Enthalpy
If a system has a decrease in enthalpy would it be exothermic or endothermic?
Exothermic, as the change in enthalpy value will be negative
What is entropy a measure of?
Disorder
What main equation is used to find change in Gibbs Free Energy of a reaction/system?
ΔG = ΔH – T ΔS
The temperature at which a reaction changes from spontaneous to nonspontaneous can be calculated with what equation?
T = ΔH/ ΔS
The addition of a catalyst will have which of the following effects on a chemical reaction?
I. The enthalpy change will decrease
II. The entropy change will decrease
III. The activation energy will decrease
III only
Consider the following reaction:
2NO(g) + O2(g) → 2NO2(g) ∆H = –112 kJ
Is the reaction endothermic or exothermic as written?
Exothermic, negative ∆H means heat is released
Which reaction will be more likely to occur: one in with the entropy is positive or one in which the entropy is negative?
The one in which the entropy is positive
The entropy and enthalpy of a high temperature reaction are both negative. Is the reaction spontaneous?
The reaction is nonspontaneous
If a reaction has a positive ΔH and a positive ΔS, it will be spontaneous under what temperature conditions?
spontaneous when the temperature is low
If the reaction A + B ⇄ C has an equilibrium constant, K, greater than 1. The reaction is spontaneous/nonspontaneous and favors the reactants/products.
Spontaneous, products
If a reaction is endothermic: ΔH has a __ sign and ΔS has a __ sign.
ΔH: positive, ΔS: negative
Solid nitrogen spontaneously sublimes into nitrogen vapor. Explain how this would be represented in a change in entropy (states of matter).
Entropy will increase because gases have greater entropy than solids
SF4(g) + F2(g) → SF6(g)
The reaction above is thermodynamically spontaneous at 298, but becomes non spontaneous at higher temperatures. Which conditions are true ΔG is positive/negative, the ΔH is positive/negative and ΔS is positive/negative.
negative, positive, negative
Which of the following must be true for a reaction that is thermodynamically favorable from initial standard state conditions? The ΔG° ___ 0 and K ____ 1
ΔG° < 0 and K > 1
If ∆Ho and ∆So are both negative, then ∆Go is
A. always negative
B. always positive
c. positive at low temperatures and negative at high temperatures
D. negative at low temperatures and positive at high temperatures
D. negative at low temperatures and positive at high temperatures
Rank the following reactions from the most exothermic to the most endothermic:
1. C(s) + O2(g) → CO2(g) + 393.5 kJ 2. N2(g) + O2(g) + 91.3 kJ → 2 NO(g) 3. 2C(s) + 3H2 → C2H6 ΔH = –84.0 kJ 4. C (g) + CO2(g) → 2CO(g) ΔH = 173.0 kJ
1 > 3 >2 > 4
N2(g)+O2(g)→2NO(l)
In the above reaction, heat is given off and NO has an entropy value of 32 kJ, N2 an entropy value of 65 kJ, and O2 an entropy value of 80kJ. What is the change in entropy of formation of this reaction?
–81 kJ
A reaction reacts at a low temperature of 45℃ and has a ΔH of -81.22 kJ and a ΔS of -175.75 J/K. What is the ΔG for this reaction?
-25.3 kJ
When solid NH4SCN is mixed with solid Ba(OH)2 in a closed container, the temperature drops and a gas is produced. What are the correct sigs for ΔG, ΔH, and ΔS for the process?
ΔG–, ΔH+ and ΔS+
Given for the reaction Hg(l) → Hg(g) that ∆Ho = 63.0 kJ/mol and ∆So = 100 J/K mol, calculate the normal boiling point of Hg in Kelvin
630 K
The ∆H value for the reaction
(1/2)O2(g) + Cu(s) → CuO(s) is –156 kJ.
How much heat is released when 31.9 g of Cu is reacted with oxygen?
78.4 kJ
At 215 K, a reaction gives off an enthalpy value of –40 kJ and the corresponding change in Gibbs Free Energy is 20 kJ. What will the change in entropy be in this reaction?
–0.28 kJ
For a generic process A → B, ΔH° = 151 kJ/mol and ΔS°= 121 J/K-mol. Assuming that ΔH° and ΔS° do not change with temperature, at what temperature in °C does the reaction go from being non-spontaneous to spontaneous?
The process is spontaneous when T > 1248K
Calculate ΔS° of the surroundings for the synthesis of N2O + NO2 ⟶ 3 NO. (T = 298K and P = 1atm). The standard enthalpies for the compounds are N2O = 87 kJ, NO2 = 54 kJ, and NO = 92 kJ
453 J/K
If an endothermic reaction is spontaneous at 298 K, which of the following must be true for the reaction?
I. ∆G is greater than zero
II. ∆H is greater than zero
III. ∆S is greater than zero
II and III only