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Chemical Bonding
Chemistry Regents June 2005 - Question 08 Print E-mail
NYS Chemistry Regents June 2005
chemjune05q08

Correct answer (4) different properties and different crystal structures.

Why:

Diamond and Graphite are the two most common crystalline forms of carbon. At standard temperature and pressure (STP), diamond has a very rigid structure composed of strong covalent bonds while graphite forms its covalent bonds in parallel sheets relying on weak intermolecular bonds between the sheets.   Because of these structures the physical properties differ substantially.  Diamond is one of the hardest substances known to man and insulates electricity.  Graphite on the other hand is one of the softest substances known and conducts electricity.   In conclusion, even though diamonds and graphite are made from the same basic element their structure and therefore their properties are very different making (4) the correct answer.

Answering this question:

It is a fundamental principal in chemistry to remember that an element or compounds structure and composition is always a direct factor in its chemical or physical properties.  Therefore (2) and (3) should be removed as possible answers immediately regardless of what substances you're comparing.  A substance with different properties can almost always attribute those differences to having a different structure and a substance with different structures will almost always have different characteristics.  This leaves you with two possible answers: that they're similar in structure and properties or that they're different.  Diamonds and graphite are fairly common items and we know inherently they're different.  Diamonds are hard gems we wear, and are even used as hard drill bits. Graphite is in pencils that we write with and breaks easily.  So using some basic principals and general knowledge of the substances it is easy for you to know that (4) will be the best answer.

 
Chemistry Regents June 2005 - Question 09 Print E-mail
NYS Chemistry Regents June 2005

chemjune05q09

Correct Answer (1) TiO

Why:

When an element is written such as titanium(II) the number in the parentheses is usually referring to the oxidation state.  So in this case we are looking at Ti2+ combining with Oxygen to make Titanium (II) Oxide.

The periodic table is arranged in groups that also give us an idea of how many valence electrons an element has.  The "tall" groups (vertical lines) can be counted, beginning with Hydrogen, one thru eight correlating to how many valence electrons are available. By this method we know Oxygen has 6 valence electrons and wants two more to complete itself with 8.

Titanium (II) has an oxidation state of +2 and will readily share with the Oxygen making this the most stable compound for it to form.  Answer (2) has two oxygen atoms therefore needing 4 electrons and will typically only form with Titanium (IV).  Answer (4) has three Oxygen needing 6 electrons and two Titanium (III) will join to complete that.  Answer (3) would need to be Titanium (I) and that doesn't occur naturally.

Answering this question:

The only way to answer this question with 100% certainty is to know what titanium (II) oxide is.  If you don't recognize it from memory you may have a better shot at the correct answer by looking at Nomenclature.  Answer (2) should be something like Titanium dioxide.  Answer (1) is the only straightforward titanium oxide.  While this approach is better than nothing, it will not always steer you into the right answer in chemistry.

 
Chemistry Regents June 2005 - Question 10 Print E-mail
NYS Chemistry Regents June 2005
chemjune05q10

Correct Answer : (4) ammonia

Why:

An element or compound can undergo a physical of chemical change.  In a physical change the molecule is never changed. The same element or compound is present at the beginning and the end.  In a chemical change the element or compound reacts and changes into or creates a new element or compound.  A substance is decomposed when it is broken down into smaller compounds or elements.  In this question, answer (4) is the only possible answer because it is the only compound that can be broken down or decomposed.  Answers (1), (2), and (3) are all elements and have nothing they could be broken down into.

Answering this question:

The best way to answer this is to understand that a chemical change requires a change in the element or compound.  The next is to logically recognize decomposed as a term for breaking something down. With that, this question simply becomes a "which of these is not like the others" solution.  Answer (4) is the only compound that could be broken down. The rest of the answers are all elements and could not be decomposed.

 
Chemistry Regents June 2005 - Question 13 Print E-mail
NYS Chemistry Regents June 2005
chemjune05q13

Correct Answer (2) an element and a compound

Why:

A pure substance is defined as a substance with constant composition.  It can be classified as an element or a compound.  An element is a substance that can not be separated into two or more substances by ordinary chemical reactions.   A compound is a substance that contains two or more elements, but a pure compound's composition will be invariant regardless of the way it was prepared.  A mixture is the combining of dissimilar substances that do not combine to form a compound.  A solution is combining substances into a mixture with uniform concentration (homogeneous) and has a variable composition.

Answering this question:

The best way to answer this question is to know what; an element, mixture, solution, and compound are and find the best combination in the answer. We know an element is about the purest form of substance we've studied.  It is uncombined with anything and therefore we can narrow our choices to (1) and (2).  Between a mixture and a compound we may have to guess a little more, but inherently a mixture sounds impure with two substances put together.  A compound is two substances combined as well but they are actually bonded and form something new. Whereas the mixture is two or more substances combined, but each substance retains its own identity.  So (2) is the best possible answer.

 


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