One of the most important aspects of chemistry is the production and flow of energy. We eat foods to produce the energy needed to maintain our biological function. We burn fossil fuels to produce the electrical energy that is central to our modern society, to produce heat for our homes, and to produce power for planes, trains, and automobiles. We produce explosives and propellants that release large amounts of energy, with both productive and terrifying consequences. We use ice cubes to cool our drinks, and we use heat to convert raw dough into baked bread. All of these examples illustrate a general point: Chemical reactions involve changes in energy. Some reactions, such as the burning of gasoline, release energy. Others, such as the splitting of water into hydrogen and oxygen, require energy. Over 90 percent of the energy produced in our society comes from chemical reactions, mostly from the combustion of coal, petroleum products, and natural gas.
The study of energy and its transformations is known as thermodynamics (Greek: therme, "heat"; dynamis, "power"). This area of study began during the Industrial Revolution as the relationships among heat, work, and the energy content of fuels were studied in an effort to maximize the performance of steam engines. Thermodynamics is important not only to chemistry but to other areas of science and to engineering as well, as we will see in later discussions.
Here we examine the relationships between chemical reactions and energy changes. This aspect of thermodynamics is called thermochemistry.
What is THERMOCHEMISTRY?
It is the study of te heat exchange and work on chemical reactions and is concerned with the measurement and interpretation of heat changes.
How did it all begin?
Thermochemistry rests on two generalizations. Stated in modern terms, they are as follows:
- Lavoisier and Laplace’s law (1780): The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process.
- Hess' law (1840): The energy change accompanying any transformation is the same whether the process occurs in one step or many.
♥ Types of System
System - specific part of a universe that is of interest to us
- Open System - can exchange mass and energy usually in the form of heat, with its surroundings. Example:
- Close System - allows the transfer of energy (heat) but not mass
- Isolated System - does not allow the transfer of either mass or energy
Surroundings - are the rest of the universe outside the system.
so..
could you now identify which type of system does each picture belongs to?
a. kettle b. a cup of coffee c. thermos bottle
Guess you get it now! :D
♥ Two Types of Energy Transfer
Heat
- is the transfer of thermal energy between two bodies that are at different temperatures
- could be either absorbed or released
Work
- is the energy transfer between a system and surroundings due to a force acting through a distance
- may be done by the system or done on the system
The sign convention for work and heat:
- Negative (-) -Work done by the system to the surrounding
- Positive (+) - Work done on the system by the surrounding
- Positive (+) - Heat absorbed by the system from the surrounding
- Negative (-) - Heat absorbed by the surrounding from the system
♥ Enthalpy
- is equal to the amount of heat flow in a system with constant pressure
Here's a video to help you thoroughly understand enthalpy :)
Sign Convention.
In terms of energy (or enthalpy changes), there are two types of chemical reaction.
· Exothermic
Exothermic reactions do the following:
1. Get hot to the touch
2. Liberate heat energy to the surroundings.
3. Result in a general increase in the temperature of the surroundings.
· Endothermic
Endothermic reactions do the following:
1. Get cold to the touch.
2. Absorb heat energy from the surroundings.
3. Result in a general decrease in the temperature of the surroundings.
The sign convention states the following:
DH negative = exothermic change.
DH positive = endothermic change.
Internal Energy
- is the sum of the kinetic and potential energy found in the system
- a change in the difference of a system can be determined by getting the difference between the final and initial values
Ooops! Stop there! Let me test you for a little while. <Click Me>
♥ Calorimetry
- is the science of measuring heat based on the change on temperature of an observed body when it releases or absorbs heat
- calorimeter - apparatus which determines the heat flow
- Two types of calorimeter: constant-pressure and constant-volume calorimeter
Here's another video which shows further explanations about calorimetry as well as it's applications :)
Heat Capacity
- the amount of heat required to raised the temperature of a given quantity of the substance by one degree Celsius.
- it is expressed in the equation:
where C = heat capacity;
ΔQ = heat absorbed ;
ΔT = change in temperature
Specific Heat
- is the amount of heat required to raise the temperature of one gram of this substance by one degree Celsius.
Specific Heat of Some Common Substance
| Substance | Specific Heat Capacity - cp - | |
| (cal/gramoC) | (J/kgoC) | |
| Air, dry (sea level) | 0.24 | 1005 |
| Aluminum | 0.215 | 900 |
| Copper | 0.0924 | 387 |
| Ice (0oC) | 0.50 | 2093 |
| Granite | 0.19 | 790 |
| Sandy clay | 0.33 | 1381 |
| Quartz sand | 0.19 | 830 |
| Water, pure | 1.00 | 4186 |
| Wet mud | 0.60 | 2512 |
| Wood | 0.41 | 1700 |
♥ Hess's Law
- is a relationship in physical chemistry named for Germain Hess, a Swiss-born Russian chemist and physician
- it states that:
" The enthalpy change of an overall reaction is the sum of the enthalpy change of its individual steps"
Wanna learn more? Check this video out
Thermochemical Equation
- the heat change associated with a chemical reaction indicated with an indication
- show the enthalpy changes as well as the mass relationship
Another related video
Are you done reading the context of Thermochemistry? Let's see.
Hope this blog helped you! thanks for dropping by! :)
