Question

The decomposition of zinc carbonate, \(\mathrm{ZnCO}_{3}(\mathrm{~s})\), into zinc oxide, \(\mathrm{ZnO}(\mathrm{s})\), and \(\mathrm{CO}_{2}(g)\) at constant pressure requires the addition of \(71.5 \mathrm{~kJ}\) of heat per mole of \(\mathrm{ZnCO}_{3}\) (a) Write a balanced thermochemical equation for the reaction. (b) Draw an enthalpy diagram for the reaction.

Short answer

The balanced thermochemical equation for the decomposition of zinc carbonate is: \[ \mathrm{ZnCO_3}(\mathrm{s}) \longrightarrow \mathrm{ZnO}(\mathrm{s}) + \mathrm{CO_2}(g) + 71.5 \, \mathrm{kJ} \] The enthalpy diagram shows an endothermic process where 71.5 kJ of heat is absorbed per mole of zinc carbonate decomposed, with the products having a higher enthalpy level than the reactants.

Step-by-step solution

Write the unbalanced equation for the reaction.

The unbalanced equation is given by: \[ \mathrm{ZnCO_3} \longrightarrow \mathrm{ZnO} + \mathrm{CO_2} \]

Balance the equation.

The equation is already balanced. We have 1 zinc atom, 1 carbon atom, and 3 oxygen atoms on both sides of the equation.

Write the thermochemical equation for the reaction.

The thermochemical equation includes the heat of the reaction, which is 71.5 kJ per mole of \(\mathrm{ZnCO_3}\): \[ \mathrm{ZnCO_3}(\mathrm{s}) \longrightarrow \mathrm{ZnO}(\mathrm{s}) + \mathrm{CO_2}(g) + 71.5 \, \mathrm{kJ} \] For part (b):

Identify the initial and final states.

The initial state consists of solid zinc carbonate, while the final state consists of solid zinc oxide and gaseous carbon dioxide.

Indicate the enthalpy levels.

Since the process absorbs heat (endothermic), the enthalpy level of the final state (products) is higher compared to the initial state (reactants).

Draw the enthalpy diagram for the reaction.

Draw a horizontal line for reactants representing the enthalpy state of the solid zinc carbonate. Then, draw another horizontal line above the first line, representing the enthalpy state of the products (solid zinc oxide and gaseous carbon dioxide). Label this energy difference as 71.5 kJ. Finally, draw an arrow from the reactant state to the product state, indicating it as an endothermic process. Enthalpy diagram: ``` enthalpy | | |--- 71.5 kJ --- ZnO(s) + CO2(g) | ZnCO3(s) | ``` In conclusion, the balanced thermochemical equation for the decomposition of zinc carbonate is: \[ \mathrm{ZnCO_3}(\mathrm{s}) \longrightarrow \mathrm{ZnO}(\mathrm{s}) + \mathrm{CO_2}(g) + 71.5 \, \mathrm{kJ} \] And the enthalpy diagram shows an endothermic process wherein 71.5 kJ of heat is absorbed per mole of zinc carbonate decomposed.

Key concepts

Enthalpy Diagram

An enthalpy diagram visually represents the energy changes during a chemical reaction. It's a useful tool for understanding how energy is absorbed or released. In the case of the decomposition of zinc carbonate,
the diagram would start with a horizontal line at a lower level labeled as ZnCO_{3}(s), symbolizing the reactants' enthalpy. Above this line, a higher-level line would be labeled with ZnO(s) + CO_{2}(g) to represent the products' enthalpy. The difference in height between these two lines reflects the energy change; in this case, an upward arrow labeled with 71.5 kJ shows that energy is absorbed by the system. As this reaction is endothermic, the products have higher enthalpy than the reactants, indicating that the surroundings lose heat to the system. This visualization assists students in grasping the concept of energy flow in chemical reactions.

Chemical Decomposition

Chemical decomposition, also known as analysis or breakdown, is the process in which a chemical compound is broken down into simpler substances. The decomposition of zinc carbonate, ZnCO_3, into zinc oxide, ZnO, and carbon dioxide gas, CO_2, is an example of such a process. It's crucial to remember that for the reaction to proceed, it must follow the conservation of mass and the conservation of atoms, meaning the number and type of atoms must remain constant before and after the reaction. In our scenario, the equation is already balanced with one zinc, one carbon, and three oxygen atoms on both sides. Understanding the balancing of equations is fundamental to solving decomposition reactions and interpreting the reaction stoichiometry can be a useful improvement exercise for students.

Endothermic Reaction

An endothermic reaction is a type of chemical process that absorbs heat from its surroundings, as opposed to releasing it. These reactions require energy input, often in the form of heat, to proceed. The decomposition of zinc carbonate (ZnCO_3) is an example of an endothermic reaction because it consumes 71.5 kJ of energy per mole from the environment. In a classroom setting, experimenting with endothermic reactions can enhance students' comprehension of concepts like reaction energetics and thermodynamics. It's also valuable for students to explore reaction conditions that affect the rate and extent of endothermic processes such as temperature, pressure, and catalyst presence.