Enthalpy: Crash Course Chemistry #18

Understanding Energy and Enthalpy

The Importance of Energy

• Energy is described as the foundation of the universe, essential for all work and heat, and it can be stored for later use.

• Despite its significance, energy is often taken for granted; understanding it better can improve our relationship with this fundamental concept.

Internal Energy vs. Enthalpy

• Previous discussions covered internal energy and its transfer methods: heat and work, which have limitations in measurement.

• Heat refers to energy transferred to atoms/molecules, while work pertains to energy transferred to moving objects; both are pathway dependent.

• The change in energy remains constant regardless of the pathway taken, making it a "state function" that simplifies calculations.

Measuring Changes in Energy

• While it's challenging to measure total energy within a molecule due to its vastness, changes in energy can be easily discussed as they are also state functions.

• For example, burning wood primarily concerns the heat generated rather than any work done by volume increase or smoke rising.

Introduction to Enthalpy

• Enthalpy (H) combines internal energy with pressure-volume work needed to accommodate surroundings; however, measuring total enthalpy is impractical.

• Instead of focusing on total enthalpy, we examine how much it changes during reactions at constant pressure—this leads us directly into calculating ΔH (change in enthalpy).

Key Assumptions in Calculating Enthalpy

• Two main assumptions simplify calculations: atmospheric pressure remains relatively constant at Earth's surface and only pressure-volume work is considered (W = -PΔV).

• An increase in volume results in negative internal energy output from the system; thus signs for work and PΔV will always oppose each other leading to ΔH equaling the heat of reaction.

Practical Applications of Enthalpy

• Understanding enthalpy allows us to determine useful energy contained within chemical compounds through temperature change measurements during reactions.

Understanding Enthalpy and Hess's Law

The Basics of Enthalpy and Calorimetry

• Atoms and molecules react, changing energy levels, which can be measured as heat released or absorbed from the environment. This process is quantified using enthalpy, measured through calorimetry.

• In calorimetry, reactions occur in insulated vessels (like thermoses), allowing scientists to measure temperature changes that correlate directly with heat and enthalpy changes.

The Significance of Hess's Law

• Germain Hess established a fundamental principle known as Hess's Law, stating that the total enthalpy change for a reaction depends only on its initial and final states, not the pathway taken. This concept is crucial for predicting reaction outcomes without extensive experimentation.

• To study reactions accurately, Hess examined standard enthalpies of formation for reactants and products under defined conditions (25°C and 1 atm). Standard enthalpy of formation measures heat change when one mole of a compound forms from its elements at their standard states.

Calculating Enthalpy Changes

• Absolute enthalpy cannot be determined; thus, measurements are based on a zero baseline defined by the most stable form of individual elements at standard state. This allows calculation of enthalpy changes for any reaction using standard values found in charts.

• The mathematical expression for Hess's Law indicates that the enthalpy change equals the sum of standard enthalpies of formation for products minus those for reactants. Each term accounts for moles involved in the reaction to ensure accurate calculations.

Practical Application: Hand Warmers

• An example illustrates how exothermic reactions can be harnessed practically; hand warmers utilize an iron powder reacting with oxygen to produce Iron(III) Oxide while releasing heat—calculated using ΔHf values from tables according to Hess’s Law.

• For this specific reaction involving four moles of iron powder (223 grams), it releases 1652 kilojoules of heat—demonstrating chemistry’s role in everyday life by providing warmth efficiently during cold weather conditions.

Key Takeaways from Today's Lesson