Enthalpy and Entropy Flashcards
What does entropy measure?
measures how much the energy of motion of the particles in a substance changes per K of temperature.
What is the definition of entropy?
entropy is a measure of disorder and is given the symbol s
unites age JK-1 mol
What does the greater the entropy value mean?
the greater that energy is spread out per Kelvin mole
what are features of entropy values?
always positive
quite small so we measure them in J not kJ
What are the entropies for solid, liquids and gases?
In general:
o solids have the smallest entropies
o liquids have greater entropies
o gases have the greatest entropies.
What happens to disorder if you increase the number of moles from the reactants to the products?
-then you will increase the disorder. -Therefore entropy will increase.
As you go from solids to liquids to gases what happens to disorder and what does this mean?
you increase the disorder.
Therefore
entropy will increase
How do you comment on entropy changes?
-Entropy decreases/increases (delta s is positive/negative)
-Disorder decreases/increases
-0 moles of gas formed from one mole of gas (talk about most disordered)
how do you calculate entropy change?
sum of products - sum of reactants
What is the equation for bond enthalpies?
delta H= products-reactants
What is the equation for delta H using formation data?
product-reactants
What is the delta H equation using combustion data?
reactants-products
What is the equation to determine feasibility?
gibbs free energy change-
deltaG= deltaH(kJ mol-1) -TdeltaS(Jmol-1k-1)
(T=298k)
what must you do to delta s in the gibbs free energy change equation?
x by 298 (T)
divide by 1000
What is a feasible reaction?
is one which can possibly occur for thermodynamic reasons.
What are reactions that demonstrate simple reactions with a positive delta H?
ammonium chloride and barium hydroxide citric acid and sodium hydrogencarbonate (Alka-Seltzer).
What are the rules for feasibility?
If ΔG is positive then a reaction is not feasible
If ΔG is negative then a reaction is feasible
If ΔG is zero then the reaction is in flux
What does it mean if ΔH is -ve and ΔS is +ve?
ΔG always negative
reaction always feasible
What does it mean if ΔH is -ve and ΔS is -ve?
-ΔG is sometimes -ve
(more negative at lower
temperature’s)
-reaction sometimes feasible
What does it mean if ΔH is +ve and ΔS is +ve?
-ΔG Sometimes negative
(more negative at higher
temperautres)
-reaction is sometimes feasible
What does it mean if ΔH is +ve and ΔS is -ve?
ΔG Always positive
reaction never feasible
For the two cases that are sometimes negative, state and explain whether temperature must be high or low in order to make ΔG negative.
Temperature must be low enough for TΔS to be less negative than ΔH
Temperature must be high enough for TΔS to be more positive than ΔH
(check notes)
why is a reaction feasible at low temperature’s?
as TAS is less negative than delta H so delta G remains negative
The following reaction is exothermic
2NO(g) + O2(g) → 2NO2(g)
It occurs spontaneously at low temperature but does not occur at very high temperature. Explain why.
-ΔH is negative as reaction is Exothermic
- ΔS is negative as 3 moles gas become 2 moles of gas
-Reaction is feasible at low temperature as TΔS is less negative than ΔH so ΔG remains negative.
-At high temperatures TΔS is more negative than ΔH so ΔG = +ve so reaction not feasible.[2]
What does it mean if ΔG = 0?
ΔH = T ΔS
so T (+ve) = ΔH / ΔS (answer q’s on page 15, 16, 17, 18)
How is ΔG = ΔH-TΔS written as y=mx+c?
ΔG = (-ΔS) x T + ΔH
where y= ΔG, x=T, y intercept= ΔH, gradient= = (-ΔS)
What does it mean if the gradient is positive?
ΔS is negative
What does it mean if the gradient is negative?
ΔS is positive
(check notes)
Explain, with the aid of a thermodynamic equation, why this line obeys the mathematical
equation for a straight line, y = mx + C.
G = H - TAS (1) -only
thermodynamic equation we use y=AG
С=Н
m=-AS
X=1
AG = -AST + ДН (2)
ДН and AS are constant
AH = y intercept = constant
AS = gradient = constant
Explain why the magnitude of delta H decreases as T increases in this reaction
T delta S becomes more positive
