St. John s College High School Mr. Trubic AP Midterm Review Packet 2

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1 Name Date Directions: Read each question carefully and write your response in the space provided following each question. Your responses to these questions will be scored on the basis of the accuracy and relevance of the information cited. Explanations should be clear and well organized. Specific answers are preferable to broad, diffuse responses. For calculations, clearly show the method used and the steps involved in arriving at your answers. It is to your advantage to do this, since you may obtain partial credit if you do and you will receive little or no credit if you do not. Pay attention to units and significant figures. 1. Use principles of atomic structure and/or chemical bonding to explain each of the following. In each part, your answer must include references to both substances. (a) The atomic radius of Li is larger than that of Be. (b) The second ionization energy of K is greater than the second ionization energy of Ca. Page 1 of 7

2 2. Hydrogen gas burns in air according to the equation below. 2 H 2 (g) + O 2 (g) 2 H 2 O(l) (a) Calculate the standard enthalpy change, Δ H 298, for the reaction represented by the equation above. (The molar enthalpy of formation, Δ H f, for H 2 O(l) is kj/mol at 298 K.) (b) Calculate the amount of heat, in kj, that is released when 10.0 g of H 2 (g) is burned in air. Page 2 of 7

3 (c) Given that the molar enthalpy of vaporization, Δ H vap, for H 2 O(l) is 44.0 kj/mol at 298 K, what is the standard enthalpy change, Δ H 298, for the reaction 2 H 2 (g) + O 2 (g) 2 H 2 O(g)? (d) A fuel cell is an electrochemical cell that converts the chemical energy stored in a fuel into electrical energy. Some fuel cells use butane gas, C 4 H 10, rather than hydrogen gas. The overall reaction that occurs in a butane fuel cell is 2 C 4 H 10 (g) + 13 O 2 (g) 8 CO 2 (g) + 10 H 2 O(l). What is one environmental advantage of using fuel cells that are based on hydrogen rather than on hydrocarbons such as butane? Page 3 of 7

4 3. Hydrazine, N 2 H 4, reacts in air according to the equation below. N 2 H 4 (l) + O 2 (g) N 2 (g) + 2 H 2 O(g) ΔH = 534 kj/mol (a) Is the reaction an oxidation-reduction, acid-base, or decomposition reaction? Justify your answer. (If you begin your answer with Yes or No, Mr. Trubic will be very disappointed.) (b) Predict the sign of the entropy change, ΔS, for the reaction. Justify your prediction. (c) Indicate whether the statement written in the box below is true or false. Justify your answer. The large negative ΔH for the combustion of hydrazine results from the large release of energy that occurs when the strong bonds of the reactants are broken. Page 4 of 7

5 MgO(s) + 2 H + (aq) Mg 2+ (aq) + H 2 O(l) 4. A student was assigned the task of determining the enthalpy change for the reaction between solid MgO and aqueous HCl represented by the net-ionic equation above. The student uses a polystyrene cup calorimeter and performs four trials. Data for each trial are shown in the table below. Trial Volume of 1.0 M HCl (ml) Mass of MgO(s) Added (g) Initial Temperature of Solution ( C) Final Temperature of Solution ( C) (a) Which is the limiting reactant in all four trials, HCl or MgO? Justify your answer. (b) The data in one of the trials is inconsistent with the data in the other three trials. Identify the trial with inconsistent data and draw a line through the data from that trial in the table above. Explain how you identified the inconsistent data. Page 5 of 7

6 For parts (c) and (d), use the data from one of the other three trials (i.e., not from the trial you identified in part (b) above). Assume the calorimeter has a negligible heat capacity and that the specific heat of the contents of the calorimeter is 4.18 J/(g C). Assume that the density of the HCl(aq) is 1.0 g/ml. (c) Calculate the magnitude of q, the thermal energy change, when the MgO was added to the 1.0 M HCl(aq). Include units with your answer. (d) Determine the student s experimental value of ΔH for the reaction between MgO and HCl in units of kj/mol rxn. Page 6 of 7

7 (e) Enthalpies of formation for substances involved in the reaction are shown in the table below. Using the information in the table, determine the accepted value of ΔH for the reaction between MgO(s) and HCl(aq). Substance Δ H f (kj/mol) MgO(s) 602 H 2 O(l) 286 H + (aq) 0 Mg 2+ (aq) 467 (f) The accepted value and the experimental value do not agree. If the calorimeter leaked heat energy to the environment, would it help account for the discrepancy between the values? Explain. Page 7 of 7