Enthalpy and Entropy

Question 1

What does entropy measure?

Answer 1

measures how much the energy of motion of the particles in a substance changes per K of temperature.

Question 2

What is the definition of entropy?

Answer 2

entropy is a measure of disorder and is given the symbol s
unites age JK-1 mol

Question 3

What does the greater the entropy value mean?

Answer 3

the greater that energy is spread out per Kelvin mole

Question 4

what are features of entropy values?

Answer 4

always positive
quite small so we measure them in J not kJ

Question 5

What are the entropies for solid, liquids and gases?

Answer 5

In general:
o solids have the smallest entropies
o liquids have greater entropies
o gases have the greatest entropies.

Question 6

What happens to disorder if you increase the number of moles from the reactants to the products?

Answer 6

-then you will increase the disorder. -Therefore entropy will increase.

Question 7

As you go from solids to liquids to gases what happens to disorder and what does this mean?

Answer 7

you increase the disorder.
Therefore
entropy will increase

Question 8

How do you comment on entropy changes?

Answer 8

-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)

Question 9

how do you calculate entropy change?

Answer 9

sum of products - sum of reactants

Question 10

What is the equation for bond enthalpies?

Answer 10

delta H= products-reactants

Question 11

What is the equation for delta H using formation data?

Answer 11

product-reactants

Question 12

What is the delta H equation using combustion data?

Answer 12

reactants-products

Question 13

What is the equation to determine feasibility?

Answer 13

gibbs free energy change-
deltaG= deltaH(kJ mol-1) -TdeltaS(Jmol-1k-1)
(T=298k)

Question 14

what must you do to delta s in the gibbs free energy change equation?

Answer 14

x by 298 (T)
divide by 1000

Question 15

What is a feasible reaction?

Answer 15

is one which can possibly occur for thermodynamic reasons.

Question 16

What are reactions that demonstrate simple reactions with a positive delta H?

Answer 16

ammonium chloride and barium hydroxide citric acid and sodium hydrogencarbonate (Alka-Seltzer).

Question 17

What are the rules for feasibility?

Answer 17

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

Question 18

What does it mean if ΔH is -ve and ΔS is +ve?

Answer 18

ΔG always negative
reaction always feasible

Question 19

What does it mean if ΔH is -ve and ΔS is -ve?

Answer 19

-ΔG is sometimes -ve
(more negative at lower
temperature’s)
-reaction sometimes feasible

Question 20

What does it mean if ΔH is +ve and ΔS is +ve?

Answer 20

-ΔG Sometimes negative
(more negative at higher
temperautres)
-reaction is sometimes feasible

Question 21

What does it mean if ΔH is +ve and ΔS is -ve?

Answer 21

ΔG Always positive
reaction never feasible

Question 22

For the two cases that are sometimes negative, state and explain whether temperature must be high or low in order to make ΔG negative.

Answer 22

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

Question 23

(check notes)

Question 24

why is a reaction feasible at low temperature’s?

Answer 24

as TAS is less negative than delta H so delta G remains negative

Question 25

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.

Answer 25

-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]

Question 26

What does it mean if ΔG = 0?

Answer 26

ΔH = T ΔS so T (+ve) = ΔH / ΔS (answer q’s on page 15, 16, 17, 18)

Question 27

How is ΔG = ΔH-TΔS written as y=mx+c?

Answer 27

ΔG = (-ΔS) x T + ΔH where y= ΔG, x=T, y intercept= ΔH, gradient= = (-ΔS)

Question 28

What does it mean if the gradient is positive?

Answer 28

ΔS is negative

Question 29

What does it mean if the gradient is negative?

Answer 29

ΔS is positive

Question 30

(check notes)

Question 31

Explain, with the aid of a thermodynamic equation, why this line obeys the mathematical equation for a straight line, y = mx + C.

Answer 31

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

Question 32

Explain why the magnitude of delta H decreases as T increases in this reaction

Answer 32

T delta S becomes more positive