What is energy ?
Energy is the ability to do work or produce heat. It exists in two basic forms of energy :
- Potential energy
- Kinetic energy
The potential Energy can be divided into two types, the first one is the energy related to composition which is called chemical potential energy and the second is due to the position of an object and it is called gravitational potential energy. Kinetic Energy is related to the motion.
Law of conservation of energy
This law States that in any chemical reaction, energy is neither created nor destroyed, it can be converted from one type to another type. For example, in a voltaic cell chemical energy is transformed to electrical energy.
Chemical potential energy
Chemical potential energy is a type of energy which is stored inside the substance because of its composition and the chemical bonds between atoms. For example, there is a big amount of chemical potential energy stored inside the various chemicals that make gasoline, this energy is converted to another type of energy. In the engine of a car gasoline is burned in a controlled way, the stored chemical potential energy is converted to mechanical energy, so the car can function.
Heat exchange in a chemical reaction
Chemical reactions involve energy because chemical bonds are being broken and made! Most of chemical reactions involve heat exchange. Depending on this exchange we can identify three types of chemical reactions which are :
- Endothermic reaction
- Exothermic reaction
- Athermic reaction
Exothermic reaction is a chemical change that releases heat energy. ‘Exo’ means outside. For example Burning natural gas (methane, CH4) is an exothermic reaction because heat is one of the products. CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g) + Heat
Endothermic reaction is a chemical change that absorb heat energy. ‘Endo’ means inside. For example decomposing HgO into mercury and oxygen is an endothermic reaction because heat is one of the reactants.
2HgO(s) + Heat —> 2Hg(l) + O2 (g)
In an athermic reaction there is no heat exchange.
Chemists use the term “enthalpy” to describe heat changes in a chemical reaction at constant pressure. Enthalpy is equal to the energy of a substance plus a small adjustment for pressure-volume work.
Enthalpy (H) = E + PV = Internal Energy + Pressure-Volume Work
Enthalpy Change (ΔH)
Enthalpy change, ΔH, for a chemical reaction that occurs at constant pressure is the quantity of heat released or absorbed in this chemical reaction.
ΔH is just the difference between the enthalpy of the products minus the enthalpy of the reactants:
ΔH = Hproducts – Hreactants
For exothermic reactions, Hproducts is lower than Hreactants so ΔH is negative. Heat is released, it is lost by the system. But for endothermic reactions, Hproducts is higher than Hreactants so ΔH is positive. Heat is absorbed, it is gained by the system.
ΔH of the reaction of burning methane is negative, so it is an exothermic reaction.
CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g) + Heat ΔH = -890kJ
For the reaction of decomposition of HgO, ΔH is positive, so it is an endothermic reaction.
2HgO(s) + Heat —> 2Hg(l) + O2 (g) ΔH = +181kJ