Chapter 22: Entropy
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What is entropy? Include its symbol and units. Required.
• A measure of the dispersal of energy in a system, that is greater the more disordered a system is • S and J K^-1 mol^-1
How does entropy change across the states of matter, and why?
• It increases from solid to liquid to gas • In solids, particles vibrate around a spot • In liquids, particles move over each other • Liquids are therefore more disordered than solids • In gases, particles move freely and faster, giving them the highest disorder
How can you tell if entropy increases or decreases in a reaction? Why?
• If the number of gaseous molecules increase, there is an increase in entropy as energy is spread out more • The reverse is also true
What is used to calculate the entropy change of a reaction? What is its definition?
• S⦵: standard entropy • The entropy of one mole of a substance under standard conditions
What is special about values of standard entropy?
• They’re always positive • (Entropy at 0K is always 0, so entropy increases from 0K to 298K)
How is the entropy change of a reaction calculated?
• ΔS⦵ = ΣS⦵(products) - ΣS⦵(reactants)
What is the term feasibility used to describe?
• Whether a reaction can take place
What 2 factors decide the feasibility of a reaction?
• The enthalpy change • The entropy change at the temperature of the reaction
What equation can be used to calculate whether a reaction is feasible? Include units.
• The Gibbs’ equation • ΔG = ΔH - TΔS • ΔG in kJ mol^-1 • ΔH in kJ mol^-1 • T in K • S in kJ K^-1 mol^-1
What is ΔG?
• The free energy change • An energy change that takes into account both the entropy change and the enthalpy change
What has to be done before free energy can be calculated?
• The entropy change has to be changed from J K^-1 mol^-1 to kJ K^-1 mol^-1 (by dividing by 1000)
How does the value of the free energy change tell you if a reaction is feasible?
• For a reaction to be feasible, ΔG must be negative
How can you find the minimum temperature required for a reaction to be feasible?
• The Gibbs’ equation is equated to 0
What limitation is there to using values of ΔG to determine whether a reaction takes place?
• Even if ΔG < 0, the reaction may not take place if the activation energy is very high, as this means the reaction will have an extremely slow rate
What does a graph of free energy look like? What does it show?
• ΔG is plotted against T • Since ΔG = ΔH - TΔS, this means that the y-intercept is ΔH, and that the gradient is -ΔS • The x-intercept would be the temperature at which the reaction becomes feasible, as ΔG = 0