6.1:
Energy Basics
Why does sodium metal react with water to make hydrogen gas, but sodium chloride simply dissolves? This can be explained by studying the relationship between chemistry and energy, called ‘thermochemistry’.
All forms of matter are associated with energy, which can be measured as heat or work.
The two main types of energy are potential and kinetic energy.
Potential energy describes the positional forces acting on the object. A rock at the top of a hill has more potential energy than a rock at the bottom because the rock at the top is further from the Earth’s center of mass.
Kinetic energy is the energy of a moving object. If a rock is pushed off the hill, it loses potential energy but it gains kinetic energy as it moves.
This also applies to the atoms or molecules that make up an object. An object at rest still has kinetic energy because its component atoms can vibrate.
This form of kinetic energy is called thermal energy. An object has higher kinetic energy and consequently higher thermal energy when the atoms move more vigorously. This object is perceived as hot.
Molecules also have potential energy called chemical energy, which is associated with the relative position of electrons and nuclei.
During a reaction, chemical energy is converted into other forms of energy, such as light, or energy is released, such as in the form of heat.
The amount of energy transformed depends on the molecular structure of reactants.
In a gas lamp, acetylene burns into carbon dioxide and water, releasing a large amount of energy as light.
On the other hand, adding sodium hydroxide to hydrochloric acid, releases a smaller amount of energy, as heat.
To study energy changes, it helps to divide the universe into a ‘system’, which contains the process being observed, and the ‘surroundings’, which is everything else.
The system can be open, meaning that both energy and mass can be exchanged between the system and surroundings. The system can be closed, where only energy is exchanged, or the system can be isolated, where neither mass nor energy is exchanged.
6.1:
Energy Basics
Chemical reactions, such as those that occur when you light a match, involve changes in energy as well as matter.
Chemical changes and their accompanying changes in energy are important parts of everyday life. The macronutrients in food undergo metabolic reactions that provide the energy to keep bodies functioning. A variety of fuels (gasoline, natural gas, coal) is burned to produce energy for transportation, heating, and the generation of electricity. Industrial chemical reactions use enormous amounts of energy to produce raw materials (such as iron and aluminum). Energy is then used to manufacture those raw materials into useful products, such as cars, skyscrapers, and bridges.
Over 90% of the energy used by humans comes originally from the sun. Every day, the sun provides the earth with almost 10,000 times the amount of energy necessary to meet all of the world’s energy needs for that day. The challenge remains to find ways to convert and store incoming solar energy so that it can be used in reactions or chemical processes that are both convenient and nonpolluting. Plants and many bacteria capture solar energy through photosynthesis. Humans release the energy stored in plants when burning wood, coal, petroleum, or other plant products such as ethanol. They also use this energy to fuel their bodies by eating food that comes directly from plants.
Thermochemistry
The basic ideas of an important area of science concerned with the amount of heat absorbed or released during chemical and physical changes — is called thermochemistry. The concepts are widely used in almost all scientific and technical fields. Food scientists use thermochemistry to determine the energy content of foods. Biologists study the energetics of living organisms, such as the metabolic combustion of sugar into carbon dioxide and water. The oil, gas, and transportation industries, renewable energy providers, and many others endeavor to find better methods to produce energy for commercial and personal needs. Engineers strive to improve energy efficiency, find better ways to heat and cool homes, refrigerate food and drinks, and meet the energy and cooling needs of computers and electronics, among other applications. Understanding thermochemical principles is essential for chemists, physicists, biologists, geologists, every type of engineer, and just about anyone who studies or does any kind of science.
Energy
Energy can be defined as the capacity to supply heat or do work. One type of work (w) is the process of causing matter to move against an opposing force. For example, when inflating a bicycle tire — matter is moved (the air in the pump) against the opposing force of the air already in the tire.
Like matter, energy comes in different types. One scheme classifies energy into two types: potential energy, the energy an object has because of its relative position, composition, or condition, and kinetic energy, the energy that an object possesses because of its motion.
Water at the top of a waterfall or dam has potential energy because of its position; when it flows downward through generators, it has kinetic energy that can be used to do work and produce electricity in a hydroelectric plant. A battery has potential energy because the chemicals within it can produce electricity that can do work.
This text is adapted from OpenStax Chemistry 2e, Section 5.1: Energy Basics.