Problem 46 - Chemistry The Molecular Science

Question

Calcium carbide, \(\mathrm{CaC}_{2}\), is manufactured by reducing lime with carbon at high temperature. (The carbide is used in turn to make acetylene, an industrially important organic chemical.) $$ \begin{aligned} \mathrm{CaO}(\mathrm{s})+3 \mathrm{C}(\mathrm{s}) \longrightarrow \mathrm{CaC}_{2}(\mathrm{~s})+\mathrm{CO}(\mathrm{g}) \\ \Delta_{\mathrm{r}} H^{\circ}=464.8 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$ Is the reaction endothermic or exothermic?

Step-by-Step Solution

Answer

The reaction is endothermic.

Step 1: Understand the Reaction

We are given a chemical reaction converting lime (CaO) and carbon (C) into calcium carbide (CaC₂) and carbon monoxide (CO). We are also given the standard enthalpy change, \( \Delta_{\mathrm{r}} H^{\circ} = 464.8 \, \mathrm{kJ/mol} \).

Step 2: Identify the Sign of Enthalpy Change

The enthalpy change \( \Delta_{\mathrm{r}} H^{\circ} \) is positive, meaning it requires an input of energy to proceed. This indicates the reaction absorbs heat from the surroundings.

Step 3: Determine Reaction Type

Since the enthalpy change is positive, the reaction is endothermic. In an endothermic reaction, heat is absorbed from the environment to proceed.

Key Concepts

Calcium Carbide

Calcium carbide, chemically expressed as \( \text{CaC}_2 \), is a significant compound in the chemical industry. Its primary use is in the production of acetylene gas, which is vital for processes such as welding and the manufacture of plastics. The production of calcium carbide involves a reaction between lime (\( \text{CaO} \)) and carbon (\( \text{C} \)) at high temperatures. This reaction is key in converting readily available raw materials into a valuable industrial product.

• Raw Materials: Lime (calcium oxide) and carbon are the main reactants.
• Synthesis Environment: High temperature is crucial to facilitate the reaction.
• Products: The process yields calcium carbide and carbon monoxide (\( \text{CO} \)).

The significance of calcium carbide extends beyond acetylene production as it also finds utility in steelmaking and as a reducing agent in other chemical reactions.

Endothermic Reaction

An endothermic reaction is characterized by the absorption of heat from its surroundings. This means that for the reaction to proceed, it requires an input of energy. In the case of the synthesis of calcium carbide, the reaction is endothermic.
The positive sign of the enthalpy change \( (\Delta_{\text{r}} H^{\circ} = 464.8 \text{ kJ/mol}) \) indicates that energy is absorbed during the reaction. Here are some key points about endothermic reactions:

• Energy Intake: These reactions absorb energy in the form of heat.
• Temperature Impact: The surrounding environment may feel cooler as a result.
• Reaction Requirement: High energy input is often needed to initiate and sustain the reaction.

In practical applications, understanding whether a reaction is endothermic or exothermic helps in designing industrial processes that efficiently manage energy use.

Standard Enthalpy Change

Standard enthalpy change, denoted as \( \Delta_{\text{r}} H^{\circ} \), is a measure of the heat absorbed or released during a reaction at standard conditions. For reactions like the formation of calcium carbide, knowing the enthalpy change gives critical insight into the energy dynamics of the process.

• Standard Conditions: This refers to a pressure of 1 atm and a temperature of 298.15 K (25 °C).
• Heat Measurement: A positive \( \Delta_{\text{r}} H^{\circ} \) signals that the reaction absorbs heat.
• Reaction Insights: Calculating the enthalpy change helps predict whether a reaction will proceed without continuous heat addition.

It is an essential concept in thermochemistry, as it enables chemists to quantify the energy demands of chemical reactions and optimize conditions for industrial applications.