Why does an endothermic reaction happen

In this lesson, you will learn about a second driving force for chemical reactions called entropy. The vast majority of naturally occurring reactions are exothermic.

In an exothermic reaction, the reactants have a relatively high quantity of energy compared to the products. As the reaction proceeds, energy is released into the surroundings. Low energy can be thought of as providing a greater degree of stability to a chemical system. Since the energy of the system decreases during an exothermic reaction, the products of the system are more stable than the reactants.

We can say that an exothermic reaction is an energetically favorable reaction. If the drive toward lower energy were the only consideration for whether a reaction is able to occur, we would expect that endothermic reactions could never occur spontaneously. In an endothermic reaction, energy is absorbed during the reaction, and the products thus have a larger quantity of energy than the reactants. This means that the products are less stable than the reactants.

Therefore, the reaction would not occur without some outside influence such as persistent heating. However, endothermic reactions do occur spontaneously, or naturally. There must be another driving force besides enthalpy change which helps promote spontaneous chemical reaction. A very simple endothermic process is that of a melting ice cube. Energy is transferred from the room to the ice cube, causing it to change from the solid to the liquid state.

The solid state of water, ice, is highly ordered because its molecules are fixed in place. The melting process frees the water molecules from their hydrogen-bonded network and allows them a greater degree of movement. Water is more disordered than ice. The change from the solid to the liquid state of any substance corresponds to an increase in the disorder of the system. Everyday uses of endothermic reactions include instant ice packs which can be used to treat sports injuries.

The slideshow describes an exothermic reaction between dilute sodium hydroxide and hydrochloric acid and an endothermic reaction between sodium carbonate and ethanoic acid. Sodium hydroxide solution is poured into a beaker of hydrochloric acid which contains a thermometer showing room temperature.

The beaker now contains sodium chloride and water, and the thermometer is showing a rise in temperature, so the neutralisation reaction is exothermic. Sodium carbonate powder is tipped into a beaker of ethanoic acid which contains a thermometer showing room temperature. The beaker now contains sodium ethanoate, water and carbon dioxide, and the thermometer is showing a fall in temperature, so this was an endothermic reaction.

The Le Cha telier's principle states that if a stress, such as changing temperature, pressure, or concentration, is inflicted on an equilibrium reaction, the reaction will shift to restore the equilibrium.

For exothermic and endothermic reactions, this added stress is a change in temperature. The equilibrium constant shows how far the reaction will progress at a specific temperature by determining the ratio of products to reactions using equilibrium concentrations. The equilibrium expression for the following equation. If the products dominate in a reaction, the value for K is greater than 1.

The larger the K value, the more the reaction will tend toward the right and thus to completion. Suppose that the following reaction is at equilibrium and that the concentration of N 2 is 2 M, the concentration of H 2 is 4 M, and the concentration of NH 3 is 3 M.