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In studying energy, scientists use the term “system” to refer to the matter and its environment involved in energy transfers. Everything outside of the system is called the surroundings. Single cells are biological systems. Systems can be thought of as having a certain amount of order. It takes energy to make a system more ordered. The more ordered a system is, the lower its entropy. Entropy is a measure of the disorder of a system. As a system becomes more disordered, the lower its energy and the higher its entropy become.
A series of laws, called the laws of thermodynamics, describe the properties and processes of energy transfer. The first law states that the total amount of energy in the universe is constant. This means that energy can’t be created or destroyed, only transferred or transformed. The second law of thermodynamics states that every energy transfer involves some loss of energy in an unusable form, such as heat energy, resulting in a more disordered system. In other words, no energy transfer is completely efficient and tends toward disorder.
[link] Look at each of the processes shown, and decide if it is endergonic or exergonic. In each case, does enthalpy increase or decrease, and does entropy increase or decrease?
[link] A compost pile decomposing is an exergonic process; enthalpy increases (energy is released) and entropy increases (large molecules are broken down into smaller ones). A baby developing from a fertilized egg is an endergonic process; enthalpy decreases (energy is absorbed) and entropy decreases. Sand art being destroyed is an exergonic process; there is no change in enthalpy, but entropy increases. A ball rolling downhill is an exergonic process; enthalpy decreases (energy is released), but there is no change in enthalpy.
[link] If no activation energy were required to break down sucrose (table sugar), would you be able to store it in a sugar bowl?
[link] No. We can store chemical energy because of the need to overcome the barrier to its breakdown.
Consider a pendulum swinging. Which type(s) of energy is/are associated with the pendulum in the following instances: i. the moment at which it completes one cycle, just before it begins to fall back towards the other end, ii. the moment that it is in the middle between the two ends, iii. just before it reaches the end of one cycle (just before instant i.).
C
Which of the following comparisons or contrasts between endergonic and exergonic reactions is false?
D
Which of the following is the best way to judge the relative activation energies between two given chemical reactions?
B
Explain in your own words the difference between a spontaneous reaction and one that occurs instantaneously, and what causes this difference.
A spontaneous reaction is one that has a negative ∆G and thus releases energy. However, a spontaneous reaction need not occur quickly or suddenly like an instantaneous reaction. It may occur over long periods due to a large energy of activation, which prevents the reaction from occurring quickly.
Describe the position of the transition state on a vertical energy scale, from low to high, relative to the position of the reactants and products, for both endergonic and exergonic reactions.
The transition state is always higher in energy than the reactants and the products of a reaction (therefore, above), regardless of whether the reaction is endergonic or exergonic.
In the International System of Units (SI), the unit of work or energy is the Joule (J). For very small amounts of energy, the erg (erg) is sometimes used. An erg is one ten millionth of a Joule:
Power is the rate at which energy is used. The unit of power is the Watt (W), named after James Watt, who perfected the steam engine:
Power is sometimes measured in horsepower (hp):
Electrical ene rgy is generally expressed in kilowatt-hours (kWh):
It is important to realize that a kilowatt-hour is a unit of energy not power. For example, an iron rated at would consume of energy in .
Heat energy is often measured in calories. One calorie (cal) is defined as the heat required to raise the temperature of of water from 14.5 to 15.5 ºC:
An old, but still used unit of heat is the British Thermal Unit (BTU). It is defined as the heat energy required to raise the energy temperature of 1 pound of water from to .
| Physical Quantity | Name | Symbol | SI Unit |
| Force | Newton | N | |
| Energy | Joule | J | |
| Power | Watt | W |
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