Calculate the amount of heat needed to melt 68.0 g of solid ethanol (CH3CH2OH) and bring it to a temperature of 15.9 degrees Celsius. What is the unit symbol for this answer?

Question

I really am no good at this, I don't know what equation to use or how to begin solving this. This is a SOS

Answer 1

What's a SOS? At any rate, these are actually not that difficult to do if you take them in steps. For this problem, there isn't enough information provided, but it can be obtained. They should give you the specific heat for ethanol, and the ∆H_fusion for ethanol. You also need to know the starting temperature of the solid ethanol, which I will assume to be the freezing point of ethanol (-114ºC). You can tell whoever gave you this problem that there isn't enough info provided. The general equation to use for these kinds of problems is

q = mC∆T where q is the amount of heat; m is the mass; C is the specific heat and ∆T is the change in temperature. If there is a PHASE CHANGE, then you simply use q = m∆H as explained below. The values I found on the internet are as follows and you may find slightly different values depending on your source:

specific heat liquid ethanol (C) = 2.46 J/g/deg

∆H_f = 106.5 J/g

melting point ethanol = -114ºC

So, here are the steps that you use:

(1) find the heat (q) needed to melt the solid. This is a PHASE CHANGE so there is NO change in temperature.

You only need what is called the enthalpy of fusion, ∆H_f.

(2) find the heat (q) needed to raise the temperature of the now melted solid (a liquid) from the current temperature to the desired temperature.

(3) sum up all the values of q to get the total heat

Step (1) q = m∆H_f = (68.0 g)(106.5 J/g) = 722 J Note, since there is no change in temperature during a phase change, you don't need a ∆T term. Just use the ∆H and the mass.

Step (2) q = mC∆T = (68.0 g)(2.46 J/g/deg)(15.9 - -114) = (68.0 g)(2.46 J/g/deg)(129.9 deg) = 21,730 J

Step (3) Add them up: 21,730 J + 722 J = 22,452 J = 22,500 J (to 3 sig. figs.)

The unit symbol is J (which stands for joules). You can also report it in units of kJ (kilojoules) and it would then be 22.5 kJ.