AP CHEMISTRY UNIT 6 - in a chemical reaction. A graph of temperature ...

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 is shown on slide 3.

(a) Is the reaction exothermic or endothermic? Justify your answer using the data 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 energy released 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 the surroundings. Since the temperature of the surroundings increases, heat flow was from 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). Since enthalpy is negative, energy released when bonds formed is greater.

(c) See slide 4

• Consider the reaction:

2CIF3 + 2NH3 ---> N2 + 6HF + Cl2. 5H«= -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 on slide 5 • Methane combusts according to the following equation: CH4 + 2O2 ---> CO2 + 2H2O 5H«= -890.7 kJ/mol. At 25 degrees C, very little CO2 or H2O is produced after a few hours when the reactants are mixed.Which of the diagrams on slide 6 could help to explain why the reaction is not producing 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. Two possible 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? 1 / 3

(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 less energy.(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 piece of 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 with time? 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 same temperature.

(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 of metal to water.This is shown on slide 9. The red curve represents the average kinetic 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 2 / 3

°C) at 25.0 °C. What happens to the temperature, average kinetic energy and average speed of the aluminum?: The temperature, average kinetic energy and the 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 °C was placed into 100.0 grams of water (specific heat capacity = 4.184 J/g °C) at 25.0 °C. The final temperature of the two

substances was 30.8 °C. What can be said of the temperature changes for each substance? 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 water at 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

• mL of water (molar mass 18.02 g/mol) at 25.0°C, the two substances 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 the specific measurements that are required to be made during the experiment.

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