THERMOCHEMISTRY
-- study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling.
--focuses on these energy changes, particularly on the system's energy exchange with its surroundings.
-- useful in predicting reactant and product quantities throughout the course of a given reaction. It is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable.
Endothermic reactions - absorb heat.
Exothermic reactions - release heat.
-- Thermochemistry coelesces the concepts of thermodynamics with the concept of energy in the form of chemical bonds. The subject commonly includes calculations of such quantities as heat capacity, heat of combustion, heat of formation, enthalpy, entropy, free energy, and calories
THERMODYNAMICS
-- science of energy conversion involving heat and other forms of energy, most notably mechanical work. It studies and interrelates the macroscopic variables, such as temperature,volume and pressure which describe physical, thermodynamic system.
THERMODYNAMICS is thestudy of processes which involveheat transfer and the performance of work..
THERMOCHEMISTRY is the study of this heat exchange and work on chemical reactions. The law of conservation of energy states that the total energy of the universe is constant.
Energy univ = constant
Since we are looking at the universe in a system-surroundings point of view, we can restate the law as the “sum of the energy in the systemand the energy in the surroundings is constant”.
Energy sys + Energy sur = constant
Therefore, whatever energy is released by the system, the surroundings gain. Their sum is stil the same.
Systems can be classified as open, closed, or isolated.
An Open system allow energy and mass to pass across the system boundary.
An Open system allow energy and mass to pass across the system boundary.
OPEN SYSTEM
The ocean is an example of an open system. The ocean is a component of the hydrosphere and the ocean surface represents the interface between the hydrosphere and the atmosphere that lies above. Solar radiation passes through the atmosphere and is absorbed by the ocean. The absorbed energy evaporates water from the ocean. As the water vapor (mass) enters the atmosphere it carries with it the heat used to evaporate the water (called latent heat). When the water vapor enters the air it raises the air's humidity.
CLOSED SYSTEM
ENERGY TRANSFER
Movement of energy from one place to another or from one substance to another, or the conversion of energy from one form to another. For example, in a car engine, the chemical energy of the fuel is converted to kinetic (motion), heat, and sound energy. HEAT * the transfer of energy between a system and surroundings due to temperature difference. It spontaneously flows from a higher temperature to a lower temperature. It may be absorbed or released by a system depending on which has a higher temperature between the system and the surroundings. The convention for the sign of Q for the two cases is given in the table. Assigned convention for Heat Q
Q = mcΔT Where Q = heat absorbed or released, J m = mass of substance, g c = specific heat of substance, J/gC ΔT = change in temperature WORK -- defined as the force applied over a given distance. It is the energy transfer between a system and surroundings due to a force acting through a distance. Work may be done by the system or done on the system. The convention of the sign of w for the two cases is given on the table | |||||||||
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To determine the magnitude of work, we derive an expression for work where a change in volume is used since expansion and compression are involved. Note that an expansion gives a positive ΔV while a compression results to a negative ΔV, where ΔV = Vfinal – Vinitial From the definition of work Work = force applied x Δdistance Recall that, Pressure = focr/area and force applies has a negative magniotude making the expression w = -pressure applied x Δvolume The magnitude of work is in joules. Every 1 liter-atmosphere is equal to 101.3 joules. ENTHALPY Internal nergy (E) is the sum of the kinetic and potential energy found in the system. It is the total energy found on the system being studied. However, this is not quantifiable. But internal energy also changes as the system changes. A change in the internal energy of a system can be determined by getting the difference between the final and initial values. ΔE = E final – Einitial A positive ΔE means that energy was gained from the surroundings. A negative ΔE means that energy was lost to the surroundings. Another equation for change in internal energy is the sum of heat and work. ΔE = Q + w From this we can ssy that when heat is absorbed by the system and the surroundings are doing work on the system, the change in internal energy is positive. On the other hand, in a system realeasing heat and doing work, ΔE is negative. | |||||||||
The law of energy conservation is evident in the ΔE. Work, heat, or both may cause a change in ΔE. * if the system is held at constant volume, w = 0 (*from-pΔV), consequently, ΔE = Qv ( subscript signify constant volume) *If no heat transfer will be allowed (adiabatic system), ΔE = w |
HESS'S LAW
-- a relationship in physical chemistry named for Germain Hess a Swiss-born Russian Chemist and Physician
The law states that the enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps.
The law is an expression of the principle of conservation of energy, also expressed in the first law of thermodynamics and that the enthalpy of a chemical process is independent of the path taken from the initial to the final state. Hess's law can be used to determine the overall energy required for a chemical reaction, when it can be divided into synthetic steps that are individually easier to characterize. This affords the compilation of standard enthalpies of formation that may be used as a basis to design complex syntheses.
Table of data for a Hess's law calculation:
| Substance | ΔH |
|---|---|
| CH4 (g) | -75 |
| O2 (g) | 0 |
| CO2 (g) | -394 |
| H2O (l) | -286 |
Using this data, ΔH
-
- CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l)
-
- ΔH
oc = [-394 + 2(-286)] - [-75 + 2(0)] = -891 kJ/mol
- ΔH
CALORIMETRY
-- science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat. Scottish physician and scientist Joseph Black, who was the first to recognize the distinction between heat and temperature, is said to be the founder of calorimetry.
Test Proper.
General Instruction: Answer the following questions honestly. You are not allowed to go back and read again on the Lecture part after starting this test. Use capital letters only. You may use your calculator on the last part of this test.
I. Fill in the blanks the correct answer.
1. _______ is the study of processes which involve heat transfer and the performance of work.
2. A closed system is only capable of transferring energy through a _______.
3. _______ is a science of measuring heat based on the change in temperature of an observed body when it releases or absorbs heat.
4. The study of heat exchange and work on chemical reactions is called _______.
5. _______ is a part of the universe outside the system separated by a boundary.
II. Write
1. Enthalpy is a state function.
2. The Earth is a closed system.
3. Internal energy is a state function.
2. The Earth is a closed system.
3. Internal energy is a state function.
4. The formula for specific heat is q/m△t.
5. The calorimeter is the science of measuring heat based on the change in temperature of an observed body when it releases or absorbs heat.
III. Choose the correct answer.
1. It states that the enthalpy change of an overall reaction is the sum of the enthalpy change of its individual steps.
a. Rate Law
b. Hess’s Law
c. Law of Conservation of Energy
2. Type of system that allows the transfer of both energy and matter into and out of the system through a boundary or wall.
a. Open system
b. Isolated system
c. Closed system
3. A system doing work while it heats up has a
a. ± w, ±Q
b. + w, – Q
b. + w, – Q
c. – w, + Q
d. – w, – Q
4. It is a much used term that represents a rather abstract concept. It cannot be seen, touched, smelled or weighed.
a. work
b. heat
b. heat
c. energy
5. 1 Joule is equal to
a. 1 kg m²/s²
a. 1 kg m²/s²
b. 0.23901 cal
c. Both a and b
d. None of the above
IV. Solve the following problems.
1. The work done when a gas is compressed in a cylinder-like is 462 J. During the process, there is a heat transfer of 128 J from the gas to the surrounding. Calculate the energy change for this process.
2. A cup of tea lost 28.6 kJ. If the same amount of heat is used to warm a piece of Al weighing 270 g, what would be the Tf of Al if its initial temperature is 42°C?
3. A 431 g of sample of water is heated from 6.50°C to 73.20°C. Calculate the amount of heat absorbed in kJ by the water.
4. A 27 g sample dissolved in water absorbed heat, increasing its temperature from 30°C to 48°C. What is Q is equal to? Assuming that the heat capacity of solution is the same as water, 4.184 J/gK.
5. If 5.5 g compound is burned in a bomb calorimeter whose total heat capacity is 7.854 kJ/°C. A temperature change of 7.82°C occurred. What is the heat of combustion per gram of compound?
