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Understanding Thermodynamics and Equilibrium: Spontaneous Reactions Explained

This lesson delves into the concepts of thermodynamics and equilibrium, focusing on spontaneous reactions. A spontaneous reaction occurs without external influence and has a natural tendency to proceed. Key factors influencing the spontaneity include enthalpy change (ΔH), temperature (T), and entropy change (ΔS). We discuss the effects of these factors and how they determine the direction of chemical reactions. Additionally, we explore Gibbs free energy (ΔG) to predict reaction favorability and identify whether reactions are spontaneous, non-spontaneous, or at equilibrium.

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Understanding Thermodynamics and Equilibrium: Spontaneous Reactions Explained

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  1. LESSON 2 THERMODYNAMICS AND EQUILIBRIUM

  2. Recall: Spontaneous Reactions: A reaction that will proceed without any apparent external cause (eg: addition of catalyst)

  3. A favourable change or reaction is one that has a natural tendency to occur. There are three factors to consider when determining if a chemical reaction will occur in a certain direction.

  4. 1) ΔH (Enthalpy Change) • Systems tend to move towards minimum potential energy • Spontaneous reactions are exothermic A + B  C + D + kinetic energy

  5. 2) T (temperature) • Spontaneous reactions favour higher temperature 3) ΔS (ENTROPY CHANGE) What is ENTROPY CHANGE?

  6. Group Activity • Groups of 4 • Activity 1 = cards • Activity 2 = test tube + food colour

  7. Work +(energy)

  8. ΔS (Entropy Change) • Entropy is the degree of randomness in a system • Systems move towards maximum entropy or randomness As the degree of atomic molecular disorder increases, the entropy of the system increases (ΔS>0).

  9. Therefore, entropy favours side of the reaction which produces more particles since this creates greater disorder. Example: states of matter (solid, liquid, gas) Reactions tend to move in the direction that produces the most gas molecules.

  10. ΔSuniverse > 0 for a spontaneous process • ΔSuniverse < 0 for a nonspontaneous process • ΔSuniverse = 0 for a process at equilibrium

  11. Sample Problem: State whether entropy of the reaction increases or decreases: NH3(g) + HCl (g)  NH4Cl(s) decreases

  12. HOW DO YOU KNOW IF A REACTION IS MOVING IN THE FORWARD DIRECTION, THE REVERSE DIRECTION, OR IF IT IS EXACTLY AT EQUILIBRIUM?

  13. GIBBS FREE ENERGY ΔG = ΔH - TΔS • Free energy or Gibbs free energy allows us to predict whether the forward or reverse reaction is favoured. • Reactions that favour the products are considered spontaneous reactions.

  14. ΔG = ΔH - TΔS • If ΔG < 0 (  ) The forward reaction is spontaneous. • If ΔG > 0 ( + ) The forward reaction is not spontaneous. • If ΔG = 0 Equilibrium is reached.

  15. Reaction Spontaneity and the signs of ΔH, ΔS, and ΔG ΔH ΔS -TΔS ΔG Description - + - - spontaneous at all T + - + + nonspontaneous at all T + + - + or - spontan. at higher T nonspontan at lower T - - + + or - spontan. at lower T non spontan at higher T

  16. Sample Problems:

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