SECTION II, Part A部分
Directions: Questions 1–3 are long free-response questions that require about 23 minutes each to answer and are worth 10 points each.
Write your response in the space provided following each question. Examples and equations may be included in your responses where appropriate. For calculations, clearly show the method used and the steps involved in arriving at your answers. You must show your work to receive credit for your answer. Pay attention to significant figures.
1. A student investigates the enthalpy of solution,, for two alkali metal halides,LiCl and NaCl. In addition to the salts, the student has access to a calorimeter, a balance with a precision of ±0.1g,and a thermometer with a precision of ±0.1。
(a) To measure ，for LiCl, the student adds 100.0g of water initially at 15.0°C to a calorimeter and adds 10.0g of LiCl(s), stirring to dissolve. After the LiCl dissolves completely, the maximum temperature reached by the solution is 35.6°C.
(i) Calculate the magnitude of the heat absorbed by the solution during the dissolution process, assuming that the specific heat capacity of the solution is4.18 J/(g·°C).Include units with your answer.
(ii) Determine the value of for LiCl in
To explain why for NaCl is different than that for LiCl, the student investigates factors that affect and finds that ionic radius and lattice enthalpy (which can be defined as the △H associated with the separation of a solid crystal into gaseous ions) contribute to the process. The student consults references and collects the data shown in the table below.
(b) Write the complete electron configuration for the Na+ ion in the ground state.
(c) Using principles of atomic structure, explain why the Na+ ion is larger than the Li+ ion.
(d) Which salt,LiCl or NaCl, has the greater lattice enthalpy? Justify your answer.
(e) Below is a representation of a portion of a crystal of LiCl. Identify the ions in the representation by writing the appropriate formulas(Li+ or CI-) in the boxes below.
(f) The lattice enthalpy of LiCl is positive, indicating that it takes energy to break the ions apart in LiCl.However, the dissolution of LiCl in water is an exothermic process. Identify all particle-particle interactions that contribute significantly to the dissolution process being exothermic. For each interaction, include the particles that interact and the specific type of intermolecular force between those particles.
2. A student designs an experiment to study the reaction betweenandThe reaction is represented by the equation above. The student places 2.24g in a flask and adds 60.0ml of 0.875m The student observes the formation of bubbles and that the flask gets cooler as the reaction proceeds.
(a) Identify the reaction represented above as an acid-base reaction, precipitation reaction, or redox reaction. Justify your answer.
(b) Based on the information above, identify the limiting reactant. Justify your answer with calculations.
(c) The student observes that the bubbling is rapid at the beginning of the reaction and gradually slows as the reaction continues. Explain this change in the reaction rate in terms of the collisions between reactant particles.
(d) In thermodynamic terms, a reaction can be driven by enthalpy, entropy, or both.
(i) Considering that the flask gets cooler as the reaction proceeds, what drives the chemical reaction betweenand?Answer by drawing a circle around one of the choices below.
Enthalpy only Entropy only Both enthalpy and entropy
(ii) Justify your selection in part (d)(i) in terms of △G°。
(e) Theion has three carbon-to-oxygen bonds. Two of the carbon-to-oxygen bonds have the same length and the third carbon-to-oxygen bond is longer than the other two. The hydrogen atom is bonded to one of the oxygen atoms. In the box below, draw a Lewis electron-dot diagram (or diagrams) for theion that is (are) consistent with the given information.
(f) A student prepares a solution containing equimolar amounts of and The pH of the solution is measured to be 4 point seven. The student adds two drops of 3.0Mand stirs the sample,observing that the pH remains at 4 point seven. Write a balanced, net-ionic equation for the reaction betweenand the chemical species in the sample that is responsible for the pH remaining at 4 point seven.
3. To determine the molar mass of an unknown metal, M, a student reacts
iodine with an excess of the metal to form the water-soluble compoundas represented by the equation above. The reaction proceeds until all of theis consumed. Thesolution is quantitatively collected and heated to remove the water, and the
product is dried and weighed to constant mass. The experimental steps are represented below, followed by a data table.
(a) Given that the metal M is in excess, calculate the number of moles ofthat reacted.
(b) Calculate the molar mass of the unknown metal M.
The student hypothesizes that the compound formed in the synthesis reaction is ionic.
(c) Propose an experimental test the student could perform that could be used to support the hypothesis. Explain how the results of the test would support the hypothesis if the substance was ionic.
SECTION II, Part B部分