Energy, Work, and Heat in Chemical Reactions

Let's dive into the fascinating world of energy! Energy is a system's ability to perform work or generate heat. When it comes to chemistry, energy changes are seen everywhere! In this study guide, we will explore what energy, work, and heat are. Along with some practice questions regarding these topics.

🔋 Types of Energy

• Kinetic Energy: Get ready for some action! Atoms and molecules are like little dancers, always on the move and filled with kinetic energy. 🕺🏿
• Potential Energy: This energy is stored based on where things are or how they're put together. Chemical bonds act like the guardians, holding onto this potential energy. 🫙
• Chemical Energy: A form of potential energy held within chemical bonds. Imagine a secret energy treasure tucked away, waiting for the right moment to shine! ✨

In the left image, a girl stands on a ledge, tightly gripping a ball, representing potential energy. The right image shows the ball dropped, symbolizing the conversion of potential energy to kinetic energy.

Image Courtesy of Sciencelearn.org

📐 Units of Energy

• The SI unit of energy is the Joule (J).
• Another common unit is the calorie (cal), where 1 cal = 4.184 J.

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🛠️ Work and Chemical Reactions

Work (w) refers to the transfer of energy that results when a force moves an object.

$$w =−P⋅ΔV$$

In chemistry:

• Work done BY a system (gas expanding) means the system loses energy.
• Work done ON a system (gas compressed) means the system gains energy.

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↔️ Heat Transfer in Reactions

Heat (q) is like the thermal energy's travel agent, moving from one place or system to another when there's a difference in temperature.

🔥 Exothermic Reaction: It's the heat-blasting superstar (ΔH < 0) that throws warmth into the surroundings.

• Example: Think of combustion reactions setting things on fire.

🧊 Endothermic Reaction: This one's the heat magnet (ΔH > 0) that snatches warmth from its surroundings.

• Example: Imagine photosynthesis soaking up sunlight like a plant sunbathing.

A graphic showing the comparison between endothermic and exothermic reactions, showing energy being absorbed or released during chemical reactions.

Image Courtesy of ThoughtCo

Enthalpy Change (ΔH)

This equation helps us figure out if a reaction is throwing heat around or absorbing it.

$$ΔH = H_{\text{products}} - H_{\text{reactants}}$$

Internal Energy (U)

Think of internal energy (U) as the total energy happening inside a system.

$$ΔU = q + w$$

Here, think of ΔU as the system's energy scoreboard. When it's positive, it's like the system is gaining energy points (endothermic), and when it's negative, it's as if the system is giving away some energy points (exothermic). 🎮 ⚡️

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⚖️ Relationship Between Work, Heat, and Energy

So now that we know about work, heat, and energy, let’s see how they are all connected!

Work-Energy Principle

The change in an object's kinetic and potential energies is exactly equal to the work done on it or by it.

Pressure-Volume Work (P-V Work)

When a gas either expands or contracts against an external pressure, we call it pressure-volume work.

$$w = -P\Delta V$$

Left cylinder is state 1 (initial) with Volume 1 and before the piston is pushed in, and on right cylinder is state 2 (final) with Volume 2 which is lower than Volume 1 & after piston pushed in. Work is done on the system in this example.

Image Courtesy of Ch301

✏️ Practice Question

Suppose you have a sample of gas in a piston-cylinder arrangement. The initial conditions are as follows:

Consider a gas confined within a piston-cylinder system. The external pressure ($-P$) acts on the gas, causing it to undergo a volume change ($ΔV$). The work done by the external force can be calculated using the equation:

Let's say the external pressure is 5 atm and the volume of the gas increases by 10 L.

Using the formula, we can find the work done, by plugging in the values given:

$$w = -P\Delta V$$ $$w = −(5 atm)×(10 L)$$ $$w = −50atm⋅L$$

Therefore, the work done by the external force is -50 atm·L. The negative sign tells us that work is done on the system by the external force, leading to a decrease in the system's internal energy.

Heat Capacity

Do you feel that? It’s starting to get warm because we’re talking about heat next! Heat capacity is all about the amount of heat needed to raise an object's temperature by 1°C. And for more precision, there's specific heat capacity, which breaks it down per unit mass. 🔥🚀

$$Q=mcΔT$$

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💡 Enthalpy (H) and Thermochemistry

Enthalpy is like the heat content when the pressure stays constant. It's simpler to measure than changes in internal energy because many reactions take place at constant pressure.

Calculating ΔH

$$ΔH = Σ(ΔH°f_{products}) - Σ(ΔH°f_{reactants})$$

✏️ Practice Question

Consider the combustion of methane (CH₄) in the presence of oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O):

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

Given the standard enthalpies of formation (ΔHᶠ⁰) for the substances involved:

• ΔHᶠ for CH₄(g): -74.81 kJ/mol
• ΔHᶠ for O₂(g): 0 kJ/mol
• ΔHᶠ for CO₂(g): -393.51 kJ/mol
• ΔHᶠ for H₂O(g): -241.82 kJ/mol

Calculate the standard enthalpy change (ΔH⁰) for the given reaction.

Solution:

$$ΔH = Σ(ΔH°f_{products}) - Σ(ΔH°f_{reactants})$$

If we plug in the values given into the equation, we get:

$ΔH=[(−393.51kJ/mol)+2(−241.82kJ/mol)]−[(−74.81kJ/mol)+2(0kJ/mol)]$

$ΔH=(−393.51+2(−241.82))−(−74.81+0)$

This results in: $ΔH\degree = -876.15 kJ/mol$

Therefore, the standard enthalpy change for the combustion of methane is -876.15 kJ/mol. The negative sign indicates that it is an exothermic reaction, releasing heat to the surroundings.

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🌡️ Calorimetry and Measuring Heat Changes

Calorimetry is the science of measuring the heat exchanged during chemical reactions, and it does so with the help of calorimeters.

A thermometer stuck in an insulated stopper in a nested insulated cup and being utilized to gauge the temperature of a reaction mixture, allowing us to determine its degree of hotness or coldness.

Image Courtesy of LibreTexts.

There are two main types:

1. Constant-pressure calorimetry – This method gauges enthalpy changes while keeping the pressure consistent with atmospheric conditions.
2. Bomb calorimetry – This technique measures changes in internal energy under conditions where volume remains constant.

In terms of experiments, you might have to calculate ΔH using a different way:

$$q = c_p m \Delta T$$

• Cp represents the specific heat capacity
• m is the mass involved
• ΔT signifies the change in temperature.

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⭐ Closing

Whether you're tackling calorimetry problems or diving into conceptual questions about P-V work, diagrams are your secret weapon. They make understanding these processes a breeze. Happy studying, and keep embracing the awesome world of chemistry—it is everywhere around us! ✨