What happens in an endothermic reaction

Chris
Chemical reactions: They’re fundamental to chemistry; they make new things by rearranging other things. They can blow stuff up ... or freeze things quickly. In short, they are awesome.

Brittny
But why do some chemical reactions release massive amounts of energy, while others absorb energy? In a chemical reaction, the main change that occurs relates to the way atoms are connected (or bonded) to each other. In order to change those connections, bonds must be broken and new bonds must be formed. Let’s break down how energy is transferred in these reactions.

Chris
To understand the energy implications of chemical reactions, it’s important to keep in mind two key ideas:

1. It takes energy to break bonds.
2. Energy is released when bonds are formed.

To understand this, consider the chemical reaction between vinegar and baking soda. That’s right —the classic baking soda volcano experiment. The chemical reaction behind this science fair favorite involves baking soda—also known as sodium bicarbonate to chemists—and vinegar, otherwise known as acetic acid.

These compounds react to form the molecules sodium acetate, water, and carbon dioxide. The baking soda and vinegar are called the reactants. The sodium acetate, water, and carbon dioxide that are formed are called the products.

Before the atoms in acetic acid and sodium bicarbonate can be rearranged to form the products, some of the bonds between the atoms in those molecules must be broken, and because the atoms are attracted to one another, it takes energy to pull them apart.

Then, when the products are formed (sodium acetate, water, and carbon dioxide) energy is released because atoms that have an attraction for one another are brought back together.

By comparing the energy absorbed when bonds in the reactants are broken with the energy released when bonds in the products are formed, you can determine whether a chemical reaction releases energy or absorbs energy overall.

Brittny
Chemical reactions that release energy are called exothermic. In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants. Exothermic reactions are accompanied by an increase in temperature of the reaction mixture.

Chemical reactions that absorb (or use) energy overall are called endothermic. In endothermic reactions, more energy is absorbed when the bonds in the reactants are broken than is released when new bonds are formed in the products. Endothermic reactions are accompanied by a decrease in temperature of the reaction mixture.

Chris
You can use energy level diagrams to visualize the energy change during a chemical reaction. To understand these diagrams, compare the energy level of the reactants on one side with that of the products on the other side.

Consider, for example, a diagram that charts the energy change when a candle burns. Wax (C34H70) combusts in the presence of oxygen (O2) to yield carbon dioxide (CO2) and water (H2O). Because more energy is released when the products are formed than is used to break up the reactants, this reaction is exothermic.

Brittny
All of this stuff relates to thermodynamics—the study of heat and its relationship to energy and work. Using thermodynamics, you’ll learn how to calculate the precise amount of energy used or released by chemical reactions. Classifying a chemical reaction as exothermic or endothermic is simple. It comes down to weighing the energy needed to break bonds in the reactants with the energy released when the products are formed.

It’s a simple idea, but one with a lot of power.

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Any chemical reaction involves the breaking of molecular bonds and the possible formation of new bonds. A process that breaks bonds is one that releases energy, and scientists refer to it as an exergonic process. On the other hand, the formation of new bonds requires an input of energy, and scientists refer to such a process as endergonic. Energy can be released or absorbed in many forms, including light, electricity and heat. When energy is released as heat, the process is exothermic, and when heat is absorbed, the process is endothermic. An endothermic reaction is one which results in a net decrease in temperature because it absorbs heat from the surroundings and stores the energy in the bonds formed in the reaction.

TL;DR (Too Long; Didn't Read)

Endothermic reactions absorb energy from the surroundings and lower the temperature. They are a type of endogenic reaction. In biology, anabolic processes are examples of endothermic reactions.

Equation for Endothermic Reactions

The general equation for endothermic reactions is:

Reactants + Heat Energy --> Products

A reaction may involve multiple processes, and some of these may release heat, but as long as a reaction involves a net reduction in temperature, the reaction is endothermic. It's possible for this to happen because a chemical reaction always proceeds in a way that increases entropy. By contrast, exothermic reactions are those which release heat. Oxidation reactions are usually exothermic such as a burning log.

Formation of Nitric Oxide: An example of an endothermic reaction that occurs every day in Earth's atmosphere is the combination of molecular oxygen with molecular nitrogen to form nitric oxide. Chemists know exactly how much heat energy it takes for this reaction to occur. The balanced equation for this reaction is:

O2 + N2 + 180.5 KJ --> 2 NO

In words, it requires 180.5 kilojoules of energy to make this reaction happen, and it's a good thing, too. Otherwise, all the oxygen in the atmosphere would have been used up long ago. The heat energy for this reaction often comes from automobile exhaust.

Endothermic Processes Are Not All Reactions

An example of an endothermic process with which everyone knows is sweating, the process by which the body produces water on the skin as a cooling strategy. It works because water absorbs energy when it changes state from a liquid to a gas. This is an endothermic process, but it isn't a reaction, because a reaction always involves the destruction or formation of chemical bonds. On the other hand, squeezing an instant-cold ice pack does produce an endothermic reaction. A chemical in the pack reacts with water to absorb energy and freezes the water into ice.

Examples from Biology

During photosynthesis, plants absorb carbon dioxide from the atmosphere and turn it into glucose and oxygen. The process requires energy in the form of sunlight and is more endogenic than it is endothermic. The equation for the reaction is:

6CO2 (carbon dioxide) + 6H2O (water) + sunlight --> C6H12O6 (glucose) + O2 (oxygen)

A number of endothermic reactions are important for the metabolism of mammals and humans. Many of these occur inside cells, and when they do, scientists call them anabolic reactions, as opposed to catabolic reactions, which release energy. Some of these reactions include:

• Amino acids joining together to form peptides.
  
• Small sugar molecules joining to form disaccharides.
  
• Glycerol reacting with fatty acids to make lipids.

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