Problem 2 - Chemistry Matter and Change

Question

An exothermic reaction releases 86.5 kJ. How many kilocalories of energy are released?

Step-by-Step Solution

Answer

The exothermic reaction releases \(20.68 \,kcal\) of energy.

Step 1: Identify the given value and conversion factor

The given value is the released energy in kilojoules, which is 86.5 kJ. The conversion factor between kilojoules and kilocalories is 1 kcal = 4.184 kJ.

Step 2: Convert kilojoules to kilocalories using the conversion factor

Now we will convert the energy released from kilojoules to kilocalories by multiplying the released energy in kilojoules by the conversion factor. Energy released in kilocalories (kcal) = Energy released in kilojoules (kJ) × \(\frac{1 \,kcal}{4.184 \,kJ}\) Energy released in kcal = 86.5 kJ × \(\frac{1 \,kcal}{4.184 \,kJ}\)

Step 3: Calculate the energy released in kilocalories

Now, we perform the calculation. Energy released in kcal = 86.5 kJ × \(\frac{1 \,kcal}{4.184 \,kJ}\) = \(20.68 \,kcal\) Therefore, the exothermic reaction releases 20.68 kilocalories of energy.

Key Concepts

Exothermic Reaction

Exothermic reactions are fascinating processes where energy is released into the surrounding environment. This typically occurs in the form of heat. Essentially, during an exothermic reaction, the total energy required to break bonds in the reactants is less than the energy released when new bonds form in the products.
Exothermic reactions are common in many everyday occurrences and industrial processes. For example:

• Combustion of fuels like wood and gasoline.
• Mixing water with strong acids or bases.
• Respiration in living organisms.

These reactions not only generate warmth, making them crucial for heating but also play a key role in energy production. They can power vehicles and machines, ensuring functionality and efficiency. Understanding the energy changes in these reactions is an integral part of thermochemistry and its practical applications.

Kilojoules to Kilocalories

Energy can be expressed in different units depending on the context, with joules being part of the metric system while calories are often used in nutrition. To convert energy from kilojoules (kJ) to kilocalories (kcal), we use a straightforward conversion factor. The relationship between the two units is:

• 1 kilocalorie (kcal) = 4.184 kilojoules (kJ)

Using this conversion factor, we can convert energy values from one unit to another. For example, if an exothermic reaction releases 86.5 kJ of energy, multiplying by the conversion factor \(\frac{1 \, \text{kcal}}{4.184 \, \text{kJ}}\) converts it to kilocalories, giving us approximately 20.68 kcal.
This conversion is especially useful in contexts where dietary energy content or biochemical energy changes need correlation with physical science data.

Thermochemistry

Thermochemistry is the branch of chemistry that deals with the study of heat energy changes that accompany chemical reactions and physical transformations. It connects temperature and energy changes, exploring how and why reactions occur. When analyzing reactions, thermochemistry helps determine whether a process is exothermic or endothermic.
Key aspects of thermochemistry include:

• Heat flow: Measuring the amount of heat involved during reactions.
• Enthalpy changes: Understanding the intrinsic energy changes at constant pressure.
• Calorimetry: A technique for measuring energy changes primarily through the use of an instrument called a calorimeter.

By investigating these factors, we can predict whether a reaction will release or absorb energy. This is crucial in industries where energy efficiency and resource management are important. Moreover, thermochemical principles are central to developing renewable energy solutions, helping society meet growing energy demands sustainably.