All chemical changes are accompanied by heat exchange, due to the fact that energy is either needed for the chemical change to take place, or else energy is released after the energy change has taken place.
In energetic there are two main definitions:
Exothermic reactions are those reactions that release energy to the surroundings, and these would thus feel hot.
Endothermic reactions are those reactions that obtain energy from the surroundings and thus these would feel cold.
Thermochemical Reaction
The above reaction is a normal thermodynamic equation where all the important information of a reaction can be found. The most important things are the reactants and products, but then there is Δ Ho which means the heat of reaction. This is a very important variable in chemistry and is constant for every reaction. This states the energy released or absorbed for the formation of 1 mole of product at room temperature and pressure. The negative sign implies an exothermic reaction while a positive sign would show an endothermic reaction. An important note on the thermochemical equation is the fact that the units used are the kJ or kiloJoules, which is the SI unit
The energy changes occur due to the fact that in a reaction bonds are broken and then new bonds would be created, and while energy is needed to break bonds, energy would be released once new bonds are formed. It must be noted that all rules of physics must be obeyed, and therefore energy can neither be created nor destroyed, meaning that the energy released must be stored inside the chemistry, and it must be noted that the energy would be stored in the chemical bonds.
Definitions
In energetic there are a number of definitions that must be noted:
- Heat of combustion is the energy released when a substance reacts with Oxygen. It must be noted that all of these reactions are exothermic.
- Heat of solution is the energy released or obtained when a substance is dissolved in a solvent.
- Heat of Neutralisation is the energy released when an acid and a base react to form water and a salt. This reaction is always exothermic.
- Precipitation Reactions is when two solutions are mixed to form a precipitate.
Bond Breaking and Bond Making
In chemistry, a reaction takes place first by breaking bonds, which is an endothermic process, then by making bonds, which is an exothermic process. This means that for any reaction a little bit of energy is always needed, which is the activation energy, as can be seen in any reaction profile.
When the difference in energy between the bonds created and the bonds destroyed is calculated the enthalpy of the reaction can be found.
Reaction Profile
A reaction profile shows the energy change between the reactants and the products. This reaction change can be either positive or negative, and when the enthalpy change is negative the reaction is exothermic whereas when it is positive the reaction is endothermic.
The activation energy is the energy needed for a reaction to initiate, to break bonds in order to create new interactions. The higher the activation energy the higher the energy needed for a reaction to take place, no matter how kinetic favourable the product could be.
Finding the heat of reaction