Problem 50 - General Chemistry

Question

Hydrogen cyanide is used in the manufacture of clear plastics such as Lucite and Plexiglas. It is prepared from ammonia and natural gas \(\left(\mathrm{CH}_{4}\right)\). $$ 2 \mathrm{NH}_{3}(g)+3 \mathrm{O}_{2}(g)+2 \mathrm{CH}_{4}(g) \longrightarrow 2 \mathrm{HCN}(g)+6 \mathrm{H}_{2} \mathrm{O}(g) $$ The reaction evolves \(939 \mathrm{~kJ}\) of heat per \(2 \mathrm{~mol} \mathrm{HCN}\) formed. Is the reaction endothermic or exothermic? What is the value of \(q ?\)

Step-by-Step Solution

Answer

The reaction is exothermic, with \( q = -939 \; \text{kJ} \).

Step 1: Identify Reaction Type

We need to determine whether the reaction is endothermic or exothermic. The problem states that the reaction evolves (releases) 939 kJ of heat per 2 moles of HCN. When a reaction releases energy in the form of heat, it is classified as exothermic.

Step 2: Evaluate Heat Change

Since the reaction is exothermic, the heat change, denoted as \( q \), is negative because the system releases energy to its surroundings. Therefore, the value for \( q \) is \(-939 \; \text{kJ}\) per 2 moles of HCN formed.

Step 3: Conclusion

The reaction is exothermic because it releases heat, and according to conventions for thermodynamic processes, a release of heat corresponds to negative values of \( q \). Hence, \( q = -939 \; \text{kJ} \).

Key Concepts

Exothermic Reactions

Exothermic reactions are a type of chemical reaction where energy is released into the surroundings, usually in the form of heat. These reactions are quite common and can often be identified through noticeable temperature increases in the environment where the reaction occurs. To consider a reaction as exothermic, it is important to note the energy balance:

• The total energy of the products is lower than the total energy of the reactants.
• This difference in energy is released, making it available to the surroundings.

In the reaction discussed in the exercise, the reaction of ammonia and methane to form hydrogen cyanide and water releases 939 kJ of heat. Because heat is released rather than absorbed, the reaction is confirmed as exothermic. In exothermic reactions, the value of the heat change, often denoted as \( q \), is negative, illustrating the loss of energy by the system.

Heat Transfer in Chemical Reactions

Heat transfer is a key component in understanding chemical reactions, as it involves the flow of thermal energy from one place to another. During a chemical reaction, bonds between atoms are broken and new bonds are formed. These changes can absorb or release energy, contributing to how heat is transferred. There are a few main points to consider:

• In exothermic reactions, heat is transferred from the system (the reactants) to the surroundings. This transfer causes the surroundings’ temperature to rise.
• When the heat transfer value \( q \) is positive, it indicates endothermic reactions, where heat is absorbed by the system.
• In our example, \( q = -939 \; \text{kJ} \) indicates an exothermic reaction, with heat transferring out of the system.

Understanding the direction of heat transfer can help predict the outcome and conditions of a reaction, as well as appropriately managing energy in practical applications.

Thermodynamic Processes

Thermodynamic processes are essential for predicting how energy will flow in chemical reactions. These processes focus on the principles governing energy changes, allowing us to characterize reactions as either endothermic or exothermic. For a clearer understanding:

• The first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed, only transformed. This forms the basis for calculating changes in energy as heat or work.
• In chemical reactions like the one in our exercise, the release of 939 kJ conforms to this principle, as energy is transformed and released as heat.
• In an exothermic reaction, thermodynamic processes are evidenced by negative \( q \), representing energy release to the surroundings.

Thermodynamic concepts help in calculating and predicting reaction conditions, guiding the design of industrial processes as depicted in real-world applications such as the production of hydrogen cyanide for plastics.