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AP CHEMISTRY UNIT 6 ( Latest 2024 / 2025 ) Actual Questions and Answers 100% Correct
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AP CHEMISTRY UNIT 6 ( Latest 2024 / 2025 ) Actual Questions and Answers 100% Correct
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AP CHEMISTRY UNIT 6• An experiment was performed in an insulated container to determine the energy changes
in a chemical reaction. A graph of temperature versus time isshown on slide 3.
(a) Is the reaction exothermic or endothermic? Justify your answer using thedata provided.
(b) A student makes the following statement. Do you agree or disagree?
The energy required to break the bonds in the reactants is less than the energyreleased when the
bonds form in the products.
(c) Sketch an energy profile diagram for this reaction.: (a) Exothermic. The measured
temperature increases. The experiment measures the temperature of thesurroundings. Since the
temperature of the surroundings increases, heat flow wasfrom system into the surroundings.
(b) Agree. Reaction is exothermic so the enthalpy change is negative. The energy required to
break bonds (positive) is less than the energy released (negative). Sinceenthalpy is negative,
energy released when bonds formed is greater.
(c) See slide 4
• Consider the reaction:
2CIF3 + 2NH3 ---> N2 + 6HF + Cl2. 5 H«= -1196 kJ/mol. Draw a reaction pathway diagram for
this reaction. Label the products, reactants, and 5H«.: 5H«is negative, which is exothermic; the
products should be lower than the reactants. Example onslide 5
• Methane combusts according to the following equation: CH4 + 2O2 --->CO2 + 2H2O
H«= -890.7 kJ/mol. At 25 degrees C, very little CO2 or H2O is produced after a few hours
5
when the reactants are mixed.
Which of the diagrams on slide 6 could help to explain why the reaction is notproducing yield
and why?: Diagram 2 because the Ea is so high there would be a lower percent of particles
that have enough energy to complete the reaction.
• The decomposition of A2B is shown by the equation A2B ---> 2A + B. Twopossible
reaction pathways are shown in the diagram on slide 7.
(A) Is this reaction endothermic or exothermic?
(B) Describe the direction of heat flow in terms of the system and the sur-roundings.
(C) What is the enthalpy of reaction for the decomposition of A2B in kJ/mol?
(D) Why would path 2 require less energy than Path 1? Explain.
(E) What is the activation energy of the forward reaction?
,(F) What is the activation energy of the reverse reaction?
(G) What happens to 5H«when Path 2 is followed rather than Path 1?: (A)
Endothermic
(B) Heat is transferred from the surroundings to the system.
(C) 120 kJ/mol
(D) It has a lower activation energy so each particle would be able to react with lessenergy.
(E) 220 kJ/mol
(F) 100 kJ/mol
(G) It remains the same
• What is temperature is measure of?: average kinetic energy
• An experiment was designed to monitor the flow of heat from a hot pieceof metal to
water. Shown on slide 8
(a) What is the change in temperature for metal?
(b) What time is thermal equilibrium achieved? Justify your answer.
(c) Does the average speed of the metal particles increase or decrease withtime? Use particle
level reasoning to justify your answer.: (a) -62 degrees Celsius
(b) 120 seconds. The temperature becomes constant. Prior to the thermal equi- librium the
faster molecules are transferring energy to the slower molecules. At thermal equilibrium, all
molecules have the same kinetic energy and thus the sametemperature.
(c) Decrease. The temperature decreases.
Temperature is a measure of the average kinetic energy of metal atoms. Since tem-perature
decreases, the average kinetic energy decreases, therefore the average speed of the metal
atoms decreases.
• An experiment was designed to monitor the flow of heat from a hot piece ofmetal to water.
This is shown on slide 9. The red curve represents the averagekinetic energy of the water
molecules and the green curve represents the average kinetic energy of the metal atoms.
Sketch the curve that represents the metal atoms and water molecules at thermal equilibrium.:
Slide 10
• 50.0 grams of Aluminum (specific heat capacity = 0.900 J/g °C) at 85.0
°C was placed into 100.0 grams of water (specific heat capacity = 4.184 J/g
, °C) at 25.0 °C. What happens to the temperature, average kinetic energy andaverage speed of
the aluminum?: The temperature, average kinetic energy andthe average speed of the
aluminum atoms decrease as both substances reach thermal equilibrium.
• 50.0 grams of Aluminum (specific heat capacity = 0.900 J/g °C) at 85.0 °Cwas placed
into 100.0
grams of water (specific heat capacity = 4.184 J/g °C) at 25.0 °C. The finaltemperature of the
two
substances was 30.8 °C. What can be said of the temperature changes for eachsubstance? What is
true of the amount of thermal energy exchanged?: The change in temperature of Aluminum is
greater than the change in temperature of Water; Same magnitude of thermal energy but
opposite in signs.
• 15.0 grams of calcium chloride, CaCl2, is dissolved into 100.0 mL of waterat 22.5°C, the
final
temperature of the solution was 32.2°C. After the dissolution took place,consider the water
and what had happened to:
a) The temperature?
b) The average kinetic energy?
c) The average speed?
d) Was the dissolution reaction endothermic or exothermic?: (a) increased
(b) Increased
(c) Increased
(d) Exothermic
• When 39.0 grams of copper metal at 92.5°C (molar mass = 63.546 g/mol) is dropped into
200. mL of water (molar mass 18.02 g/mol) at 25.0°C, the twosubstances reach thermal
equilibrium. Which substance has:
a) Greater kinetic energy?
b) Particles with the greatest average speed?
c) Highest temperature?: (a) the same
(b) water (lighter)
(c) the same
• (a) The equipment shown on slide 11 is provided so that the student can determine the
value of the molar heat of solution for calcium chloride (CaCl2).Knowing that the specific heat
of the solution is 4.18 J/(g*degrees C), list thespecific measurements that are required to be
made during the experiment.
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