Thermochemistry & Kinetics Unit Test Review
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Define the following terms:
a. Activated complex
l. Gibb’s Free Energy
x. Second Law of
b. Activation energy
m. Heat
Thermodynamics
c. Calorimetry
n. Heat Capacity
y. Specific Heat
d. Catalyst
o. Heat of Condensation
Capacity
e. Chemical potential
p. Heat of Fusion
z. Spontaneous
energy
q. Heat of Reaction
Reaction
f. Collision theory
r. Heat of Solidification
aa. Surroundings
g. Endothermic
s. Heat of Vaporization
bb. System
h. Enthalpy
t. Kinetic Energy
cc. Thermochemical
i. Entropy
u. Kinetics
Equation
j. Exothermic
v. Potential Energy
dd. Transition State
k. First Law of
w. Reaction Rate
Thermodynamics
Explain the relationship between qsys and qsurr.
qsys = -qsurr : as the system gives off energy, the surroundings gain an equal amount of energy or vice versa.
What always happens when two objects of different temperatures come in contact?
The object with the higher temperature will transfer energy to the object with lower temperature. This causes the warmer object
to become colder and the colder object to become warmer. This transfer of energy will continue until both objects are at the
same temperature.
Two substances in a flask chemically react and the glass beaker becomes too cold to touch.
a. Is this an endothermic process of exothermic process? endothermic
b. If the two substances are defined as the system, what makes up the surroundings? Beaker, hand, everything else
Classify each statement as being endothermic, exothermic, or both types of processes.
Exothermic
Endothermic
Both
The system loses energy.
X
qsurr is negative.
H is negative.
X
X
Energy is neither created nor destroyed.
CH3OH(l) + O2(g)  HCO2H(g) + H2O(g) + 87.5 kJ
X
X
At constant pressure, the change in enthalpy is equal to the heat absorbed or lost
by the system.
CaF2(s) + H2SO4(aq)  CaSO4(s) + 2 HF(g) H = 157.7 kJ
X
X
The melting of a solid.
X
qsys is positive.
The phase change of a gas to a liquid (condensation).
You are holding a test tube that has a chemical reaction in progress inside and
you feel the test tube get hot.
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Using dimensional analysis, convert 1252.3 Joules to calories. 299.3 cal
Using dimensional analysis, convert 8450 calories to Joules. 35400 J
In a reaction in a calorimeter the temperature of 32.0 grams of water increases from 19.50C to 22.83C. What is the amount of
heat absorbed by the water? 446 J
9. If you were to heat 453.6 grams of lead, how many Joules of heat would be required to raise the temperature from 25C to
227C? (the specific heat of lead = 0.159 J/gC) 14600 J
10. Suppose that you drop a 50.0 g piece of metal with a temperature of 100.0C into 100.0 mL of water at 25C in a calorimeter.
What is the specific heat of the metal if the final temperature of both the metal and water is 30.0C? -0.60 J/gC
11. Energy is stored in the body in the form of adenosine triphosphate, ATP. It forms when adenosine diphosphate, AD, and
phosphoric acid react in the body according to the reaction that follows.
ADP(aq) + H3PO4(aq) + 38 kJ  ATP(aq) + H2O(l)
Is the reaction endothermic or exothermic? Explain. Endothermic – energy is a reactant, showing that it must be
absorbed for the reaction to occur
b. Write the equation in the other style (with H = following the equation).
ADP(aq) + H3PO4(aq)  ATP(aq) + H2O(l) H = 38 kJ
Given the reaction 2POCl3(g)  2PCl3(g) + O2(g) H = 543kJ
a. Is this reaction endothermic or exothermic? Explain. Endothermic
b. Write the equation in the other style (with the enthalpy change as a reactant or product).
2POCl3(g) + 543 kJ  2PCl3(g) + O2(g)
c. If 995kJ of energy are used in the reaction, how many moles of PCl 3 are produced? 3.66 mol PCl3
d. What mass of POCl3 can react if 155kJ of heat is added to the system? 87.5g POCl3
Use the equation below to answer the following questions
CaCl2(s)  Ca2+(aq) + 2 Cl-(aq) H = -325.0 kJ
a. Is the process endothermic or exothermic? How can you tell the difference? exothermic
b. Write the equation in the other style (with the enthalpy change as a reactant or product).
CaCl2(s)  Ca2+(aq) + 2 Cl-(aq) + 325.0 kJ
c. If you have 15.0 grams of the solid, how much heat is released or gained in kilojoules as it dissolves? -44 kJ
A 7.51g sample of NH4NO3 is placed into 100.0 g of water in a calorimeter. The material dissolves in the water and the
temperature of the water drops from 24.8C to 19.0C during the process.
a. Calculate the heat lost by the water (qwater) -2400 J
b. What is the heat absorbed by the process (qsys)? 2400 J
c. What is the change in enthalpy in units of J? 2400 J
d. How many moles of NH4NO3 were involved in this process? 0.0939 mol NH4NO3
e. What is the change in enthalpy (H) in units of J/mol? 26,000 J/mol
f. This reaction is spontaneous. What is the sign of H that you found in (e)? + Based on this, can you tell what the
sign of S must be? Yes If so, what is it? +
Phase Change Enthalpy Calculations
a. Which molar heat (of fusion, solidification, condensation, and vaporization) corresponds to each of the following phase
changes?
i. Condensation (gas to liquid) molar heat of condensation
ii. Freezing (liquid to solid) molar heat of solidification
iii. Melting (solid to liquid) molar heat of fusion
iv. Vaporizing or boiling (liquid to gas) molar heat of vaporization
b. Use this information about water to answer the following questions.
Hfus = 6.02 kJ/mol and Hvap = 40.7 kJ/mol
i. If you add 78.2kJ of energy to water at 100.C, how many moles will be vaporized? 1.92 mol H2O
ii. How many kJ of energy are released when you freeze 0.78 moles of water? -4.7 kJ
iii. What quantity of heat is needed to melt 225.0g of ice at 0C? 75.3 kJ
What is the difference between molar heat of fusion and molar heat of solidification? Molar heat of fusion is the amount of
energy needed for one mole to melt from a solid to a liquid; molar heat of solidification is the amount of energy needed for one
mole to solidify from a liquid to a solid. They are equal in magnitude, but have opposite signs.
Is entropy increasing or decreasing for each of the following?
a. Ice Melting increasing
b. Gas condensing to a liquid decreasing
c. Forming a mixture from two pure substances increasing
d. N2(g) + 3 Cl2(g)  2 NCl3(l) decreasing
e. PCl5(g)  PCl3(g) + Cl2(g) increasing
What are the two driving forces that can make a reaction spontaneous? Enthalpy and entropy
What must be true for a reaction to be spontaneous? Exothermic and increasing entropy
Describe how calculating G can allow you to determine if a reaction is spontaneous at a given temperature. If the free energy
is positive, energy is absorbed by the process and is not spontaneous; if the free energy is negative, energy is released by the
process and it is spontaneous.
Carbon disulfide reacts with hydrogen at 25.0C to form methane gas and hydrogen sulfide vapor according to the following
balanced equation.
CS2(g) + 4 H2(g)  CH4(g) + 2 H2S(g)
a. Using the Gibbs Free Energy equation determine whether this reaction is spontaneous or not (Note: H = -232.3
kJ/mol and S = -162.9 J/mol K) The reaction is spontaneous (G = -183.7 kJ)
b. Which term (H or S) is the driving force for the reaction? Explain. Change in enthalpy – it is negative, indicating
that the reaction is exothermic.
c. What is the temperature at which the spontaneity of this reaction changes? 1426 K
Draw a potential energy diagram for each of the following: see notes for energy diagrams with labels
a. Exothermic reaction
b. Endothermic reaction
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Label the diagrams with the following: reactants, products, activation energy, activated complex, H.
On each of the previous diagrams draw a dashed line to show hoe the reaction would change with the addition of a
catalyst.
Compare the chemical potential energy of products to that of the reactants for an exothermic reaction. The products have a
lower chemical potential energy than the reactants in an exothermic reaction.
Compare the chemical potential energy of products to that of the reactants for an endothermic reaction. The products have a
higher chemical potential energy than the reactants in an endothermic reaction.
Explain how each factor affects reaction rate (Be sure to mention if the rate increases or decreases. Be sure to explain why this
occurs in terms of the Collision Theory:
a. Temperature Increase in temp = increase in reaction rate; increasing the temperature increases the speed of the
particles, causing more collisions and it increases the particles kinetic energy, resulting in more collisions have the
energy needed to produce products
b. Surface Area Increase in surface area = increase in reaction rate; increasing surface area increases the number of
places for collisions between reactants
c. Concentration Increase concentration = increase in reaction rate; increasing concentration increases the number of
particles in a given volume that can collide to cause a reaction
d. Catalyst presence of a catalyst = increase in reaction rate; catalysts lower the activation energy, allowing more
collisions to have the necessary energy to form products
Give two reasons why many collisions between particles do not lead to a chemical reaction. The collisions do not have the
correct orientation to form products or they do not have enough energy needed to form the products.
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