Hess’ Law

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6 Terms

1
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Define Hess‘ Law

enthalpy change for any chemical reaction will be the same regardless of the ‘path’ taken as long as the starting conditions and final conditions, and reactants and products, are the same

<p>enthalpy change for any chemical reaction will be the same regardless of the ‘path’ taken as long as the starting conditions and final conditions, and reactants and products, are the same</p>
2
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<p>How would you use Hess’ Law in this instance?</p>

How would you use Hess’ Law in this instance?

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3
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What are the rules of Enthalpy changes?

  1. ∆H must accord with energy flow

    1. if flow reversed, then flip sign

  2. ∆H is proportional to the number of moles, so if moles are manipulated, then ∆H must be equally manipulated

4
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Tell me about Standard Enthalpy Changes of Reactions

  • occur under standard conditions

  • The reactions are either formation, ∆Hf, or combustion, ∆Hc

  • indicate stability of substance and used to calculate enthalpy changes of all reactions (real and hypothetical)

5
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Tell me about Enthalpy Changes of Formation Reactions

  • denoted by ∆Hf

  • ∆Hf of an element at standard state is (SATP) is zero

<ul><li><p>denoted by ∆Hf</p></li><li><p>∆Hf of an element at standard state is (SATP) is <span style="color: yellow"><strong>zero</strong></span></p></li></ul><p></p>
6
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Tell me about Enthalpy Changes of Combustion Reactions

  • one mole of a substance is completely burned in oxygen

  • denoted by ∆Hc

  • products are carbon dioxide and water

  • can use fractions to balance

<ul><li><p>one mole of a substance is completely burned in oxygen</p></li><li><p>denoted by <span style="color: red"><strong>∆H<sub>c</sub></strong></span></p></li><li><p>products are carbon dioxide and water</p></li><li><p>can use <span style="color: purple"><strong>fractions</strong></span> to balance</p></li></ul><p></p>