Unit VI Exam [Entropy and Free Energy]
Situations associated with NO formation involve all but one of the following 1) negative entropy change 2) high temperatures 3) lightning strikes 4) highly endothermic
1) negative entropy change
. Assumptions made in calculations dealing with quicktime formation include all of the following except 1) ∆G switches from positive to negative at 835°C 2) The CO2 formed remains in the reaction vessel 3) ∆H and ∆S values are independent of temperature 4) no products are formed below 835°C
2) The CO2 formed remains in the reaction vessel
One of the following is not a characteristic of exothermic reactions 1) reactants have high energy compared to products 2) products have lower stability than reactants 3) products are more stable than reactants 4) energy is released into the surroundings
2) products have lower stability than reactants
One of the following is not a characteristic of an endothermic reaction 1)energy is absorbed during the reaction 2)reaction is energetically favorable 3)products have larger quantity of energy than reactants 4)products are less stable than reactants
2) reaction is energetically favorable
All of the following are characteristics of non-spontaneous reactions except 1) reaction is endothermic 2) reactants are favored over products 3) entropy increases 4) randomness decreases
3) entropy increases
At the temperature at which a change of state occurs, all of the following are true except 1) the two states are at equilibrium 2) ∆G = zero 3) -T = temperature at which transition occurs 4) ∆Sfus = zero
4) ∆Sfus = zero
Why would a decrease in entropy be required for a non-spontaneous reaction to occur?
A decrease in entropy implies an increase in organization, which goes against the natural tendency of matter to increase in disorganization unless energy is put into the system from outside.
If ∆H is negative and ∆S is positive, then ∆G is
Always negative
One of the following does not show an increase in entropy A.) ice becomes liquid water B.) solid reactant forms liquid product C.) CO2 condenses to form dry ice D.) iodine crystals form a vapor
CO2 condenses to form dry ice
Is entropy increasing or decreasing in the reaction 2CO(g) + O2(g) ---> 2CO2(g)
Decreasing
Is entropy increasing or decreasing in the reaction 3Li2CO3(aq) + 2Al(NO3)3(aq) ---> 6LiNO3(aq) + Al2(CO3)3(s)
Decreasing
Is entropy increasing or decreasing in the reaction 2Al(s) + 3Br2(g) --->> 2AlBr3(s)
Decreasing Decreasing. The products have fewer particles (two solid molecules) than the reactants (two solid particles and three gas particles).
Units for ∆G are
kJ/mol
True or False: The entropy change for the vaporization of water is negative.
False he entropy change for the vaporization of water can be found as follows: ΔS° = S°(H2O(g)) - S°(H2O(l)) ΔS° = 188.7 J/K•mol - 69.9 J/K•mol = 118.8 J/K•mol
A decrease in entropy is seen when
Hydrogen and oxygen form water
Is entropy increasing or decreasing in the reaction Ca(s) + 2H2O(l) ---> Ca(OH)2(aq) + H2(g)
Increasing
Is entropy increasing or decreasing in the reaction 2Cl2O5(g) ---> 2Cl2(g) + 5O2(g)
Increasing
Is entropy increasing or decreasing in the reaction 2HgO(s) ---> 2Hg(l) + O2(g)
Increasing Increase, products have more particles (two liquid particles and one gas particle) than the reactants (two solid particles). Also there is higher entropy in the liquid and gas states of the products.
True or False: All molecular motion ceases at 0 degrees K.
True
True or False: Changes in enthalpy and entropy are the driving forces behind all chemical reactions.
True
True or False: Formation of carbonic acid after CO2 is added to water is a non-spontaneous reaction
True
True or False: Most naturally occurring reactions are exothermic.
True
True or False: Spontaneous reactions release free energy as they proceed.
True
True or False: The formation of NO from nitrogen and oxygen in the atmosphere is non-spontaneous at normal temperatures and pressures.
True
True or False: The standard entropy of a substance is given by S°.
True
True or False: The ∆H for the quicktime reaction is positive at 25°C.
True
True or False: When Keq is large, ∆G should be negative.
True
True or False: ∆G must be negative for a spontaneous reaction.
True
Free energy is energy that is
available to do work
When the T∆S term becomes larger than the ∆H term, the ∆G value will be
negative if ∆S is positive ΔG° = ΔH° - TΔS°. If ∆S is positive then you will be subtracting a larger positive number from ∆H.
If ∆H is positive and ∆S is negative, then ∆G is
never negative
A combustion reaction is considered to be spontaneous because
reaction is exothermic
A decrease in entropy can be seen when
sugar is crystallized out of solution
Calculate ∆S° for: CaCO3(s) ---> CaO(s) + CO2(g) S° values: CaCO3 = 92.9 J/K•mol, CaO = 38.2 J/K•mol, CO2 = 213.6 J/K•mol
∆S° = 38.2 J/K•mol + 213.6 J/K•mol - 92.9 J/K•mol = 158.9 J/K•mol [adding so maintain least precision -- to tenths]
Standard entropy change can be calculated using
∆S°= ΣnS°(products)- ΣnS°(reactants)
True or False: The ∆G value = zero at the equilibrium point in the ice-water system.
True
True or False: When a reaction is exothermic, the ∆H term needs to be larger than the T∆S term for the reaction to be spontaneous.
True
True or False: Work must be done to decrease entropy.
True
True or False: When Keq = 1, then ∆G is positive.
False When Keq = 1, ln(Keq) = 0, then ∆G is zero.
True or False: Chemical reactions tend to proceed in a way as to decrease the entropy of the system.
False Chemical reactions also tend to proceed in such a way as to [increase] the total entropy of the system.
True or False: A spontaneous reaction is one that occurs rapidly.
False One must be careful not to confuse the term spontaneous with the notion that a reaction occurs rapidly. A spontaneous reaction is one in which product formation is favored, even if the reaction is extremely slow.
True or False: The energy of the system increases during an exothermic reaction.
False Since the energy of the system [decreases] during an exothermic reaction, the products of the system are more stable than the reactants.
True or False: Entropy decreases during most combustion reactions.
False The products of a fire are composed mostly of gases such as carbon dioxide and water vapor, so [the entropy of the system increases] during most combustion reactions.
True or False: The entropy for the quicktime reaction decreases as the reaction proceeds.
False The reaction is endothermic with an increase in entropy due to the production of a gas.
True or False: The symbol for free energy is in honor of the American scientist James Gibbs.
False The symbol for free energy is G, in honor of American scientist Josiah Gibbs (1839-1903), who made many contributions to thermodynamics.
True or False: The entropy change during the melting of ice can be calculated using ∆Hvap.
False The symbol ΔSfus represents the entropy change during the melting process, while Tf is the freezing point of water.
The units for entropy are
J/K•mol
A combustion reaction is considered to be spontaneous, but firewood does not burst into flame until an external source of heat is provided. Why is burning wood considered an spontaneous reaction?
Once the activation energy is achieved with the external source of heat, the exothermic reaction drives the process to completion without further external input of energy.
The symbol for entropy is
S
For a reaction with a ∆H of 14.2 kJ/mol and a ∆S of 46.8 J/K•mol, at what temperature will the reaction be spontaneous?
T = ∆H/∆S = 14.2 kJ/mol/ 0.0468 kJ.K x mol= 303 K = 29.85 C Any temperature above 29.85 C will be spontaneous
The variable R is the
ideal gas constant
In the quicklime manufacturing process
measurable CO2 is detected above 700°C
To determine the temperature at which ∆G changes sign, we calculate
the equation for ∆G = zero
The release of CO2 when carbonic acid is dissolved in water is spontaneous because
the equilibrium favors the forward reaction
The relationships between Gibbs free energy and the equilibrium constant is written
∆G = -RT ln(Keq)
The Keq for the reaction 2SO3(g) arrows 2SO2(g) + O2(g) is 1.4 x 10-25. Calculate the ∆G° for this reaction at 298 K.
∆G = -RTlnKeq = -(8.314 J/K•mol)(298K)ln(1.4 x 10-25) = (-2.48 kJ/mol)(-57) =140 kJ/mol [1.4 has only two significant digits]
For the reaction N2(g) + 3H2(g) arrows 2NH3(g), the ∆G at 298K = -32.96 kJ Calculate the Keq.
∆G = -RTlnKeq so lnkeq = ∆G/-RT lnKeq = (-32.96 kJ/mol)/(-8.314 J/K•mol)(298K) = (-32960 J/mol)/(-2480 J/mol) = 13.3 lnKeq = 13.3 so Keq = 5.97 x 105
AgCl(s) + H2O arrows Ag+(aq) + Cl-(aq) ∆G = 55.6 kJ. Calculate the Keq for the reaction at 298 K
∆G = -RTlnKeq so lnkeq = ∆G/-RT lnKeq = (55.6 kJ/mol)/(-8.314 J/K•mol)(298K) = (55600 J/mol)/(-2477.6 J/mol) = -22.4 lnKeq = -22.4 so Keq = 1.87 x 10-10
For the equilibrium reaction A + B A arrows B at 298 K ∆H =-55.8 kJ/mol ∆S = +80.8 J/K•mol Calculate ∆G for this process
∆G° = ∆H° - T∆S° = -55.8 kJ/mol - (0.0808 kJ/K•mol)(298K) = -55.8 kJ/mol - 24.1 kJ/mol = -79.9 kJ/mol
For a given reaction ∆H° = 35.6 kJ/mol and ∆S° = 113 J/K•mol. Is the reaction spontaneous at 280 K? At what temperature will the reaction be spontaneous?
∆G° = ∆H° - T∆S° = 35.6 - (0.113)(280) = 35.6 - 31.6 = 4.0 Not spontaneous since ∆G° is not negative. 0 = 35.6 - T(0.113) or T(0.113) = 35.6 and T = 315K. The reaction will be spontaneous above 315K.
The melting point of Hg is 234 K and the ∆Hfus = 2.30 kJ/mol. What is the ∆Sfus?
∆Sfus= ∆Hfus/Tv = 2.30 kJ/Mol/234K= 0.00983 kJ/K x mol = 9.83 J/K x mol [2.30 has three significant digits]
Metallic sodium boils at 1158 K and the ∆Hvap is 98 kJ/mol. What is the ∆Svap?
∆Svap = ∆Hvap/Tv= 98.0 kJ/mol / 1158 K =0.0846 kJ/K x mol = 85 J/K x mol [98 has only two significant digits]
Calculate ∆S° for: CO(g) + 2H2(g) --> CH3OH(l) ∆S° values: CO = 197.9 J/K•mol, H2 = 131.0 J/K•mol, CH3OH = 126.8 J/K•mol
∆S° = 126.8 J/K•mol - 197.9 J/K•mol - 2(131.0 J/K•mol) = -333.1 J/K•mol