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Topic: "General Range of Molar Heats"  (Read 3942 times)

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Offline gt5hz

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"General Range of Molar Heats"
« on: September 17, 2009, 08:25:46 PM »
This phrase appeared on my chem worksheet. I've no idea what it means, though.

The question is like this.

PHYSICAL CHANGE: H20 (l) --> H20 (s), HEAT OF REACTION is -6.03 kJ, GENERAL RANGE OF MOLAR HEATS 10^1 to 10^2 kJ.

How was 10^1 to 10^2 attained?

Offline renge ishyo

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Re: "General Range of Molar Heats"
« Reply #1 on: September 17, 2009, 08:38:21 PM »
That is a puzzling statement indeed. I thought that maybe it was a range to compare the molar enthalpy of fusion for water to other standard compounds (saying maybe that other compounds fall in the range of 10-100kj/mol), but the problem there is that the molar enthalpy of fusion for many common compounds is below 10 KJ/mol for many compounds and certainly not near 100 (see the table on: http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Enthalpy-of-Fusion-and-Enthalpy-of-Vaporization.html).

Is there any other information present on the problem page or was this it?

Offline gt5hz

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Re: "General Range of Molar Heats"
« Reply #2 on: September 17, 2009, 08:49:53 PM »
COMPARING PHYSICAL, CHEMICAL AND NUCLEAR CHANGES is the title.

For centuries man has relied mostly on chemical change for his source of energy. But with the increasing demand for energy, the availability of the chemical energy has been threatened.

In the 20th Century, nuclear energy was discovered. Nuclear reactions involve the breaking and forming of bonds between protons and neutrons with the nuclei of the atoms. Nuclear forces are so much stronger than the electrostatic forces between positive nuclei and negative electrons found in chemical bonds. Thus the amount of energy involved during a nuclear change is much greater than in a chemical change.

Theres also 2 more examples.

CHEMICAL: H2 (g) + 1/2 O2 (g) --> H20 (g), HEAT OF REACTION IS -242.0 kJ, GENERAL RANGE OF MOLAR HEAT IS 10^2 to 10^3 kJ.

And another is a equation for the fuison of deuterium and tritium. Heat of RXN is 1.7 x 10^9 kJ, range is 10^6 to 10^9 kJ.

Offline renge ishyo

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Re: "General Range of Molar Heats"
« Reply #3 on: September 17, 2009, 09:48:55 PM »
I still think from those examples that it is giving you a range for the given types of transformations. I.e. the idea is this:

Physical changes (such as freezing a solid):
Energy Released 10-100 kJ/mol (or would that be 1-100 kj/mol?)

Chemical changes (such as combining two elements to form a compound):
Energy Released 100-1,000 kJ/mol

Nuclear changes (such as converting two hydrogen atoms into helium):
Energy Released 1,000,000-1,000,000,000 kJ/mol

You can see that physical changes are weaker than chemical changes which are both far weaker than nuclear changes. The individual reactions are not important to the concept, they are just specific examples of what each change would be (like what I placed in parenthesis above).

Hope this helped.

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