- Characteristics of an endothermic reaction
- ΔH> 0
- They cool their surroundings
- Equations
- Examples of common endothermic reactions
- Dry ice evaporation
- Baking bread or cooking food
- Sunbathing
- Reaction of atmospheric nitrogen and ozone formation
- Electrolysis of water
- Photosynthesis
- Solutions of some salts
- Thermal decompositions
- Ammonium Chloride in Water
- Sodium triosulfate
- Car engines
- Boiling liquids
- Cook an egg
- Food cooking
- Heating food in the microwave
- Glass molding
- Consumption of a candle
- Hot water cleaning
- Heat sterilization of food and other objects
- Fight infections with fever
- Water evaporation
- References
An endothermic reaction is one that to take place must absorb energy, in the form of heat or radiation, from its surroundings. Generally, but not always, they can be recognized by a drop in temperature in their surroundings; or on the contrary, they need a source of heat, such as that obtained by a burning flame.
The absorption of energy or heat is what all endothermic reactions have in common; their nature, as well as the transformations involved, are very diverse. How much heat should they absorb? The answer depends on its thermodynamics: the temperature at which the reaction occurs spontaneously.
Melting ice stalactite. Source: Pixabay
For example, one of the most emblematic endothermic reactions is the change of state from ice to liquid water. Ice needs to absorb heat until its temperature reaches approximately 0ºC; at that temperature its melting becomes spontaneous, and the ice will absorb until it has completely melted.
In hot spaces, such as on the shores of a beach, temperatures are higher and therefore the ice absorbs heat more quickly; that is, it melts faster. The melting of glaciers is an example of an undesirable endothermic reaction.
Why does it happen this way? Why can't ice appear as a hot solid? The answer lies in the average kinetic energy of the water molecules in both states, and how they interact with each other through their hydrogen bonds.
In liquid water, its molecules have greater freedom of movement than in ice, where they vibrate stationary in its crystals. To move, molecules must absorb energy in such a way that their vibrations break the strong directional hydrogen bonds in the ice.
For this reason, ice absorbs heat to melt. For "hot ice" to exist, hydrogen bonds would have to be abnormally strong to melt at a temperature well above 0ºC.
Characteristics of an endothermic reaction
The change of state is not properly a chemical reaction; However, the same thing happens: the product (liquid water) has higher energy than the reactant (ice). This is the main characteristic of an endothermic reaction or process: the products are more energetic than the reactants.
While this is true, it does not mean that products must necessarily be unstable. In the case that it is, the endothermic reaction ceases to be spontaneous under all conditions of temperature or pressure.
Consider the following chemical equation:
A + Q => B
Where Q represents heat, usually expressed in the units of joule (J) or calories (cal). As A absorbs heat Q to transform into B, it is then said that it is an endothermic reaction. Thus, B has more energy than A, and must absorb enough energy to achieve its transformation.
Endothermic reaction diagram for A and B. Source: Gabriel Bolívar
As can be seen in the diagram above, A has less energy than B. The amount of heat Q absorbed by A is such that it overcomes the activation energy (the energy needed to reach the purple dotted top). The difference in energy between A and B is what is known as the enthalpy of the reaction, ΔH.
ΔH> 0
All endothermic reactions have the above diagram in common, since the products are more energetic than the reactants. Therefore, the energy difference between them, ΔH, is always positive (H Product -H Reactive > 0). As this is true, there must be an absorption of heat or energy from the surroundings to supply this energy need.
And how are such expressions interpreted? In a chemical reaction, bonds are always broken to create new ones. To break them, the absorption of energy is necessary; that is, it is an endothermic step. Meanwhile, the formation of the bonds implies stability, so it is an exothermic step.
When the bonds formed do not provide stability comparable to the amount of energy required to break the old bonds, it is an endothermic reaction. This is why additional energy is needed to promote the breaking of the most stable bonds in the reactants.
On the other hand, in exothermic reactions the opposite occurs: heat is released, and ΔH is <1 (negative). Here the products are more stable than the reactants, and the diagram between A and B changes shape; now B is below A, and the activation energy is lower.
They cool their surroundings
Although it does not apply to all endothermic reactions, several of them cause a decrease in the temperature of their surroundings. This is because the absorbed heat comes from somewhere. Consequently, if the conversion of A and B were to take place inside a container, it would cool down.
The more endothermic the reaction, the colder the container and its surroundings will become. In fact, some reactions are even capable of forming a thin layer of ice, as if they had come out of a refrigerator.
However, there are reactions of this type that do not cool their surroundings. Why? Because the surrounding heat is insufficient; that is, it does not provide the necessary Q (J, cal) that is written in chemical equations. Therefore, this is when fire or UV radiation enters.
A little confusion may arise between the two scenarios. On the one hand, the heat from the surroundings is enough for the reaction to proceed spontaneously, and cooling is observed; and on the other, more heat is needed and an efficient heating method is used. In both cases the same thing happens: energy is absorbed.
Equations
What are the relevant equations in an endothermic reaction? As already explained, the ΔH must be positive. To calculate it, the following chemical equation is first considered:
aA + bB => cC + dD
Where A and B are the reactants, and C and D are the products. The lower case letters (a, b, c and d) are the stoichiometric coefficients. To calculate the ΔH of this generic reaction, the following mathematical expression is applied:
ΔH Products - ΔH Reagents = ΔH rxn
You can proceed directly, or do the calculations separately. For ΔH Products the following sum must be calculated:
c ΔH f C + d ΔH f D
Where ΔH f is the enthalpy of formation of each substance involved in the reaction. By convention, substances in their most stable forms have ΔH f = 0. For example, molecules of O 2 and H 2, or a solid metal, have ΔH f = 0.
The same calculation is now done for the reactants, ΔH Reagents:
a ΔH f A + b ΔH f B
But since the equation says that ΔH Reagents must be subtracted from ΔH Products, then the above sum must be multiplied by -1. So you have:
c ΔH f C + d ΔH f D - (a ΔH f A + b ΔH f B)
If the result of this calculation is a positive number, then it is an endothermic reaction. And if it is negative, it is an exothermic reaction.
Examples of common endothermic reactions
Dry ice evaporation
Dry ice. Source: Nevit, from Wikimedia Commons
Anyone who has ever seen those white fumes emanating from an ice cream cart has witnessed one of the most common examples of an endothermic "reaction".
Beyond some ice creams, these vapors released from white solids, called dry ice, have also been part of the scenarios to create the haze effect. This dry ice is nothing more than solid carbon dioxide, which when absorbing temperature and external pressure begins to sublimate.
An experiment for a child audience would be to fill and seal a bag with dry ice. After a while, it will end up inflating due to the gaseous CO 2, which generates work or presses the internal walls of the bag against atmospheric pressure.
Baking bread or cooking food
Baked bread. Source: Pixabay
Baking bread is an example of a chemical reaction, since now there are chemical changes due to heat. Anyone who has smelled the aroma of freshly baked breads knows that an endothermic reaction is occurring.
The dough and all its ingredients need the heat of the oven to carry out all the transformations, essential to become bread and exhibit its typical characteristics.
In addition to breads, the kitchen is full of examples of endothermic reactions. Whoever cooks deals with them daily. Cooking pasta, softening grains, heating corn grains, cooking eggs, seasoning meats, baking a cake, making tea, heating up sandwiches; each of these activities are endothermic reactions.
Sunbathing
Turtles getting a sun bath. Source: Pixabay
As simple and common as they may seem, sunbaths taken by certain reptiles, such as turtles and crocodiles, fall into the category of endothermic reactions. Turtles absorb heat from the sun to regulate their body temperature.
Without the sun, they retain the heat of the water to keep warm; which ends in cooling the water in your ponds or fish tanks.
Reaction of atmospheric nitrogen and ozone formation
Lightning. Source: Pixabay
Air is mainly made up of nitrogen and oxygen. During electrical storms, such energy is released that it can break the strong bonds that hold the nitrogen atoms together in the N 2 molecule:
N 2 + O 2 + Q => 2NO
On the other hand, oxygen can absorb ultraviolet radiation to become ozone; allotrope of oxygen that is very beneficial in the stratosphere, but detrimental to life at the ground level. The reaction is:
3O 2 + v => 2O 3
Where v means ultraviolet radiation. The mechanism behind that simple equation is very complex.
Electrolysis of water
Electrolysis uses electrical energy to separate a molecule into its forming elements or molecules. For example, in the electrolysis of water, two gases are generated: hydrogen and oxygen, each one in different electrodes:
2H 2 O => 2H 2 + O 2
Also, sodium chloride can undergo this same reaction:
2NaCl => 2Na + Cl 2
In one electrode you will see the formation of metallic sodium, and in the other, greenish bubbles of chlorine.
Photosynthesis
Plants and trees need to absorb sunlight as an energy supply to synthesize their biomaterials. For this, it uses CO 2 and water as raw materials, which through a long series of steps, are converted into glucose and other sugars. In addition, oxygen is formed, which is released from the leaves.
Solutions of some salts
If sodium chloride is dissolved in water, no appreciable change will be noticed in the external temperature of the glass or container.
Some salts, such as calcium chloride, CaCl 2, increase the temperature of the water as a result of the great hydration of the Ca 2+ ions. And other salts, such as ammonium nitrate or chloride, NH 4 NO 3 and NH 4 Cl, lower the temperature of the water and cool its surroundings.
In classrooms, home experiments are often done by dissolving some of these salts to demonstrate what an endothermic reaction is.
The drop in temperature is due to the fact that the hydration of the NH 4 + ions is not favored against the dissolution of the crystalline arrangements of their salts. Consequently, the salts absorb heat from the water to allow the ions to be solvated.
Another chemical reaction that is usually very common to demonstrate this is the following:
Ba (OH) 2 8H 2 O + 2NH 4 NO 3 => Ba (NO 3) 2 + 2NH 3 + 10H 2 O
Note the amount of water formed. When both solids are mixed, an aqueous solution of Ba (NO 3) 2 is obtained, with the smell of ammonia, and with a drop in temperature such that it literally freezes the external surface of the container.
Thermal decompositions
One of the most common thermal decompositions is that of sodium bicarbonate, NaHCO 3, to produce CO 2 and water when heated. Many solids, including carbonates, often break down to release CO 2 and the corresponding oxide. For example, the decomposition of calcium carbonate is as follows:
CaCO 3 + Q => CaO + CO 2
The same is true for magnesium, strontium, and barium carbonates.
It is important to note that thermal decomposition is different from combustion. In the first there is no presence of ignition or heat is released, while in the second there is; that is, combustion is an exothermic reaction, even when it requires an initial heat source to take place or occur spontaneously.
Ammonium Chloride in Water
When a small amount of ammonium chloride (NH4Cl) is dissolved in water in a test tube, the tube becomes colder than before. During this chemical reaction, heat is absorbed from the environment.
Sodium triosulfate
When crystals of sodium thiosulfate (Na 2 S 2 O 3.5H 2 O), commonly called hypo, dissolve in water, a cooling effect occurs.
Car engines
Burning gasoline or diesel in car, truck, tractor or bus engines produces mechanical energy, which is used in the circulation of these vehicles.
Boiling liquids
By putting a liquid to heat, it gains energy and goes into a gaseous state.
Cook an egg
When heat is applied, the egg proteins are denatured, forming the solid structure that is usually ingested.
Food cooking
In general, always when cooking with heat to change the properties of food, endothermic reactions are occurring.
These reactions are what cause food to become softer, generate malleable masses, release the components they contain, among other things.
Heating food in the microwave
Due to microwave radiation, water molecules in food absorb energy, begin to vibrate, and increase the temperature of the food.
Glass molding
The absorption of heat by the glass makes its joints flexible, making its shape easier to change.
Consumption of a candle
Candle wax melts by absorbing the heat from the flame, changing its shape.
Hot water cleaning
When using hot water to clean objects that have been stained with grease, such as pots or clothes, the grease becomes thinner and is easier to remove.
Heat sterilization of food and other objects
When heating objects or food, the microorganisms they contain also increase their temperature.
When much heat is supplied, reactions within microbial cells occur. Many of these reactions, such as the breaking of bonds or the denaturation of proteins, end up killing the microorganisms.
Fight infections with fever
When a fever occurs, it is because the body produces the heat necessary to kill the bacteria and viruses that cause infections and cause disease.
If the heat generated is high and the fever is high, the body's cells are also affected and there is a risk of death.
Water evaporation
When water evaporates and turns into steam, it is due to the heat it receives from the environment. As heat energy is received by each water molecule, its vibration energy increases to the point where it can move freely, creating steam.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Wikipedia. (2018). Endothermic process. Recovered from: en.wikipedia.org
- Helmenstine, Anne Marie, Ph.D. (December 27, 2018). Endothermic Reaction Examples. Recovered from: thoughtco.com
- Khan Academy. (2019). Endothermic vs. exothermic reactions. Recovered from: khanacademy.org
- Serm Murmson. (2019). What Happens on the Molecular Level During an Endothermic Reaction? Hearst Seattle Media. Recovered from: education.seattlepi.com
- QuimiTube. (2013). Calculation of the enthalpy of reaction from the enthalpies of formation. Recovered from: quimitube.com
- Quimicas.net (2018). Examples of Endothermic Reaction. Recovered from:
quimicas.net.