Heat in a chemical reaction
Energy associated with chemicals is held within the bonds formed between atoms. When bonds are formed, energy is stored. When bonds are broken, energy is released.
- A reaction is said to be endothermic when a bond breaks and energy is absorbed. In a chemical reaction, energy being absorbed is shown as a reactant, and the sign on the enthalpy value is positive.
- A reaction is said to be exothermic when a bond is formed and energy is released. In a chemical reaction, energy being formed is shown as a product, and the sign on the enthalpy value is negative.
In a closed system, the law of conservation of energy is followed, in that energy is neither created nor destroyed in any physical or chemical change, it will only change forms. The energy released or absorbed in a chemical reaction is measured in terms of enthalpy (ΔH). Enthalpy or heat of reaction is a measure of the amount of energy associated with a substance in a chemical reaction. It is defined as being the energy of the bonds formed in the products minus the energy of the bonds broken in the reactants.
Check out the video at the following site: http://www.videojug.com/film/a-guide-to-exothermic-reactions
The energy associated with the reactants and products can be representated graphically in order to describe how the energy changes during a chemical reaction. You can explore how chemists represent these kinds of energy changes in the following hotspot. Keep in mind, it frequently costs energy to get a chemical reaction moving, even if the products are lower in energy.
|Scroll your mouse over the two energy diagrams to learn about important parts of the diagrams.|