Energy Calculations

Energy Calculations Specific Heat vs. Molar Heat Capacity

Different substances respond differently to being heated. • This response can be described in terms of either “specific heat” (cp) or “molar heat capacity” (C). • Both are physical properties and can be used to describe/identify the substance.

Both are physical properties and can be used to describe/identify the substance. • Specific heat relates the amount of energy needed to raise the temperature of a sample to its mass • Molar heat capacity relates the amount of energy needed to raise the temperature of a sample to moles.

Both can be measured in either calories (cal) or joules (J). • The unit for specific heat is “joules per gram degree Celsius”: J/gºC (or J g-1 ºC -1); • The unit for molar heat capacity is “joules per mole degree Kelvin”: J/molK (or J mol-1 K-1)

Both can be measured in either calories (cal) or joules (J). • The unit for specific heat is “joules per gram degree Celsius”: J/gºC (or J g-1 ºC -1); • The unit for molar heat capacity is “joules per mole degree Kelvin”: J/molK(or J mol-1 K-1)

Specific heat reports the temperature change in degrees Celsius, but molar heat capacity reports this change in degrees Kelvin • Both involve a change in temperature (T = Tfinal - Tinitial) and not a change in state. • [Note: a change in degrees Celsius is the same as a change in degrees K because one Celsius degree is equivalent (the same size) as one Kelvin degree]

[Note: a change in degrees Celsius is the same as a change in degrees Kelvin because one Celsius degree is equivalent (the same size) as one Kelvin degree]

Directions: Answer, fill in or calculate the following. Show work for the calculations. Include units when appropriate. Use your notes, text (including the Appendix) and tables provided.

Remember-show work! units! • Specific Heat (definition)- • Molar Heat Capacity (definition)- • How do you convert between Celsius and Kelvin? (give the conversions) • What temperature is absolute zero? (report in terms of Celsius and Kelvin) • How does the size of the Kelvin and the Celsius degree compare? Explain.

6. What is the change in temperature (T) for the following?(indicate + or – , report in ºC and K) • a rise in temperature from 25K to 50K T _____ºC; T _____K • a drop in temperature from 96ºC to 11ºC T _____ºC; T _____K • a rise in temperature from -15º to 5ºC T _____ºC; T _____K

Energy needed to raise the temperature of a sample can be calculated using the following equation: q = mT cp (___ stands for heat, ____ stands for mass, ____ stands for change in temperature and ____ stands for specific heat) Rearrange the above equation to solve for each of the following: m = T = cp = Energy needed to raise the temperature of a sample can be calculated using the following equation: q = nT C (___ stands for heat, ____ stands for mole, ____ stands for change in temperature and ____ stands for molar heat capacity) Rearrange the above equation to solve for each of the following: n = T = C=

Calculations • Perform the following calculations. Attach your answer sheet to the back of this handout. Show your work. Circle your answers, include the unit. Report to the correct number of significant figures. • Remember, you can use molar mass to convert between moles and mass.

1. The specific heat of iron is .46 J/gºC. How many joules will it take to make the temperature of a 150 g bar go up from 20.ºC to 70.ºC?

2. What is the specific heat of silver if an 80.0g sample is heated from 24.0ºC to 49.0ºC by adding 468.2 J?

3. What is the molar heat capacity of copper if a 75 g sample is heated from 20.ºC to 24.ºC by adding 117 J?

4. What is the specific heat capacity of the sample in question #3?

5. The specific heat of aluminum is .88 J/gºC. How many joules will it take to make a 50. g nugget go up by 50 K?

6. How many joules are needed to change the temperature of 22 g of water from 18ºC to 33ºC? (the specific heat of water is 4.18 J/gºC and its molar heat capacity is 75.3 J/molK) ?

Energy Calculations