Energy in Bonds
What does bond energy refer to?
Energy to break a mole of a compound
The overall energy change in a chemical reaction is -24 kJ/mol. What does this tell us about the reaction?
It is exothermic
Entropy refers to...
the measurement of the degree of disordered particles in a system (the number of configurations available in a system)
If the standard Gibb's free energy for a reaction is positive, what is the most likely interpretation of the reaction favorability?
Reactants are favored
When K is greater than 1, are the reactants or products more favored?
product favored
True or False (if false, explain why):
bond breaking = endothermic = takes in energy
bond making = exothermic = give out energy
True
What temperature change takes place in an endothermic reaction?
The temperature of the surroundings decreases
The temperature of the surroundings increases
The temperature of the surroundings stays the same
The temperature of the surroundings decreases
Determine the sign of Δ S for the following reactions:
1) 2 H2O2 (aq) --> 2 H2O (aq) + O2 (g)
2) Cooling one mole of ideal gas
1) +
2) -
The transformation of a system from one state to another, at constant temperature and pressure, is spontaneous if the Gibb's free energy:
increases, decreases, attains equilibrium, or remains constant
Decreases
For a product favored reaction, draw the concentration vs. time graph.
Graph: products should start at a low concentration, increasing over time. reactants should start at a high concentration, decreasing over time. The two lines should intersect at which point, the concentrations are equal. After this point, products will be at higher concentration than reactants.
Which of the following bonds has the highest energy?
Se-Se, Te-Te, S-S, O-O
S-S
In the reaction of hydrogen with oxygen to make water, 1370 kJ of energy is taken in and 1856 kJ of energy is given out. Which of these statements can be concluded from this information?
The reaction is exothermic
The reaction is endothermic
The reaction has a high activation energy
The reaction is exothermic
For the substances below, choose the one with the higher molar So at 298 K:
NH3 (g) or Ne (g)
NH3 (g)
ΔH is positive, ΔS is positive. What is the nature of this reaction?
Draw the ΔG vs T graph for this reaction.
Spontaneous only at high temperatures.
Graph: slope line should decrease with ΔG positive at first, crossing x-axis into ΔG negative at higher temps.
The elementary reaction 2 NH3 (g) <-> 3 H2 (g) + N2 (g), the equilibrium concentrations were found to be [H2] = 0.300 M, [NH3] = 0.250 M, and [N2] = 0.620 M. What is the equilibrium constant for this reaction?
What is the directionality of this reaction at the equilibrium constant?
0.268 M
reactant-favored
The bond that requires the most energy to break is:
triple bond between non-polar atoms
OR
single bond between non-polar atoms
triple bond between non-polar atoms
What is the energy change for the reaction: H2 + F2 -> 2 HF
(Bond energies : H-H = 436 kJ, F-F = 158 kJ, H-F = 585 kJ)
-576 kJ/mol
Calculate the standard entropy of the following reaction:
N2 (g) + 3 H2 (g) --> 2 NH3 (g)
(So = J/mol*K = N2 = 191.6, H2 = 130.7, NH3 = 192.4)
-198.9 J/mol*K
At 0 oC, ice and water are in equilibrium and ΔH = 6.00 kJ/mol for the process H2O (s) --> H2O (l). What will be ΔS and ΔG for the conversion of ice to liquid water?
ΔS = 21.97 J/K*mol
ΔG = 0 kJ
Draw both the ΔG vs T and K vs T graphs for a reaction that is product favored at low temperatures.
What are the signs of ΔH and ΔS for this reaction?
ΔG vs T graph:slope line should start at a negative ΔG, increasing over temperature. At higher temps, the line should be at a positive ΔG
K vs T graph: line should start at a value K>1. This line should exponentially decrease at K < 1 values with increasing temperatures.
ΔH = -
ΔS = -
Which of these statements is true for the reaction profile of an endothermic reaction?
The reactants and products are at the same energy level
The reactants are at a higher energy level than the products
The products are at a higher energy level than the reactants
The products are at a higher energy level than the reactants
Calculate the enthalpy for the following:
NaOH (s) + HCl (g) --> NaCl (s) + H2O (g)
(Δ Hf in kJ/mol = NaOH (s) = -426.7, HCl (g) = -92.3, NaCl (s) = -411.0, H2O (g) = -241.8)
-133.8 kJ/mol
A reaction has the following PEC diagram analysis:
The products are on the left, lower side while the reactants are on the right, upper side.
Predict the signs of ΔH and ΔS. Without considering ΔG, is this reaction favoring the reactants or products more?
ΔH = -
ΔS = -
Reaction predictability is dependent on temperature.
The ΔH and ΔS for a reaction at one atm are +30.558 kJ and 0.066 kJ/K, respectively. At what temperature will ΔG be zero? What would the nature of this reaction be when below the found temperature?
463 K; non-spontaneous
What is the value of K for this aqueous reaction at 298 K? Is this reaction reactant- or product-favored?
A + B <-> C + D ΔGo = 18.20 kJ/mol
6.452 x 10-4
reactant-favored