AP Chemistry Final

2. b) A second student is given two solution, 75.0mL of 1.00M HCl and 75.0mL of 1.00M NaOH, each at 250°C. The student pours the solutions into an insulated cup, stirs the mixture, covers the cup, and records the maximum temperature of the mixture. i) The student calculates the amount of heat evolved in the experiment to be 41.1kJ. Calculate the student's experimental value for the enthalpy of reaction, in kJ/mol(rxn).

75.0mL x 1.00mol HCl (or NaOH)/ 1000mL = 0.0750mol HCl (or NaOH) change(triangle) in H = -4.1kJ/0.0750mol of reactants = -55kJ/mol(rxn)

4. Step 1: NO₂+F₂→NO₂F+F (slow) Step 2: NO₂+F↔NO₂F (fast reversible) a. Step 1 of the proposed mechanism involves the collision between NO₂ and F₂ molecules. This step is slow even though such collisions occur frequently in a mixture of NO₂ and F₂. Consider a specific collision between a molecule of No₂ and a molecule of F₂. ii)Identify and explain one other factor that affects whether the collision will result in a reaction.

A factor that affects the reaction of the collision is the orientation of the molecules. Two molecules must hit in the correct orientation so that the collision breaks the F₂ bond and forms an N-F bond.

2. b) A second student is given two solution, 75.0mL of 1.00M HCl and 75.0mL of 1.00M NaOH, each at 250°C. The student pours the solutions into an insulated cup, stirs the mixture, covers the cup, and records the maximum temperature of the mixture. ii) The student assumes that the thermometer and the calorimeter do not absorb energy during the reaction. Does this assumption result in a calculated value of the enthalpy or reaction that is higher than, lower than, or the same as it would have been had the capacities of the thermometer and calorimeter been taken into account? Justify

Calculated value of the enthalpy of reaction will be lower (less negative) than it would have been if the thermometer and calorimeter been taken into account. The thermometer and calorimeter will absorb some heat of the reaction. This lost heat is not taken into account of the original calculation of (triangle)change in H(rxn), making it less negative in magnitude.

3. A 4.32g sample of liquid SO₂Cl₂ is placed in a rigid, evacuated 1.50L reaction vessel. As the container is heated to 400K, the sample vaporizes completely and starts to decompose according to the equation above. The decomposition reaction is endothermic. d. The temperature of the equilibrium mixture is increased to 425K. Will the value of Kp increase, decrease, or remain the same. Justify

If the temperature is increased to 425K, the value of Kp will increase because the reaction is endothermic. For endothermic reactions, Le Chatelier's principle states raising temperature of endothermic reaction adds more heat, so more products will form and there will be an increase in Kp.

2. a) The student selects two 100mL beakers, uses them to measure 50mL each of 1.00M HCL(aq) solution and 1.00M NaOH(aq) solution, and measures an initial temperature of 24.5°C for each solution. Then the student pours the two solutions into an insulated cup, stirs the mixture, covers the cup, and records a maximum temperature of 29.9°C. i) Is the experimental design sufficient to determine the enthalpy of reaction to a precision of two significant figures? Justify

No. Use of beakers that are ⁺₋10mL precise to measure 50mL solution limits the precision of the volume measurements and the calculation to ⁺₋20% off or 1 significant figure.

4. Step 1: NO₂+F₂→NO₂F+F (slow) Step 2: NO₂+F↔NO₂F (fast reversible) a. Step 1 of the proposed mechanism involves the collision between NO₂ and F₂ molecules. This step is slow even though such collisions occur frequently in a mixture of NO₂ and F₂. Consider a specific collision between a molecule of No₂ and a molecule of F₂. i) One factor that affects whether the collision will result in a reaction is the magnitude of the collision energy. Explain

Only collisions with enough energy are able to overcome the activation energy barrier, Ea, and create a reaction. If they don't have energy the molecules will just bump into one another without any reaction.

3. A 4.32g sample of liquid SO₂Cl₂ is placed in a rigid, evacuated 1.50L reaction vessel. As the container is heated to 400K, the sample vaporizes completely and starts to decompose according to the equation above. The decomposition reaction is endothermic. b) When the system has reached equilibrium at 400K, the total pressure in the container in 1.26atm. Calculate the partial pressures, in atm, of SO₂Cl₂(g), SO₂(g), and Cl₂(g) in the container at 400K.

SO₂Cl₂↔ SO₂ + Cl₂ I: .701 0 0 C: -x +x +x E: .701-x x x (.701-x)+x+x=1.26atm x=.56 P SO₂Cl₂= .701-.56=.14atm P SO₂=PCl₂=.56atm

3. A 4.32g sample of liquid SO₂Cl₂ is placed in a rigid, evacuated 1.50L reaction vessel. As the container is heated to 400K, the sample vaporizes completely and starts to decompose according to the equation above. The decomposition reaction is endothermic. e. In another experiment, the original partial pressures of SO₂Cl₂, SO₂, and Cl₂ are 1.0atm each at 400K. Predict whether the amount of SO₂Cl in the container will increase, decrease or remain the same. Justify

The amount in the container will decrease. If the partial pressures are at 1atm than the Q=1.0atm. K for this reaction is equal to 2.2atm which is greater than Q. Because of this, the reaction will shift right and make more products. And by making more products, the reactant SO₂Cl₂ will decrease.

4. Step 1: NO₂+F₂→NO₂F+F (slow) Step 2: NO₂+F↔NO₂F (fast reversible) b. Consider the following potential rate laws for the reaction. Circle the rate law below that is consistent with the mechanism proposed above. Explain the reasoning behind your choice in terms of the details of the elementary steps of the mechanisms. rate=k[NO₂]²[F₂] rate=k[NO₂][F₂]

The correct rate law is (rate=[NO₂][F₂]). This is because the slow reaction of step 1 is the rate determining step, or rate law for the overall reaction. Since the reactants in step one has one NO₂ and one F₂ the rate law is [NO₂][F₂]. The coefficients of the slow step is the exponents in the rate law.

2. b) A second student is given two solution, 75.0mL of 1.00M HCl and 75.0mL of 1.00M NaOH, each at 250°C. The student pours the solutions into an insulated cup, stirs the mixture, covers the cup, and records the maximum temperature of the mixture. iii) One assumption in interpreting the results of the experiment is that the reaction between HCl(aq) and NaOH(aq) goes to completion. Justify the validity of this assumption in terms of the equilibrium constant for the reaction.

The reaction between a strong base and strong base (HCl and NaOH). Both of those reactants are 1x10⁷. Combining those neutralizes the reaction so it becomes the an equilibrium constant of water or (1.0x10¹⁴). Because this number is so big it is essentially a limiting reactant which means the reaction goes to completion.

2. c) A third student calculates a value for the enthalpy of reaction that is significantly higher than the accepted value. i) Identify a specific error in procedure made by the student that will result in a calculated value for the enthalpy of reaction that is higher than the accepted value.

The student read the thermometer wrong(Tfinal too high/Tinitial too low) such that the calculated value for triangleT was too high.

2. a) The student selects two 100mL beakers, uses them to measure 50mL each of 1.00M HCL(aq) solution and 1.00M NaOH(aq) solution, and measures an initial temperature of 24.5°C for each solution. Then the student pours the two solutions into an insulated cup, stirs the mixture, covers the cup, and records a maximum temperature of 29.9°C. ii) List two specific changes to the experiment that will allow the student to determine the enthalpy of reaction to a precision of three significant figures. Explain

Use the graduated cylinders to measure volumes of the acid and base allowing a volume of precision ⁺⁻0.1mL or 3 significant figures for a volume of 50.0mL Use the 2.00M HCl and 2.00M NaOH solutions(instead of 1.00M) to increase the amount of heat released for the same volume. This should cause the change in Temperature (triangle T) for the reaction to improve the precision ⁺₋0.1%

3. A 4.32g sample of liquid SO₂Cl₂ is placed in a rigid, evacuated 1.50L reaction vessel. As the container is heated to 400K, the sample vaporizes completely and starts to decompose according to the equation above. The decomposition reaction is endothermic. c. For the decomposition at 400K i)write the equilibrium constant expression for Kp for the reaction ii)calculate the value of the equilibrium constant Kp

i) Kp=(PSO₂)(PCl₂)/(PSO₂Cl₂) ii) Kp=.56²/.14=2.2

2. c) A third student calculates a value for the enthalpy of reaction that is significantly higher than the accepted value. ii) Explain how the error that you identified in part c.i. leads to a calculated value for the enthalpy of reaction that is higher than the accepted value

molar enthalpy of reaction is Molar (triangle)Hrxn = - mass(soln) x c x (triangle)T/n(rxn) If there is a measurement error that makes triangle T too high, the magnitude of calculated molar enthalpy will also be too high

3. A 4.32g sample of liquid SO₂Cl₂ is placed in a rigid, evacuated 1.50L reaction vessel. As the container is heated to 400K, the sample vaporizes completely and starts to decompose according to the equation above. The decomposition reaction is endothermic. a) If no decomposition occurred, what would be the pressure, in atm, of the SO₂Cl₂(g) in the vessel at 400K?

n=m/M→4.32g/134.96(so₂cl₂)=0.0320mol PV=nRT→P=nRT/V= (.0320mol)(.0821)(400K)/(1.5L) P=0.701atm