# chemistry

Ethanol (C2H5OH) melts at -114 °C. The enthalpy of fusion is 5.02 kJ/mol. The specific heats of solid and liquid ethanol are 0.97 J/g-K and 2.3 J/g-K, respectively. How much heat (kJ) is needed to convert 25.0 g of solid ethanol at -125 °C to liquid ethanol at -50 °C?

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1. There are two equations used here. The one (or ones) you use depend upon if the ethanol is within one phase or if it is changing phase. What does that mean? If ethanol is a solid and is changing temperature but stays a solid OR if it is a liquid and is changing temperature but staying liquid OR if it is a vapor and changing temperature but staying a vapor, that is WITHIN one phase and the formula is
q = mass ethanol x specific heat in that phase x (Tfinal-Tinitial) = ?

If the phase is changing; i.e., it is solid and changes to a liquid (or the reverse) or it is liquid changing to a vapor (or the reverse) that is a phase change and the formula is
q = mass ethanol x heat fusion = ? if from solid to a liquid or liquid to a solid or
q = mass ethanol x heat vaporization (or condensation) = ? if from liquid to vapor or vapor to liquid.
So you go through each step from where ethanol is now to where it will be at the end and add each q to get a q total. This process will work any problem of this type.
For example: the FIRST step in warming ethanol from its current temperature of - 125 C as a solid to its melting point of -114 C is
q1 = mass ethanol x specific heat solid ethanol x (Tfinal-Tinitial) =
25.0 g x 0.97 J/g*C x [-114 -(-125)] = ?
and continue to the next step of melting the solid ethanol @ -114 C to liquid ethanol @ -114 etc. Post your work if you get stuck.

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DrBob222

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