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Spontaneous Reactions

Spontaneous Reactions. Proceed forward on their own without outside or external cause. Certain conditions can help us predict whether a reaction will occur spontaneously or not. Factors that Determine Spontaneity. Change in Enthalpy ( ∆H) Exothermic ( - ∆H) reactions are more likely to

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Spontaneous Reactions

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  1. Spontaneous Reactions • Proceed forward on their own without outside or external cause. • Certain conditions can help us predict whether a reaction will occur spontaneously or not.

  2. Factors that Determine Spontaneity Change in Enthalpy (∆H) Exothermic (- ∆H) reactions are more likely to be spontaneous. A + B → C + D + energy • Why? • Release energy, producing lower energy more stable products. • Require less activation energy, so are easier to start

  3. Factors that Determine Spontaneity Change in Entropy (∆S) • Reactions with an INCREASE in Entropy are more likely to be spontaneous. • Entropy is a measure of the disorder, randomness or lack of organization in a system. • + ∆S = increase entropy or disorder

  4. Entropy deals with the Second Law of Thermodynamics, which basically states that all systems in the universe progress to a state of higher disorder. +∆S -∆S

  5. Entropy = Chaos and Disorder http://www.youtube.com/watch?v=CgppGozbFd4&safe=active http://www.youtube.com/watch?v=uQSoaiubuA0&safe=active

  6. How to Determine the Change in Entropy • +∆S entropy increases • - ∆S entropy decreases • Changes to Phases of Matter Solids Liquids Gases Most ordered Least ordered Ex: H2O(s) → H2O (l) +∆S Ex: H2O(l) → H2O (s) -∆S

  7. Gas particles are highly disbursed with random motion and high entropy. • Liquids have much lower entropies • Solids are most ordered and lowest entropy.

  8. If Phases Stay the Same Look at: Is there an increase in number of moles of gas? +∆S More highly disorganized gas = more chaos Is the substance breaking apart or dissolving? +∆S More pieces, more disbursed = more chaos

  9. What is the Sign of +∆S?

  10. Given the balanced equation representing a phase change: C6H4Cl2(s) + energy → C6H4Cl2(g) Which statement describes this change? (1) It is endothermic, and entropy decreases. (2) It is endothermic, and entropy increases. (3) It is exothermic, and entropy decreases. (4) It is exothermic, and entropy increases.

  11. The entropy of a sample of H2O increases as the sample changes from a (1) gas to a liquid    (2) gas to a solid  (3) liquid to a gas (4) liquid to a solid

  12. Which 1-mole sample has the least entropy? (1) Br2(s) at 266 K     (2) Br2(l) at 266 K  (3) Br2(l) at 332 K (4) Br2(g) at 332 K

  13. In terms of energy and entropy, systems in nature tend to undergo changes toward (1) higher energy and higher entropy (2) higher energy and lower entropy (3) lower energy and higher entropy (4) lower energy and lower entropy

  14. Which list of the phases of H2O is arranged in order of increasing entropy? (1) ice, steam, and liquid water (2) ice, liquid water, and steam (3) steam, liquid water, and ice (4) steam, ice, and liquid water

  15. Given the balanced equation: I2(s) + energy → I2(g) As a sample of I2(s) sublimes to I2(g), the entropy (1) increases because the particles are less randomly arranged (2) increases because the particles are more randomly arranged (3) decreases because the particles are less randomly arranged (4) decreases because the particles are more randomly arranged

  16. At STP, a sample of which element has the highest entropy? (1) Na(s)  (2) Hg(l) (3) Br2(l) (4) F2(g)

  17. Which of these changes produces the greatest increase in entropy? (1) CaCO3(s)  → CaO(s) + CO2(g) (2) 2 Mg(s) + O2(g)  → 2 MgO(s) (3) H2O(g)  → H2O(l) (4) CO2(g)  → CO2(s)

  18. Even though the process is endothermic, snow can sublime. Which tendency in nature accounts for this phase change? (1) a tendency toward greater entropy (2) a tendency toward greater energy (3) a tendency toward less entropy (4) a tendency toward less energy

  19. Which process is accompanied by a decrease in entropy? (1) boiling of water (2) condensing of water vapor (3) subliming of iodine (4) melting of ice

  20. Answer: A

  21. Answer: 2

  22. Answer: 1

  23. Is Reaction Spontaneous? • A reaction will always be spontaneous if: • - ∆H • + ∆S • A reaction will never be spontaneous if: • + ∆H • - ∆S

  24. Sometimes Spontaneous • + ∆H • + ∆S • More likely at higher temperatures • - ∆H • - ∆S • More likely at lower temperatures • These situations depend on the temperature conditions. • You would have to do a calculation to determine.

  25. Gibbs Free Energy Gibbs Free Energy (∆G) A portion of energy from a spontaneous reaction that can perform useful work. • Ex: battery, glowstick Spontaneous Reactions are - ∆G They give off free energy Nonspontaneous Reactions are + ∆G They require the input of free energy to occur • When a rxn is no longer spontaneous and producing free energy the system is at equilibrium and ∆G = 0

  26. (- ∆H) (+ ∆S) Always Spontaneous so - ∆G (+ ∆H) (- ∆S) Never Spontaneous so + ∆G What about sometimes spontaneous? How would you know if it is or not? Use the Gibbs Free Energy Equation ∆G = ∆H - T ∆S Kelvin Temperature

  27. Gibbs Free Energy of Formation (∆Gf) (Honors Only) • Quantity of free energy involved in the formation of one mole of a compound from it’s elements. • Way to determine overall ∆G of a reaction ∆Grxn = Σ (∆GfProducts) - Σ (∆GfReactants) This should look familiar!!! Same as ∆Hfproblems from last test.

  28. An Endothermic reaction may proceed spontaneously if there is an increase in A. Potential Energy B. Order C. Concentration D. Entropy

  29. A reaction must be spontaneous if its occurrence is A. exothermic with an decrease in entropy B. exothermic with an increase in entropy C. endothermic with an decrease in entropy D. endothermic with an increase in entropy

  30. Above 0°C, ice changes spontaneously to water according to the following equation: H2O(s) + heat --> H2O(l) The change in H2O(s) involve A. an absorption of heat and a decrease in entropy B. a release of heat and a decrease in entropy C. an absorption of heat and a increase in entropy D. a release of heat and a increase in entropy

  31. The ΔG of a chemical reaction refers to the change in  A. entropy B. free energy C. state D. activation energy

  32. The change of the reactants into products will always be spontaneous if the products, compared to the reactants, have A. lower enthalpy and higher entropy B. higher enthalpy and higher entropy C. higher enthalpy and lower entropy D. lower enthalpy and lower entropy

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