[sam12103]

1. The freezing point of mercury is -38.8 degrees C. What quantity of energy, in joules, is released to the surroundings if 1.00 mL of mercury is cooled from 23.0 degrees Celsius to -38.8 degrees Celsius and then frozen to a solid? (The density of liquid mercury is 13.6 g/cm^3. Its specific heat capacity is 0.140 J/g*K and its heat of fusion is 11.4 J/g.)

answer:273 J

2. Ethanol C2H5OH, boils at 78.29 degrees celsius. How much energy, in joules, is required to raise the temperature of 1.00 kg of ethanol from 20.0 degrees Celsius to the boiling point and then to change the liquid to vapor at that temperature? ( The specific heat capacity of liquid ethanol is 2.44 J/g*K and its enthalpy of vaporization is 855 J/g.)

answer:9.97*10^5 J


can you please help me solve these two problems, I have been working on them for a while and I haven't been able to figure out how to solve them to get the answer provided in the back of my textbook. In particular for these problems, I don't know which formula to apply.

[Yggdrasil]

For problems like these you need to split the process into two (or more steps):

1)  For steps where there is no phase change (e.g. no melting, freezing, boiling, etc.) and your substance is only changing temperature, you calculate the amount of heat absorbed/released as:

q = mCΔT

where q is the amount of heat, m is the mass of the substance, C is the specific heat capacity, and ΔT is the change in temperature.

2)  For steps involving a phase change, you would use the following formula:

q = mΔH

where q is the amount of heat, m is the mass of the substance, and ΔH is the change in enthalpy associated with that particular phase change.