Ендотермічні реакції

Endothermic and Exothermic Reactions

Reactions always involve energy. You can consider energy as the "currency of change." Why? Because it will be exchanged in a reaction to cause a physical or chemical changes. We use descriptions like "exothermic" and "endothermic" to describe which direction the energy goes.

In the case of an exothermic reaction, energy is transferred from the system to the surroundings in the form of heat. This is evident by the prefix, exo-, which means outside. Remember, the temperature is the state variable for heat. So, in exothermic reactions the temperature increases.

Exothermic reactions can take a variety of forms. For example, the synthesis of magnesium oxide emits an immense amount of light and heat. Sometimes, adding water to anhydrous compounds causes some water to boil. Combustion (explosions!) is also a class of exothermic reactions.

Exothermic reactions tend to involve the formation of bonds. When atoms form a molecule, the potential energy they previously had as individuals is released as thermal energy.

Exothermic reactions sometimes require energy to be started. This is known as activation energy. After surpassing this "energy hurdle", the reaction will release energy. In the end, more energy will be lost from the system than gained.

There are various ways that we express exothermic reactions, and various ways we use these expressions. In a chemical equation, you would express exothermic reactions as reactants → products + energy. When calculating the net enthalpy change, you express it as a negative number, because the system is losing energy. In energy diagrams, potential energy will be lower at the end than it was at the beginning, while thermal energy will be higher.

In endothermic reactions, energy is transferred from the surroundings to the system. The prefix endo- means inside.

Endothermic reactions also require activation energy to start, just like exothermic reactions. The difference is that this amount will be a lot higher for the endothermic process (compared to its reverse exothermic reaction). In addition, the endothermic reaction will not release energy once it surpasses the activation energy hurdle.

Endothermic reactions tend to involve breaking of bonds. You can actually predict whether a reaction will be endothermic or exothermic by looking to see if bonds are formed (exothermic) or broken (endothermic).

You would express endothermic reactions as reactants + energy → products. When calculating the net enthalpy change, you should get a positive number because the system is gaining energy. In energy diagrams, potential energy will be higher at the end than it was at the beginning, while thermal energy will be lower.

More complicated reactions may be difficult to predict, as they involve multiple processes--some which are exothermic, and some which are endothermic. You can use Hess's law to add all of the energy changes from each "step" of a reaction. The final result is the change in enthalpy; if this is a negative number, the reaction is exothermic, because the system loses energy. If the number is positive, the reaction is endothermic. Many physical chemistry textbooks and websites provide appendices with the enthalpy change of individual energy changes, which can be used in your calculations. Be sure not to miss any steps!

An endothermic reaction is a reaction that absorbs heat energy, and typically breaks more bonds than it forms. This means that the system becomes warmer, and the surroundings becomes cooler. Mathematically this is expressed in the following manner, $$đQ>0$$

This is the same as the enthalpy ($H$) for many chemical reactions and processes. Enthalpy is given by the equation below.

$$dH=TdS+VdP$$

This equation shows the relationship between enthalpy, entropy, temperature, and pressure-volume work. Remember, for a reversible process, at constant pressure the work term disappears. So for most reactions, $dH=TdS$. But, we defined $TdS=đQ$. So, $dH=đQ$. For this reason, enthalpy is commonly known as the Heat of Reaction.

An exothermic reaction is a reaction that releases heat. Typically, more bonds form than break. This means that the system becomes cooler, and the surroundings become warmer. Mathematically this is expressed as, $đQ<0$.

(Video) Endothermic and Exothermic Reactions

by Bozeman Science

In this video, Mr. Andersen explains the difference between exothermic and endothermic reactions. He shows how to use the temperature change to determine it. Then he explains it graphically with an energy diagram. By the end of this video, you'll understand exothermic vs. endothermic reactions!