13. Energetics (Entropy)
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Define what is meant by entropy + give the units
Entropy is a measure of disorder of a system The symbol is S and is always positive Units are J K-1 mol-1 (note, we need /1000 to make it KJ k-1 mol-1) Entropy change, however, can be positive or negative
When would entropy change ΔS be positive or negative
ΔS would be positive if entropy increases (ie if there are more moles of products than reactants/ products are gasses and reactants are solids) ΔS would be negative if entropy decreases (ie number of moles of product decreases/ reactants are gasses and products are solids)
What is the natural direction of entropy
To increase total entropy Thus, solid<liquid<gas
Give the equation used to calculate entropy change of the system
Entropy change of the system = sum of entropies of products - sum of entropies of reactants ΔSsystem = ΣSproducts - ΣSreactants
Give the equation used to calculate total entropy change
Total entropy change = entropy change of system + entropy change of surroundings If positive, the reaction is spontaneous. If negative the reaction is non spontaneous
Give the equation used to work out entropy change of the surroundings
–enthalpy change / temperature
Give the Gibbs free energy equation
Gibbs free energy = enthalpy change - (temperature x entropy change of system) Gibs free energy has units of kilojoules per mole ,KJmol-1, so convert entropy change ΔS (J mol⁻¹ K⁻¹ ) to kJ by ÷1000 Gibbs free energy combines the factors of enthalpy change and entropy change. If ΔG is negative, the reaction is feasible at that temperature. Another thing that will indicate if a reation is feasible is if the ΔS total (total entropy) is positive. Often will be asked to calculate min/ max temp that a reaction can be feasible (ie occurs spontaneously), so for this you assume ΔG is 0. Also assume ΔG is 0 for a reaction said to be at equilibrium
Why might a reaction not occur despite a negative ΔG value indicating it is feasible?
The reaction may be inhibited by kinetic factors: - The reaction may have a large activation energy. - The rate of reaction may be so slow that the reaction does not appear to happen at all.
Is dissolving ammonium nitrate in water an endothermic or exothermic process
Endothermic -system absorbs heat from its surroundings. As heat is taken in, the surroundings lose heat, so the temperature you measure goes down. –> entropy change increase as there is an increase of disorder as ionic lattice breaks down and hydrated ions move independently
Give the equation that relates Gibbs free energy with the equilibrium constant - Kc/Kp
Gibbs free energy = - gas constant x temperature x natural log of equilibrium constant - ΔG = - RT lnK ΔG has to be Jmol-1 here The more negative the ΔG, the more positive the total entropy (A negative ΔG directly corresponds to a positive change in total entropy), so reaction is more feasible, so equilibrium constant K will be higher, so equilibrium shifts to right and reaction is spontaneous
What do the following results happen, ie what do they tell you about ΔH and ΔSsystem: • Reaction is always feasible as ΔG is negative • Reaction is never feasible as ΔG is always positive • Only at a lower temperature is reaction likely feasible • Only at a higher temperature is a reaction feasible
• A negative ΔH and positive ΔSsystem • A positive ΔH and negative ΔSsystem • A negative ΔH and negative ΔSsystem • A positive ΔH and positive ΔSsystem
Give the equation that links gibbs free energy to entropy and temperature
ΔG= - T∆S(total)
Can rate constant K ever be negative
No