Chapter 17 - Thermodynamics
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what is entropy
the randomness of a system
give an example of a reaction where the entropy increases
when the products have more moles in total than the reactants or when the products have more moles of gas than the reacants
what is the entropy change of the universe, how can you calculate it
the change in entropy of the system plus the surroundings. it can only increase, thus for a reaction to be feasible the entropy change of the universe must be greater or equal to zero
what does it mean for a reaction to be feasible
that it can happen, it is possible at a certain temperature
what is the equation for gibbs free energy
G = H - TS
what is entropy change denoted by
delta S
what is enthalpy change denoted by
delta H
for a reaction to be feasible, what must the value of deta G be
negative
what is gibbs free energy
a measure of a way to determine a reactions feasibility
at what values of H and S is a reaction always feasible, refer to the equation for gibbs free energy
when H is negative and S is positive, G will always be negative we multiply S by -T to give a negative value, and negative plus negative is always negative
at what values of H and S is a reaction never feasible, refer to the equation for gibbs free energy
when H is positive and S is negative, G will always be positive. we multiply S by -T to give a positive value, and positive plus positive is always positive
why do some reactions not occur, even if G is negative
some reactions may have a very high actiavtion energy which means that only very few particles have enough energy to react (refering to maxwell boltzmann distributions), therefore it may take centuries for a reaction to go to completion, and we say that it doesnt occur becaus the rate of reaction is really slow
when using the equation for gibbs free energy to plot a graph, what are the axis, gradient and y-intercept
G = S - TH in the form y=mx +c is G = (-H)T + S this means that the x axis is T, the Y-axis is G, the y intercept is the enthalpy change and the gradient is entropy change times by -1
what is the x intercept on a GFE graph
the minimum temperature at which a reaction is feasible
how can you calculate the minimum temperature at which a reaction is feasible
T = H/S make sure that all units are in either j or kj
what are the limitations when finding the minimum temperature at which a reaction is feasible using T = H/S
a negative value will mean that a reaction is either always feasible or never feasible the units need to be the same we dont know if the reaction is feasible above or below this temperature
how can you tell if a reaction is feasible above or below the temperature when G = 0
if the value of S is negative then it is feasible below that temperature, and if its positive then above that temperature. (this is because of the gradient of the graph - visualise it)
using the graph of gibbs free energy againts T, when will a reaction be feasible
when the line is below the X axis (this means that G is negative)
without a graph, how can you tell if a reaction is feasible above or below a certain temperature
if S is negative and H is positive, then the reaction is feasible below the temperature if S is positive and H is negative, then the reaction is feasible above the temperature
if T when G = 0 is negative, how can you tell if its always or never feasible
if H is negative and S is positive then the reaction is always feasible if its the opposite then the reaction is never feasible
what is the exam procedure when given a reaction, its feasbility at a temperaature, and its enthalpy change. asking to explain its feasibility at thos temperature
determine if the value of G is negative or positive, if its feasible then its negative. determine the sign of the entropy change of the reaction and give a reason. look at the number of moles and the state symbols on either side of the equation. compare the sign of the entropy to the sign of the enthalpy and state whether TS must be more positive or negative than H
how can you determine the sign of the entropy change of a reaction given only the equation
determine the number of moles on each side, if there are more moles of products than reactants, the “randomness” has increased so the entropy is likely to be positive if there are more moles of gas on the products side then the randomness has increased and the entropy is likely to be positive
define first electron affinity
the energy required to add one mole of electrons to one mole of gaseous atoms to form one mole of one minus ions
which electron affinities are exothermic and endothermic
the first is always exothermic, every other one is endothermic