A chemical reaction is a process in which one or more substances are chemically changed into one or more new substances. A chemical reaction may involve the motion of electrons in the forming and breaking of chemical bonds. Therefore when a chemical reaction takes place, energy is transferred to, or from, the environment. A Chemical reaction can either be exothermic or endothermic. Exothermic Reactions
Reaction a transfer of energy to the surrounding, usually in the form of heat, takes place. This causes an increase in the temperature of the surrounding environment. The process can be easily explained using the change of phases of Water (H2O): Liquid water had to have energy put into it to become steam, and that energy is not lost. Instead, it is retained by the gaseous water molecules. When these molecules condense to form liquid water again, the energy put into the system must be released.</p> <p style="text-align: justify;">And this stored energy is let out as exothermic heat. The same explanation can be used for the process of freezing: energy is put into a liquid during melting, so freezing the liquid into a solid again returns that energy to the surroundings. The most stable state is where all energy has been released. When going to a more stable state, energy will be released. On an enthalpy level diagram, higher positions will be less stable therefore, if the product is lower, heat is released. Therefore we can say that the formation of bonds causes an energy release (exothermic). Examples of exothermic reactions:</p> <p style="text-align: justify;">-making ice cubes<br /> -formation of snow in clouds<br /> -condensation of rain from water vapour<br /> -a candle flame<br /> -rusting iron<br /> -burning sugar<br /> -forming ion pairs<br /> -Combining atoms to make a molecule in the gas phase<br /> -mixing water and strong acids</p> <p style="text-align: justify;">Endothermic Reactions<br /> An Endothermic reaction is a reaction that takes place when energy is taken in from the surrounding environment. This energy is usually transferred as heat energy, causing the reaction mixture and its surrounding to decrease in temperature. The main reason a reaction is endothermic is because it requires the absorption of heat to take place. Because we must add heat, boiling water is a process that chemists call endothermic. When going to a less stable state, energy will be gained (from the surroundings). On an enthalpy level diagram lower levels will be more stable therefore if the product is higher, heat is gained. We can therefore say that breaking of bonds requires energy (endothermic).</p> <p style="text-align: justify;">Other examples of Endothermic reactions include:<br /> -melting ice cubes<br /> -baking bread<br /> -cooking an egg<br /> -producing sugar by photosynthesis<br /> -melting solid salts<br /> -mixing water and ammonium nitrate</p>
The release of energy in chemical reactions occurs when the reactants have higher chemical energy than the products. The chemical energy in a substance is a type of potential energy stored within the substance. This stored chemical potential energy is the heat content or enthalpy of the substance. The collection of substances that is involved in a chemical reaction is referred to as a system and anything else around it is called the surroundings. If the enthalpy decreases during a chemical reaction, a corresponding amount of energy must be released to the surroundings.
Conversely, if the enthalpy increases during a reaction, a corresponding amount of energy must be absorbed from the surroundings. This is simply the Law of Conservation of Energy. Endothermic reactions increase their enthalpy by absorbing heat. They feel cold to the touch after they have occurred. Exothermic reactions decrease their enthalpy by releasing heat. They will get warm, and may even burn or explode if they release enough heat. You are already familiar with enthalpy: melting ice is exothermic and freezing water is endothermic. Examples
When methane burns in air the heat given off equals the decrease in enthalpy that occurs as the reactants are converted to products. The enthalpy difference between the reactants and the products is equal to the amount of energy released to the surroundings. A reaction in which energy is released to the surroundings is called an exothermic reaction. In this type of reaction the enthalpy, or stored chemical energy, is lower for the products than the reactants. When ammonium nitrate is dissolved in water, energy is absorbed and the water cools.
This concept is used in “cold packs”. The enthalpy difference between the reactants and the products is equal to the amount of energy absorbed from the surroundings. A reaction in which energy is absorbed from the surroundings is called an endothermic reaction. In endothermic reactions the enthalpy of the products is greater than the enthalpy of the reactants. Because reactions release or absorb energy, they affect the temperature of their surroundings. Exothermic reactions heat up their surroundings while endothermic reactions cool them down.
The study of enthalpy, along with many other energy-related topics, is covered in the Thermodynamics Unit. Activation Energy Think about the combustion of methane. It releases enough heat energy to cause a fire. However, the reaction does not occur automatically. When methane and oxygen are mixed, an explosion does not instantly occur. First, the methane must be ignited, usually with a lighter or matchstick. This reveals something about reactions: they will not occur unless a certain amount of activation energy is added first.
In this sense, all reactions absorb energy before they begin, but the exothermic reactions release even more energy. This can be explained with a graph of potential energy: This graph shows an exothermic reaction because the products are at a lower energy than the reactants (so heat has been released). Before that can happen, the energy must actually increase. The amount of energy added before the reaction can complete is the activation energy, symbolized Ea.