lecture 5 thermo Flashcards
What is the second law of thermodynamics?
The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. Isolated systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy.
Spontaneity is connected to what?
Change in enthalpy, Temperature and Entropy
What are the two classes of exothermic reactions?
- Reactions that are spontaneous at all T except O K
2. Reactions that can be spontaneous or non-spontaneous depending on T
What are the two classes of endothermic reactions
- Reactions that are non-spontaneous at all T
2. Reactions that can be spontaneous or non-spontaneous depending on T
Disorder is a key factor in what?
Spontenaeity
True or False? Things tend to become disordered and randomised
True
What is Entropy (S)?
A measure of disorder
What does disorder depend on?
How fast and far atoms/ions/molecules are moving around, how much rotation, vibration, bond stretching and, bending they are doing, and how many particles are in the system
What is the unit for Entropy?
J K-1 mol-1
The larger the S what?
The greater the disorder
S is always what?
+ve
What is the calculation for predicting entropy change in a reaction?
(Change in)S = S(final) - S(initial)
Will increasing the temperature of a substance increase or decrease its Entropy? explain
It will increase the entropy as greater motion = more disorder
True or False? There is a large decrease in Entropy when changing states
False, There is a large change in entropy when there is a change of state for example ice to water to steam causes large increases in entropy
True or False? gas - liquid - solid = Greater entropy
True
True or False? Smaller molecules have more entropy than larger molecules under the same conditions
False, Larger molecules have more entropy that smaller molecules
Predict the entropy change for this reaction. 2H2(g) + O2(g) > 2H2O(g) ?
Decrease in entropy as the number of moles of O2 gas decreases
Predict the entropy change for this reaction. NH4CL(g) > NH3(g) + HCl (g) ?
Increase in Entropy
Predict the entropy change for this reaction. NaCl (s) > Na+(aq) + Cl-(aq) ?
Increase in entropy
Predict the entropy change for this reaction. C(graphite) + O2(g) > CO2 (g) ?
Small increase as its going to fully gas
Predict the entropy change for this reaction. H2(g) + Cl2(g) > 2HCl(g) ?
Cant say (moles technically stay the same)
Predict the entropy change for this reaction. Water Freezes
Decrease in Entropy
True or False? The amount of disorder is not always obvious
True
What is the equation for delta rS?
Delta rS = products - reactants
What is the 2nd law of thermodynamics?
The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process
What is the equation for the 2nd law of thermodynamics?
Delta S universe > 0 for spontaneous reaction
What is the equation for delta rS of the system?
(Products) - (Reactants)
What is the equation for delta rS of the surroundings?
Because delta r H and T are involved in sponteneity they must be involved in delta rS for the surroundings therefore, Delta rS surroundings = -deltaH system / T
What is Gibbs free energy?
A thermodynamic potential that can be used to calculate the maximum of reversible work that may be performed by a thermodynamic system at a constant temperature and pressure. DeltarH sys - T x deltarS sys
When Delta G is <0, is the reaction spontaneous or non-spontaneous?
Spontaneous (-ve)
When Delta G is >0, is the reaction spontaneous or non-spontaneous?
Non-Spontaneous (+ve)
At equilibrium Delta r G = what?
0
how do you convert between celsius and kelvin?
Kelvin = Celsius + 273.15.
