Problem 52 - General Chemistry

Question

When 2 mol of potassium chlorate crystals decompose to potassium chloride crystals and oxygen gas at constant temperature and pressure, \(78.0 \mathrm{~kJ}\) of heat is given off. Write a thermochemical equation for this reaction.

Step-by-Step Solution

Answer

The thermochemical equation is: \( 2 \text{KClO}_3 (s) \rightarrow 2 \text{KCl} (s) + 3 \text{O}_2 (g) \), \( \Delta H = -78.0 \text{ kJ} \).

Step 1: Understanding the Chemical Equation

The chemical reaction for the decomposition of potassium chlorate (\( \text{KClO}_3 \)) into potassium chloride (\( \text{KCl} \)) and oxygen gas (\( \text{O}_2 \)) is: \[ 2 \text{KClO}_3 (s) \rightarrow 2 \text{KCl} (s) + 3 \text{O}_2 (g) \]. This equation shows that 2 moles of potassium chlorate decompose to produce 2 moles of potassium chloride and 3 moles of oxygen gas.

Step 2: Identify the Heat Involved

According to the problem statement, 78.0 kJ of heat is given off during this decomposition. This information tells us that the reaction is exothermic, as heat is released.

Step 3: Writing the Thermochemical Equation

A thermochemical equation includes the balanced chemical equation and the enthalpy change (\( \Delta H \)) of the reaction. Since heat is given off and the reaction is exothermic, \( \Delta H \) is negative. We write this as: \[ 2 \text{KClO}_3 (s) \rightarrow 2 \text{KCl} (s) + 3 \text{O}_2 (g) \quad \Delta H = -78.0 \text{ kJ} \]

Key Concepts

Chemical Decomposition

Chemical decomposition, also known as chemical breakdown, is a process where a compound breaks apart into two or more smaller compounds or elements. It is akin to taking a large molecule and breaking it down into its building blocks.

In the context of our exercise, the compound involved is potassium chlorate (\(\text{KClO}_3\)). When it decomposes, it breaks down into potassium chloride (\(\text{KCl}\)) and oxygen gas (\(\text{O}_2\)). Thus, during the decomposition:

• 2 moles of potassium chlorate yields 2 moles of potassium chloride and 3 moles of oxygen gas.

Chemical decomposition reactions are essential in various fields, including chemistry and industrial manufacturing, as they allow the extraction of useful materials and substances from complex compounds.

Exothermic Reactions

Exothermic reactions are a type of chemical reaction where energy is released to the surroundings, usually in the form of heat. This results in the surrounding temperatures increasing. You'll often feel these reactions as they release heat.

The decomposition of potassium chlorate in our exercise is exothermic because it releases 78.0 kJ of energy. This release of heat is typically indicated in thermochemical equations with a negative enthalpy change (\(\Delta H\)).

• Exothermic reactions are common in combustion processes, such as when burning wood.
• They are also found in chemical hand warmers or self-heating cans.

Recognizing exothermic reactions is crucial as they often provide energy for further reactions or applications.

Enthalpy Change

Enthalpy change (\(\Delta H\)) is the heat change that occurs during a chemical reaction at constant pressure. It is a measure of the total energy gained or lost by a system. In essence, it tells us how much heat is absorbed or released during a reaction.

In exothermic reactions, like the decomposition of potassium chlorate, the enthalpy change is negative because heat is released. This concept is crucial in understanding how energy flows in chemical processes, which can affect everything from the stability of products to the feasibility of industrial processes.

• Negative \(\Delta H\) values indicate energy release (exothermic).
• Positive \(\Delta H\) values show energy absorption (endothermic).

Being able to calculate and interpret \(\Delta H\) helps chemists and engineers design reactions that are safe and efficient in terms of energy use.

Chemical Reactions

Chemical reactions involve the transformation of one set of substances known as reactants into another set of substances, known as products. During a chemical reaction, bonds between atoms in the reactants are broken and new bonds are formed to create the products.

In our example, the chemical reaction is the decomposition of potassium chlorate into potassium chloride and oxygen gas:

• The balanced equation is \(2 \text{KClO}_3 (s) \rightarrow 2 \text{KCl} (s) + 3 \text{O}_2 (g)\).

A balanced equation is crucial because it ensures that the law of conservation of mass is obeyed, meaning the number of atoms of each element is the same on both sides of the equation. Understanding chemical reactions is fundamental in chemistry, as they describe how substances interact and change, impacting everything from energy production to the development of new materials.