelier's Principle, if the temperature is increased around an exothermic reaction, the system will shift equilibrium in favor of the reverse reaction to alleviate the stress of the added heat.

• Exothermic reactions release heat, causing a shift in equilibrium towards reactants if temperature is increased.
• These reactions have a negative \( \Delta H^\circ \).

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal in a closed system, meaning that no net change in the concentrations of the reactants and products takes place over time. At equilibrium, the reaction hasn't stopped; instead, both the forward and reverse processes are occurring simultaneously but at identical rates, creating a balance.It's essential for students to understand this dynamic process when discussing how temperature changes can influence chemical reactions. Le Chatelier's Principle provides insight into this by stating that if a system at equilibrium is disturbed by changes in temperature, pressure, or concentration, the system will adjust to counteract the disturbance. It is important to relate this concept back to endothermic and exothermic reactions because changes in temperature will respectively shift the equilibrium to favor the reaction that absorbs or releases heat, maintaining the principle of dynamic balance.

• Chemical equilibrium is dynamic and can be shifted by changing conditions.
• Temperature changes can shift equilibrium in favor of endothermic or exothermic reactions based on Le Chatelier's Principle.

Enthalpy Change

Enthalpy change \( \Delta H^\circ \) refers to the amount of heat absorbed or released by a system at constant pressure during a chemical reaction. It is a central concept in thermodynamics and plays a key role in predicting the energy changes occurring in chemical processes. A positive \( \Delta H^\circ \) indicates an endothermic reaction, while a negative value shows an exothermic reaction.Understanding enthalpy is crucial when applying Le Chatelier's Principle to predict how a reaction system will respond to changes in temperature. If enthalpy change is not explicitly given, it can be deduced from the bond energies of the reactants and products. This concept ties directly to the textbook problem, where enthalpy changes determine the direction in which equilibrium shifts when temperatures are varied. Mastering the understanding of enthalpy changes allows students to make educated predictions about the behavior of reactions under different conditions.

• Enthalpy change signifies the heat absorbed or released by a reaction at constant pressure.
• Positive or negative \( \Delta H^\circ \) are indicative of endothermic and exothermic reactions, respectively.