Enthalpy: Crash Course Chemistry #18

CrashCourse
17 Jun 201311:23
EducationalLearning
32 Likes 10 Comments

TLDRThis video explains the concept of enthalpy in chemistry, which measures the total heat content of a system and the energy required to create a system. It allows chemists to calculate the energy change in chemical reactions. We learn about state functions like internal energy and enthalpy, which depend only on the initial and final states, not the pathway. Using enthalpy instead of internal energy simplifies calculations. The video discusses how to calculate enthalpy changes using Hess's Law and standard enthalpies of formation. An example shows how this allows inventors to choose chemical reactions to produce desired amounts of heat, like for hand warmers.

Takeaways
  • ๐Ÿ˜ƒ Energy powers the universe but we often take it for granted. We should appreciate and understand it more.
  • ๐Ÿ”‹ Chemical bonds contain energy that can be released as heat during reactions.
  • ๐Ÿ”ฅ Enthalpy measures heat changes during reactions at constant pressure.
  • ๐Ÿ“ˆ Enthalpy change depends only on the initial and final states, not the reaction pathway.
  • ๐Ÿงช Calorimetry allows measuring temperature changes to determine enthalpy changes.
  • ๐Ÿ“Š Hess's Law relates enthalpy change to standard enthalpies of formation.
  • ๐Ÿค“ Standard enthalpy of formation measures heat change when 1 mole of compound forms.
  • ๐Ÿงฎ Use Hess's Law to calculate enthalpy change from standard enthalpies of formation.
  • โฑ Iron reacting with oxygen in a hand warmer releases heat slowly to warm hands.
  • ๐Ÿ™Œ Chemistry allows predicting exact heat produced in chemical reactions.
Q & A

  • What is enthalpy?

    -Enthalpy is a state function that accounts for the internal energy of a system plus the energy required to make room for the system's volume against external pressure.

  • Why is enthalpy useful in chemistry?

    -Enthalpy allows us to easily calculate the amount of heat energy produced or absorbed in a chemical reaction at constant pressure, which applies to most reactions on Earth's surface.

  • What assumptions allow us to derive the enthalpy change equation?

    -We assume constant pressure conditions and that the only work done is pressure-volume work. This allows the pressure-volume terms to cancel out, leaving only heat.

  • What is calorimetry and how is it used to measure enthalpy?

    -Calorimetry involves monitoring the temperature change of a reaction carried out in an insulated container. This allows direct linkage between the measured heat and enthalpy change.

  • What is a standard state in chemistry?

    -The standard state refers to conditions of 25ยฐC and 1 atmosphere. This allows consistent comparison of thermodynamic values for different substances.

  • What is standard enthalpy of formation?

    -It is the enthalpy change associated with forming one mole of a compound from its constituent elements in their standard states.

  • What does Hess's Law state?

    -Hess's Law states that the total enthalpy change for a reaction depends only on the initial and final states, not the reaction pathway.

  • How can we use Hess's Law to calculate enthalpy changes?

    -We can look up standard enthalpies of formation for reactants and products and plug them into Hess's Law to derive the total reaction enthalpy change.

  • What practical application utilizes calculated enthalpy changes?

    -Hand warmers work by using an iron oxidation reaction with a known highly exothermic enthalpy change to provide heat.

  • Why are state functions like enthalpy useful?

    -State functions like enthalpy only depend on the initial and final states, not the pathway. This simplifies calculations of changes between states.

Outlines
00:00
๐Ÿ˜ƒ Introduction: Overview of key energy concepts

The first paragraph introduces the concept of energy as the foundation of the universe that enables all work and heat. It notes that energy is conserved and transformed, existing in chemical bonds. The limitations of heat and work as energy transfer mechanisms are discussed, motivating the use of the state function enthalpy.

05:01
๐Ÿ˜ฏ Explaining importance and applications of enthalpy

The second paragraph explains why enthalpy is useful, allowing the energy change in chemical reactions to be calculated by measuring heat. It introduces calorimetry for experimentally determining enthalpy changes. The work of Germain Hess in defining standard enthalpies of formation is described, enabling calculation of reaction enthalpies using Hess's Law.

10:02
๐Ÿ˜€ Demonstrating calculation of reaction enthalpy

The third paragraph provides a detailed example applying Hess's Law to calculate the enthalpy change for the reaction in a hand warmer. It shows step-by-step how the standard enthalpies of formation are used with the balanced chemical equation to determine the heat released.

Mindmap
Keywords
๐Ÿ’กEnthalpy
Enthalpy is a measure of the total heat content of a thermodynamic system. It represents a state function that can be used to calculate the amount of heat exchanged in a reaction happening at constant pressure, which is the case on Earth's surface. In the video, enthalpy change is shown to be equal to the heat gained or lost by the system, which makes it very useful for studying chemical reactions without having to consider more complex terms of internal energy and heat/work calculations.
๐Ÿ’กCalorimetry
Calorimetry is the science of measuring heat changes during chemical reactions. As explained in the video, calorimetry involves having a reaction take place inside an insulated container and measuring the temperature change to calculate enthalpy changes. The data from calorimetry experiments allows predicting the amount of heat different reactions will produce.
๐Ÿ’กEnergy
Energy is referred to as the foundation of the universe in the script. It enables all work and heat transfers. The video emphasizes chemical energy stored in molecular bonds and how enthalpy allows measuring bond energy changes when those chemical bonds are formed or broken in reactions.
๐Ÿ’กBonds
Bonds connect atoms in molecules and represent stored chemical energy. As stated in the video, bonds are literally energy. When bond formation/breakage occurs during a chemical reaction, energy is absorbed from or released to the surroundings, allowing calculation of enthalpy change.
๐Ÿ’กHess's Law
Hess's Law is a key principle stating that total enthalpy change for a reaction only depends on the initial and final state, irrespective of the reaction pathway. This law allows determining enthalpy change from standard enthalpy of formation values for the reactants and products.
๐Ÿ’กHeat
Heat is energy transferred between systems due to temperature difference. According to the video, heat and enthalpy change are equivalent for reactions happening at constant pressure. By measuring heat flows, the enthalpy change of a reaction can be determined through calorimetry.
๐Ÿ’กStandard Enthalpy
The standard enthalpy of formation of a compound gives its enthalpy change for 1 mole of the substance being formed from its elements in their standard states. As described in the video, standard enthalpies allow calculating the enthalpy change for any reaction from Hess's Law.
๐Ÿ’กWork
Work is a way of transferring energy through the application of a force over a distance. The script mentions that for reactions at constant pressure, the only useful work considered is the pressure-volume work.
๐Ÿ’กState Function
A state function depends only on the initial and final state, not the pathway. The video emphasizes that energy and enthalpy are state functions, allowing easy calculation of net change by simple subtraction of end and start points.
๐Ÿ’กPressure
The video assumes constant pressure while deriving the relationship between heat and enthalpy change. Pressure relates to the definition of enthalpy and allows simplification of the math by enabling the Pฮ”V terms to cancel out.
Highlights

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Transcripts

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