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5.3 Entropy

5.3 Entropy . 5.3.1 State and explain the factors that increase the entropy in a system. 5.3.2 Predict whether the entropy change (∆S o ) for a given reaction or process is positive or negative.

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5.3 Entropy

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  1. 5.3 Entropy 5.3.1 State and explain the factors that increase the entropy in a system. 5.3.2 Predict whether the entropy change (∆So) for a given reaction or process is positive or negative. 5.3.3 Calculate the standard entropy changes for a reaction (∆So )using standard entropy values (So)

  2. Entropy (S) • A thermodynamic quantity related to the number of ways the energy of a system can be dispersed through the motions of its particles. • More freedom of particle movement and dispersed energy (spread over more energy levels) the higher the entropy

  3. Probability • The entropy of a system is a measure of the degree of disorder or randomness of a system. • The less order of the state (more chaotic) the greater its probability of occurring, so the greater its entropy, S. • Associated with states. S will be higher for gases and lower for solids. • Gases have more movement (vibrational, translational and rotational) than any other state, so they have higher entropy values.

  4. Predicting relative values of Entropy • Temperature increases, so does So • Vaporisation has a greater So increase than fusion. • Dissolved from (s) or (l) in (l) have a greater So than their elements. • Dissolved from (g) in (l) has a lower So • Greater the atomic size or more complex the molecule, the greater the So • PHYSICAL STATE DOMINATES OVER COMPLEXITY!

  5. Methods to increase Entropy • Increase volume, or decrease gas pressure • Reactions that produce more gas reactants • Increasing the temperature of a reaction • At absolute zero, there is no randomness so entropy would be zero. • Reactions that decrease the number of moles of solid products formed. NH4Cl(s)NH3(g) + HCl (g)∆S = +285 J K-1mol-1

  6. Calculating the ∆Srxn • ∆Srxn = ΣnS (products) - ΣnS (reactants) • Look up the values of S on data tables. • CH4(g) + 2O2(g)  CO2(g) + 2 H2O(l)

  7. Answer CH4(g) + 2O2(g)  CO2(g) + 2 H2O(l) ∆Srxn =ΣnS (products) - ΣnS (reactants) = [ (1mol x 214 J/K mol) + (2 mol x 70 J/K mol)] – [(1 mol x 186 J/K mol) + (2 mol x 205 J/K mol)] = -242 J/K mol Since the value of ∆Srxn is negative, there is an increase of order to the system. Due to the decrease of moles of gas produced, and the formation of the more ordered liquid water.

  8. Spontaneous change • Spontaneous = process that occurs by itself. • The system changes (physically or chemically) by itself under specific conditions without an ongoing input of energy from the surroundings. • Enthalpy change and its sign areNOT a criteria for spontaneity (or predicting direction of the change) • Direction doesn’t mean flow of energy, its related to the direction of a reaction (forwards or backwards)

  9. Examples • Spontaneous does NOT mean instantaneous. • Some spontaneous reactions occur so slowly or have such high activation energies to overcome that they don’t appear to move forward. • If a change is spontaneous in one direction under certain conditions it is NOT spontaneous in the other direction. • Melting snow at 8 oC and 1 atm. • Shingle falls off the roof of the building

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