Site Map 

Instructions for Exercises 
*Problem Sets 
*Questions and Problems 
*Multiple Choice Questions 
*Spreadsheets 

Using E-mail to Discuss Chemistry 

Post your free response questions and problems and their answers on our list server for discussion.  Send me your final versions and I will post them on our web site so we have all questions and answers in a single location.  Please give the complete question or problem and follow it with the complete answer.  This will simply life for for participants who want to use them in their course.  Since e-mail does not handle superscripts, subscripts, greek letters, or arrows, use the conventions described in using e-mail to discuss chemistry.    

Tables containing commonly used chemistry equations are provided with each examination for students to use when taking the free-response section. The availability of these equations means that in the scoring of the free-response sections, little or no credit will be awarded for simply writing down correct equations or for ambiguous answers unsupported by explanations or logical development. 

Instructions for Free Response Questions  
Instructions for Problems  

Write your free response questions in the style of the AP examination.   

AP free response questions are set so that students really must understand the material to answer the question, they can not simply use memorized definitions and explanations.  These questions: 

• use words in such a way that students who try to recognize a question topic from the language without understanding (i.e., students who use code words to prompt responses) could be misled. 
• may contain too much information. 
• require more than simple recall.
• often require students to use concepts that come from topic areas that are not directly related to the main topic of the question.

7)  For the gaseous equilibrium represented below, it is observed that greater amounts of PCl₃ and Cl₂ are produced as the temperature is increased. 

PCl₅(g) <===> PCl₃(g) + Cl₂(g)

a) What is the sign of S° for the reaction? Explain.  

b) What change, if any, will occur in G° for the reaction as the temperature is increased. Explain your reasoning in terms of thermodynamic principles.  

c) If He gas is added to the original reaction mixture at constant volume and temperature, what will happen to the partial pressure of Cl₂?  Explain.  

d) If the volume of the original reaction is decreased at constant temperature to half the original volume, what will happen to the number of moles of Cl₂ in the reaction vessel?  Explain.

• The question starts off like an equilibrium question ("For the gas phase equilibrium represented below..."), however, parts a) and b) turn out to be thermodynamics questions. 
• The question indicates that the amount of products increases as the temperature is increases but that information is not necessary to answer parts a) - d).  In fact, the information is part of the answer to part b). 
• The majority of understanding tested concerns thermodynamics, but part c) invokes the concept of partial pressures of gases and part d) invokes gas laws (decreasing V increases P).

Finally, future AP exams will not contain questions like "What is the sign of S° for the reaction?"  This is a coin-flip question; a student has a 50/50 chance of getting the correct answer.  Future exams will focus on the explanation and not give credit for the correct sign. 

Write your problems in the style of the AP examination.   

Like the AP free response questions, AP problems are set so that students really must understand the material to solve the problem.  These questions: 

• are presented in such a way that students who solve problem by invoking an algorithm without understanding will not be successful. 
• may contain too much information. 
• require more than simple recall.
• often require students to use concepts that come from topic areas that are not directly related to the main topic of the question.

In an electrolytic cell, a current of 0.250 ampere is passed through a solution of a chloride of iron, producing Fe(s) and Cl₂(g). 

a) Write the equation for the reaction that occurs at the anode. 

b) When the cell operates for 2.00 hours, 0.521 gram of iron is deposited at one electrode. Determine the formula of the chloride of iron in the original solution. 

c) Write the balanced equation for the overall reaction that occurs in the cell. 

d) Calculate the current that would produce chlorine gas at a rate of 3.00 grams per hour. 

• Students must apply the definition of an anode, not just recall it.
• Part b) is a stoichiometry problem embedded in an electrochemistry problem and requires a nontrivial chain of logic: 
• Amps and time give moles of electrons reacting with iron ions
• Mass of iron gives moles of iron atoms and ions
• Moles of electrons and moles of iron ions give charge on iron ions.
• Write formula from the charge on an iron ion and the charge on a chloride ion
• Part d) looks like a rate problem but it is a stoichiometry problem coupled with an electrochemistry problem