Standard RXN Entropy/Enthalpy

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Rachael_1H
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Joined: Fri Sep 25, 2015 3:00 am

Standard RXN Entropy/Enthalpy

Postby Rachael_1H » Thu Jan 21, 2016 7:41 pm

When calculating the standard rxn entropy and enthalpy, using standard molar entropies and enthalpies, how do you know whether to subtract sum of products from sum of reactants, or vice verse? Our course reader says:
Standard RXN Entropy/Enthalpy=(sum of products' standard molar entropies/enthalpies)-(sum of reactants' standard molar entropies/enthalpies).
In practice quiz 1 though there is a problem that requires you to use the equation:
RXN Enthalpy=(sum of reactants' mean bond enthalpies)-(sum of products' mean bond enthalpies) to get to the correct answer. The problem is: estimate the rxn enthalpy for burning of 1 mol of Butane: C4H10(g) + O2(g) ---> CO2 + H2O(g).

Can someone please clarify when to subtract sum of products from reactants, or vice verse? Does it depend on which bonds are being broken and formed, and if so how do you know when a bond is breaking or forming? Sorry if this post does not make sense. I am very confused.

Katherine Ho 1E
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Joined: Fri Sep 25, 2015 3:00 am

Re: Standard RXN Entropy/Enthalpy

Postby Katherine Ho 1E » Fri Jan 22, 2016 3:56 pm

You subtract when they ask you to find Hrxn using mean bond enthalpies because what it's really asking for is

delta Hrxn= sum of mean bond enthalpies broken - sum of mean bond enthalpies formed

Knowing whether you use products-reactants vs reactants-products (really bonds broken-bonds formed) depends on what you're given. If you're given the enthalpy of formation, Hf, or the enthalpies of a set of reactions, then use sum of prod-sum of reactants.

However, if you are asked to find Hrxn by means of mean bond enthalpies (where the molecule is broken up and the mean bond enthalpies are used to calculate), you would use sum of mean bond enthalpies broken - sum of mean bonds formed

So for example with C4H10(g) + O2(g) ---> CO2 + H2O(g)

The bonds of the reactants C4H10 and O2 must be broken in order to form CO2 and H2O
so that would be [mean bond enthalpies of C4H10 + mean bond enthalpies of O2] (broken) - [mean bond enthalpies CO2 + mean bond enthalpies H20] formed

Rachael_1H
Posts: 31
Joined: Fri Sep 25, 2015 3:00 am

Re: Standard RXN Entropy/Enthalpy

Postby Rachael_1H » Sun Jan 24, 2016 12:01 am

Thank you. How do you know whether The reactants' bonds are breaking or forming? I understand that you look at the chemical equation, but for the equation
Ch4 + 2O2 ----> CO2 + 2H2O
For instance, are the reactants' bonds breaking? Does this make it exothermic?

Chem_Mod
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Re: Standard RXN Entropy/Enthalpy

Postby Chem_Mod » Sun Jan 24, 2016 12:45 am

Just break all of the reactants' bonds (endothermic) and form all of the products' bonds. This works because enthalpy is the state function, and this way is one path to go from the reactants to products.


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