Unit 6: Topic 4 - Heat Capacity and Calorimetry Flashcards
How can we determine the amount of heat transferred between system and surrounding when heating or cooling a specific substance?
It is difficult to determine the exact energy, but the change in energy, or the heat transfer, is equivalent to q = mc∆T, where m is the mass of the substance, c is the specific heat, and ∆T the change in temperature (negative if decrease). Specific heat for a substance is the heat required to raise the temperature of the unit mass (grams) by a given amount (usually 1 degrees Kelvin). We can experimentally determine all this through calorimetry experiments.
What is the first law of thermodynamics, and how is it applicable in chemistry?
The first law of thermodynamics states that energy in the universe cannot be created or destroyed. In any physical or chemical process, the energy of the universe is conserved by this law.
One container holds 10 grams of water, and another container holds 10 grams of mercury. If 100 Joules of heat is applied to both containers, how will the temperature increase in each container? The specific heat of mercury is 0.140 J/g C.
The formula for this is q = mc∆T. For water, the specific heat is 4.184 J/g C (and should be memorized). Since there are 10 grams of water and 100 Joules of heat applied, we have ∆T = q/mc = 100/(10)(4.184) = 2.39 C, so the water changes by 2.39 degrees Celsius. For mercury, we apply the same formula so that ∆T = q/mc = 100/(10)(0.140) = 71.42C, so the mercury changes by 71.42 degrees Celsius. Notice that since the two specific heats are different, the temperature changes will be different given equal heats and equal masses. Similarly, the amount of heat transferred will also differ given equal masses and temperature changes.
How does the formula for heat transfer be used to predict the sign of heat transfer?
q = mc∆T. SInce mass and specific heat are always positive, the sign of heat transfer will be the same as the sign of the temperature change. Therefore, if the system is heated, then the temperature change is positive, resulting in an increase in the energy of the system. Similarly, cooling the system results in a negative temperature change, so the energy of the system will decrease.
How are specific molar heat and specific heat related, and how can they be used in the formula for energy transfer?
Specific heat capacity has units J/g C, where Joules is in the numerator and grams and Celsius are in the denominator. Specific molar heat capacity has units J/mol K, and it is the heat required to raise one mole of the substance by one degree Celsius. Therefore, for a single substance, we can convert the heat capacity to the molar heat capacity by multiplying by the molar mass of the substance. Denote the molar heat capacity by c_m: c_m = c * M. Therefore, since q = mc∆T, we can also use q = (m/M)(c * M)∆T = (n)(c_m)∆T, where n is the number of moles.
What are the three main ways chemical systems change their energy?
- heating/cooling: using the formula q = mc∆T, heating or cooling will cause a change in the temperature, which will change the overall energy of the system.
- phase transitions: for water (and other substances), phase transitions are accompanied by a heat of fusion or heat of vaporization, which will cause heat and energy transfer.
- chemical reactions: when bonds are broken and formed, energy is absorbed and released, which results in a net change in energy of the system.
