# Homework Help: Volatility of alkyl halides

1. Jul 12, 2015

### gracy

1. The problem statement, all variables and given/known data
which of the following is least volatile (1)propyl fluoride (2)propyl chloride (3)isopropyl bromide (4)propyl bromide

2. Relevant equations
_______

3. The attempt at a solution
The boiling points of different alkyl halides containing the same halogen increase with increasing chain length. For a given chain length, the boiling point increases as the halogen is changed from fluorine to iodine. For isomers of the same compound, the compound with the more highly‐branched alkyl group normally has the lowest boiling point. Hence, the correct order of volatile nature of the compounds mentioned are:

Propyl fluoride > Propyl Chloride > Isopropyl bromide > Propyl bromide
right?

2. Jul 12, 2015

### Staff: Mentor

Have you tried to google for their boiling points?

3. Jul 12, 2015

### gracy

yes.it has he followng order Propyl fluoride <Propyl Chloride <Isopropyl bromide < Propyl bromide

4. Jul 12, 2015

### gracy

So it verifies that the order of volatility as given below
is correct.

5. Jul 12, 2015

### gracy

6. Jul 12, 2015

### gracy

is it right?

7. Jul 12, 2015

### Staff: Mentor

Trust the data, not random opinions on the internet.

8. Jul 12, 2015

### gracy

Data confirms this.And i don't think physics forum experts have random opinions.

9. Jul 12, 2015

### epenguin

IMHO not worth knowing independently of a rationale- can you give one?

Also for melting points while you are at it.

10. Jul 12, 2015

### gracy

11. Jul 12, 2015

### epenguin

You have learned some facts. If you meet these substances in the laboratory they might mean something purely qua facts, but then the laboratory is the place to be learning them. (Actually I do not frequent any laboratories now so if you will be kind enough to remind me what the boiling points are something may come back to me).

If you are not in a laboratory the facts can only have a value if related to a theoretical rationale, it is impossible as well as useless to remember the facts otherwise.

12. Jul 15, 2015

### Staff: Mentor

You should learn solve problems by knowing general rules or knowing how to lookup general rules. Then applying rules to specific observations to make sense of the observations.

There are two very important concepts in learning and applying Science: Inductive reasoning, Deductive reasoning.

What you did is best explained as deductive. You had a problem, you looked up a general statement that can be used to solve your problem, then, finally you researched specific properties of each compound so you could order them by some arbitrary property into a list.

Inductive reasoning can be more hazardous - you take some observables and try to make a general hypothesis and then test it against your information. If your group of observables comes from a non-representative population, is very small, or is otherwise limited AND you do not know any of this, then the hypothesis you create has likely got problems. We humans do this kind of reasoning instinctively. Because in the distant past if we saw a tiger attack an animal or a group member you would immediately thereafter avoid tigers. You did not have to witness another attack and risk being tiger chow. Since we all got past the tiger chow stage, this kind of reason is how we work day to day. A "bum-like" man walks up and pulls a knife, you run away. Thereafter you avoid people that appear to you to be the same as the guy who pulled a knife. One incident and you develop a general rule: avoid bum-like people. Use some other characteristic than "bum-like"" if that does not work for you.

As an aside: there is a really unfortunate aspect to this - people get all kinds of bizarre ideas from observing something and not having a good way to know if what they saw was representative. But. They don't care! Our tiger-chow avoidance program (inductive reasoning) kicks in. They are convinced that they know. Advertisers and politicians exploit this reasoning tendency mercilessly.