**AP CHEMISTRY EXAM ** **QUESTION PROFILER**

**VSEPR Models**

### AP EXAMS FROM 1999 - 2011

There were eleven questions that directly involved VSEPR, Lewis structures, and

polarity. Many of these questions also incorporated other topics.

There was an average of 3.7 subparts related to the above topics per question.

There was a total of 41 subparts that included VSPER.

### BREAKDOWN OF QUESTIONS RELATED TO VSEPR

**Exam Year** **Other Topics Included** **Number of **

**Subparts Related **
**to VSEPR**

2002B Bonding 5

2004 Gas Law 2

2005 -- 7

2006 Oxidation numbers 7

2006B -- 5

2007 Factors affecting reactions, including kinetic and

thermodynamic factors

4

2007B Acid-base buffer 1

2008 Ionization energy 4

2009B Thermodynamics 1

2010 Intermolecular Forces 5

2011 Inter- versus intra-molecular

forces, thermodynamics 2

### ENTRY POINTS ON FREE RESPONSE QUESTIONS

The questions are designed to have an “entry point” for the student.

An entry point is a relatively easy question (usually worth one point).

It is designed to give the students a feeling of “I can do this problem”

Build confidence in the student so they attempt to do the problem.

### ENTRY POINT FOR QUESTIONS WITH VSEPR

In each of the questions that involved VSEPR, drawing a Lewis dot diagram and/or

identifying the shape of a species is the entry point to a question.

This means: This is a topic you should feel comfortable with and be able to do with

ease.

## STUDENT PERFORMANCE

**Year**

**Possible **
**Points **

**for **
**VSEPR **
**subparts**

**Performance**

2011 2

The mean score was 3.04 out of 8
possible points, with most of the
**points earned by completing the **
Lewis diagram and identifying the
arrangement of atoms or citing the
electron-pair repulsion.

2010 6

The mean score was 4.09 out of 8
possible points. The distribution was
bimodal with one maximum between
5 and 6 and the other at zero. Most
**students earned points on the **
**Lewis diagrams.**

### STUDENT PERFORMANCE

**Year**

**Possible **
**Points for **

**VSEPR **
**subparts**

**Performance**

2008 5

The mean score was 3.92 out of 9 possible points, with scores of 4 and 5 both being modal. This question had a wide bell-shaped distribution of scores.

**Most students drew correct **
**Lewis diagrams. Performance **
varied when students attempted to
use these diagrams to reason out
the shape, the central atom

hybridization, and then molecular polarity.

2005 9 Almost all students attempted this required question. The mean score was 4.97 out of a possible 9 points.

### COMMON MISTAKES MADE BY STUDENTS

Students did not draw diagrams in the boxes provided, and sometime they drew multiple diagrams without indicating which was their final answer.

Forgetting to answer questions completely.

Miscounting valence electrons

They showed all electrons, not just valence electrons

### COMMON MISTAKES MADE BY STUDENTS

Students sometimes completed an octet for hydrogen.

Placing unbonded electron pairs in the wrong position

Students gave an overemphasis on the octet rule and “happy” electrons.

Occasionally structures included lone electrons

### COMMON MISTAKES MADE BY STUDENTS

### Unable to identify a triple bond as being stronger than a single bond

### Misunderstanding the term

### “resonance.”

### Many students did not understand the term “hybridization of the S atom.”

### Some confused this with atomic

### structure or a statement of the

### geometry

### COMMON MISTAKES MADE BY STUDENTS

### Incomplete justification given for a

### molecular geometry. A statement that lone electron pairs are present is a fact and does not provide an explanation

### for the geometry of the molecule.

### Describing the two-dimensional image

### you have drawn on the paper, citing

**relative the relative positions of **

**molecules as above or below one **

### another is not sufficient justification.

**ANALYSIS OF**

**FREE RESPONSE QUESTION**

**2006**

A choice of two questions was given on this exam. The

other question tested the student on periodic trends and ions.

There is no longer any choice on the exam – you MUST do ALL problems.

This question was

worth 15% of the Free Response section, or 7.5% of the entire exam.

Recall that halogens only form single bonds – not part of CNOPS!

Notice that both stems in this question are worded in the same manner.

Similarly, parts i, ii, and iii ask for the same things in both questions, but (a) examines a cation and (b) an anion.

You must justify your answer in order to earn credit for this portion.

**CHIEF READER’S ** **COMMENTS**

**2006**

### CHIEF READER’S COMMENTS

Key words:

• Lewis Structures

• Hybridization

• Geometry

• Effect of nonbonding pairs of electrons

• Bond angles

• Expanded valence shell hybridization and geometry

• Oxidation number on element in an ionic compound

### CHIEF READER’S COMMENTS

• This is a problem where a well-prepared student can earn full points.

• Make sure you take full advantage of problems, like this one, that focus on a basic concept and information.

### CHIEF READER’S COMMENTS

### CHIEF READER’S TIPS FOR STUDENTS

Learn how to count electrons for charged species.

Are consistent in the representation of

electron pairs – use either dotes or dashes so they do not represent more electrons than

the number of valance electrons present in the Lewis structures drawn.

Learn hybridization and the corresponding shapes.

### CHIEF READER’S TIPS FOR STUDENTS

Learn the correct names of the shapes for expanded valence geometries.

Learn the difference between bonding and nonbonding pairs.

Use the correct vocabulary for unshared pairs (not referring to them as “unpaired

electrons,” “lone electrons,” “extra pairs”)

### CHIEF READER’S TIPS FOR STUDENTS

Represent the bonding and not the

complementary angle for a structure (often there was not a clear idea as to what F—S—F angle represented).

Use models so that they can see the shapes of species (either hands-on models or

computer models).

Understand hybridization in general terms
(e.g., students should recognize that sp^{4}
hybrids do not exist).

### CHIEF READER’S TIPS FOR STUDENTS

Know the effect of nonbonding electrons on the shape of a species.

Know that the geometry of a species

represents the shape of the species that one would see if the species could be magnified.

The use of three-dimensional models in

teaching is helpful for students to understand the geometric shapes of species.

**SCORING STANDARD**

**2006**

### SCORING STANDARDS

Notice that it is okay to use dots or

dashes, but BE CONSISTENT!

Be sure to add the charge

### SCORING STANDARDS

The Readers prefer the lines for a simple reason – they are easier to see!

An exact angle does not need to be given. The question does not ask for one.

These do not need to be answered in full sentences.

If you have drawn the Lewis

structure incorrectly in part (i), you can still get credit for part (iii).

This is also true for part a(iii).

**HOW TO SUCCEED WITH ** **THESE TYPES OF **

**PROBLEMS**

**Tips from the Chief Reader**

### IN GENERAL:

Read the question carefully and answer the

question asked. Begin by writing the answer and then providing supporting evidence.

When responding to the question of whether a statement is true or false, begin by writing down one of these choices and then provide supporting evidence. Do not begin the answer with the word

“yes.”

Be sure to address each substance within a given question stem. When asked to compare two

substances, always talk about both. Simply

discussing one and expecting the reader to assume that the other is implied will not receive credit.

### IN GENERAL:

Often information is used for multiple parts of a question; be sure to refer to information

that is given.

Avoid vague generalizations when answering questions. Give details as often as possible.

Write legibly and be sure to write all answers in the lined spaces provided instead of

squeezing words between question parts.

Be sure to label each response; this is

especially true when choosing to answer the questions out of order.

### DRAWING LEWIS STRUCTURES:

Take time to calculate the total number of electrons before beginning.

*Students should clearly represent the correct *
number of electrons in a Lewis diagram with
easy-to-discern dots or dashes (to represent
electron pairs).

Know that molecules are three-dimensional and that bond angles should reflect this

### SHAPES AND ANGLES

Know the difference between electronic and molecular structure.

Appreciate the difference between the double-headed resonance arrow, which

implies that neither individual Lewis diagram is correct but that the actual structure is

between the two presented structures, and the two arrows that are presented together pointed in opposite directions which

represent a dynamic process, with bonds rapidly oscillating between two positions.

### OTHER TIPS:

Limit the use of the “octet rule” and avoid discussing “happy” molecules, ions, and electrons

Practice determining hybridization and

number of sigma and pi bonds in molecules

### OTHER TIPS:

Students need to practice the reasoning sequence employed in understanding molecular structure: complete a Lewis

electron-dot diagram, use this to determine the electron-pair orientation and thus the molecular shape, and then use the shape to find the central atom hybridization and

molecular polarity. Symmetry plays a major role in determining polarity.

### OTHER TIPS:

“People like to open books”

Shapes that can be symmetrical and therefore non-polar:

Planar

Linear

Tetrahedral

Octahedral

Bipyramidal.