8.67: Carbon graphite

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Mayvera Doaran 2H
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8.67: Carbon graphite

Postby Mayvera Doaran 2H » Tue Jan 12, 2016 9:15 pm

For parts b, c, and d, carbon in its solid phase is shown in the equation as C(gr). I know the (gr) stands for graphite, but why use (gr) instead of (s)? Why do we assume that carbon is in its solid phase, using its enthalphy of sublimation, while we assume that hydrogen and oxygen are in their gas phase when using bond enthalpies?

fatima donato 1H
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Re: 8.67: Carbon graphite

Postby fatima donato 1H » Tue Jan 12, 2016 10:35 pm

This homework question asks us to use the standard enthalpy of formation method. When we use this method, we assume that we are forming 1 mole of product by breaking up bonds between reactants in their standard states. Oxygen and hydrogen are in their standard states in the gas phase, while carbon is in its standard state as graphite. A variety of solids can be formed by carbon but graphite can only be formed by pure carbon, so using (gr) instead of (s) is a way of specifying that we are dealing with carbon in its purest (aka standard) state. From my understanding all the reactants should be in the same phase to react. We use the enthalpy of sublimation for carbon to take into account the transition between phases that has to take place in order for this reaction to proceed. For example, in part b, carbon must be changed to a gas in order to react with hydrogen gas and oxygen gas. We use the bond enthalpies for hydrogen and oxygen because we have to break the H-H and O-O bonds to form new bonds for the methanol molecule. This requires an input of energy. Since C(gr) is not bonded to anything else we only have to worry about its transition to a gas (which also requires energy).

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Re: 8.67: Carbon graphite

Postby Chem_Mod » Wed Jan 13, 2016 12:12 am

The above explanation is great! I want to clarify a bit. Pure Carbon is not only in graphite form but also in diamond form but at standard condition, graphite is more stable. The reason it use sublimation enthalpy is because if you look at the reaction, you only want 1 C atom to react; therefore, you have to form 1 C atom first buy atomize graphite. With only 1 atom, C certainly cannot stay in solid form but rather in gas state.

H2 and O2 are at gas phase is because that is their most stable form at the reaction condition.


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