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.

Chemical energy transfer In a chemical reaction, energy is transferred between reactants and products, and between the reactants or products and their surroundings. The energy released or absorbed during the chemical reaction is the energy of reaction, or energy of formation. During a chemical reaction the amount of chemical energy stored is equal to the amount of chemical energy released; this principle is known as the law of conservation of energy. It means that energy cannot be created or destroyed, but can change from one form to another such as chemical energy changing into heat energy.

The living world Life processes of all kinds involve energy transfer; there can be no life without it. Energy may be transferred between an organism and its surroundings, for example when heat is lost or gained. Energy is also transferred inside an organism's body. Energy is used by organisms to do things such as growing and moving.

Nearly all life on Earth depends on energy transferred from the Sun. Plants carry out photosynthesis in which the green chemical chlorophyll absorbs sunlight, the energy of sunlight being transferred to the plant. The plant uses this energy to make glucose, which is used as a fuel during respiration and is also used for growth. When animals eat plants, energy is transferred from the plants to the animals. Chemicals in the plant are absorbed by the animal, and these can then be used as fuel to provide energy for the animal. The same energy transfer occurs when animals eat other animals. So energy transfer is a key part of a food chain or a food web.

Energy is defined as the ability to do work. In chemicals, energy is stored in the chemical bonds between the atoms. Heat, radiation, or electric energy can be used to break the chemical bonds so that new chemical bonds can be formed. In every chemical bond there is a fixed amount of chemical energy stored. During a chemical reaction, chemical bonds are broken by absorbing heat energy (endothermic reaction) and new chemical bonds are reformed by releasing heat energy (exothermic reaction). If the overall heat energy released by the chemical bonds is greater than heat energy absorbed by the chemical bonds, then the reaction is said to be exothermic.

In an explosion the chemical reaction is rapid; therefore, the rate of energy transfer from chemical energy to heat/light/sound/kinetic energy is very fast. Such chemical reactions are highly exothermic.

Chemical bond energy data Using chemical bond energy data, it is possible to tell from a chemical equation whether heat is absorbed or released in the reaction. For example, the chemical equation for the production of hydrochloric acid (HCl) from hydrogen (H) and chlorine (Cl) is:

H2 + Cl2 → 2HCl

The chemical bond energy data for the equation are: breaking 1 mole of H-H bond requires 436 kJ/mol; breaking 1 mole of Cl-Cl bond requires 242 kJ/mol; and forming 1 mole of H-Cl bond releases 431 kJ/mol. Therefore, energy in the reaction = (436) + (242) and (2 × 431): energy required to break the chemical bonds is 678 kJ/mol; energy released on formation of the chemical bonds is 862 kJ/mol; overall heat energy released is 184 kJ/mol. The chemical reaction is, therefore, exothermic.

Heat energy transfer Fuels are another example of a class of chemical compounds that store chemical energy. The amount of heat energy given out when fuel undergoes combustion can be worked out by measuring the change in temperature before and after a chemical reaction. A formula used to work out the heat energy transferred is:

heat energy transferred = mass of solution × temperature change × specific heat capacity

or:

Q = M × change in T × C

where M is the mass of solution/fuel used, the change in temperature (T) is the difference in temperature before and after reaction, and C is the specific heat capacity of that solution.

Chemical energy to electrical energy The transference of chemical energy to electrical energy can be achieved using a battery or a fuel cell.

A battery is a device where two different metals are used in an electrolyte to produce electricity. When two different metals, such as magnesium and copper, are dipped into a conducting solution (the electrolyte), the more reactive magnesium acts as a cathode (negative electrode) and the less reactive copper acts as an anode (positive electrode). The electrolyte transfers a flow of current from the electrons lost from the more reactive magnesium to the copper; the current can be used to light a small bulb.

The two materials used in a common dry-cell battery are a carbon rod as the anode, and zinc casing as the cathode; the electrolyte is ammonium chloride.

In a fuel cell, chemical energy is converted directly to electrical energy. Fuel cells are a new technology, capable of providing an endless and efficient supply of electricity. It derives electrical power from a fuel without having to burn it. Fuel cells create electricity through the chemical reaction of hydrogen with an oxidizing agent over an electrolyte. Hydrogen-oxygen fuel cells are used in space shuttles.