reaction enthalpy

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705121606
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reaction enthalpy

Postby 705121606 » Mon Jan 27, 2020 11:50 pm

why is the enthalpy negative when bonds are formed rather than when they are broken?

Jainam Shah 4I
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Re: reaction enthalpy

Postby Jainam Shah 4I » Mon Jan 27, 2020 11:59 pm

It is negative, because bonds are being formed and the rxn is exothermic. Forming bonds is more favorable, because it doesn't require an input of energy breaking bonds does.

McKenna_4A
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Re: reaction enthalpy

Postby McKenna_4A » Tue Jan 28, 2020 9:07 am

Forming a bond is a favorable/lower energy state for atoms to be in, so that energy is released. Consider the chart we looked at in 14A-- two atoms decreased in their energies as they became closer, the lowest energy being when they formed a bond; however, when they got any closer, the energy skyrocketed due to the repelling interaction of the two nuclei.

Michelle Xie 2B
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Re: reaction enthalpy

Postby Michelle Xie 2B » Tue Jan 28, 2020 4:57 pm

Energy is released when bonds form, therefore making it exothermic and the enthalpy would be negative since the final molecule has less energy than the separate atoms.

KaleenaJezycki_1I
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Re: reaction enthalpy

Postby KaleenaJezycki_1I » Tue Jan 28, 2020 7:04 pm

Jainam Shah 4I wrote:It is negative, because bonds are being formed and the rxn is exothermic. Forming bonds is more favorable, because it doesn't require an input of energy breaking bonds does.


The reaction being endothermic or exothermic in Thermochemistry is based on what is happening to the surroundings. To break bonds the reaction itself is absorbing heat, which is lowering the temperature of the surroundings therefore this is exothermic. When forming bonds the reaction will have to release heat and therefore the surroundings with absorb heat making it endothermic.

205405339
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Re: reaction enthalpy

Postby 205405339 » Tue Jan 28, 2020 7:10 pm

To break bonds, energy has to be brought in aka endothermic reaction and to form bonds, energy must leave the system aka exothermic

Leila_4G
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Re: reaction enthalpy

Postby Leila_4G » Tue Jan 28, 2020 7:47 pm

This confuses me too because its not like it gives off energy to create a bond, right? Unless it is saying it gives off energy to break a bond.

alicechien_4F
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Re: reaction enthalpy

Postby alicechien_4F » Tue Jan 28, 2020 8:23 pm

When you form a bond, you release energy. On the other hand, when you break bonds, you must add energy. This is because atoms/molecules are more stable when they are bonded together, so to break bonds you must add. When you form bonds, electrons typically go to lower, more stable orbitals, so it releases energy.

Emily Chirila 2E
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Re: reaction enthalpy

Postby Emily Chirila 2E » Tue Jan 28, 2020 8:41 pm

Leila_4E wrote:This confuses me too because its not like it gives off energy to create a bond, right? Unless it is saying it gives off energy to break a bond.


When the bond is formed, energy is given off to the surroundings.
When the bond is broken, energy is absorbed by the system.

It is really important to clarify transfer of energy in terms of the system and surroundings, or else it can be really confusing

Elizabeth Harty 1A
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Re: reaction enthalpy

Postby Elizabeth Harty 1A » Tue Jan 28, 2020 10:32 pm

What is the difference between enthalpy and entropy?

Jessica Kwek 4F
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Re: reaction enthalpy

Postby Jessica Kwek 4F » Thu Jan 30, 2020 6:34 pm

It requires energy to be inputted in order to break bonds, which is why breaking bonds makes enthalpy positive. Therefore, forming bonds would make enthalpy negative.

Simon Dionson 4I
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Re: reaction enthalpy

Postby Simon Dionson 4I » Sat Feb 01, 2020 1:01 pm

Elizabeth Harty 1A wrote:What is the difference between enthalpy and entropy?


Entropy is the disorder of a system, Enthalpy is the measure of heat/energy related to a substance when it is changed.

Nuoya Jiang
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Re: reaction enthalpy

Postby Nuoya Jiang » Sat Feb 01, 2020 3:27 pm

It requires energy to break a bond, so the change in energy should be positive. When a bond form, the system indeed goes more stable, with less energy, so it releases energy, and the enthalpy should be negative.

Elizabeth Harty 1A
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Re: reaction enthalpy

Postby Elizabeth Harty 1A » Tue Feb 04, 2020 8:48 am

In the thermodynamics outline what does the bullet point "calculate enthalpy changes from calorimetric data" mean?

Minh Ngo 4G
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Re: reaction enthalpy

Postby Minh Ngo 4G » Tue Feb 04, 2020 9:32 am

Elizabeth Harty 1A wrote:In the thermodynamics outline what does the bullet point "calculate enthalpy changes from calorimetric data" mean?

I think he is referring to deltaH rxn so like the 3 ways and the q=mcdeltaT


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