BCScience great 10 (March 2018)10Big Idea: Energy change is forced as atom rearrange in chemistry processes.

You are watching: If an endothermic reaction absorbs heat, then why does it feel cold to the touch?

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When girlfriend hurt her ankle, thermodynamics can assist you heal! one endothermic reaction reduce the ede by cooling her injury.

Has this ever happened to you? You are running in gym class and you twist your ankle. It hurts and starts come swell up. Your teacher take away the very first aid kit and pulls the end an instant cold pack. After one good squeeze, the fill becomes really cold, virtually instantly. What is happening? exactly how do the chemicals make the load cold for this reason quickly? The answer have the right to be discovered in thermodynamics! This is a branch of scientific research that explores the move of energy. In thermodynamics, chemistry reactions have the right to be classified as either endothermic or exothermic

What space the 2 main types of thermodynamic reactions?

Exothermic reactions are reactions the release energy in the type of heat. You room probably acquainted with plenty of examples of these reactions. Because that example, burning gasoline in a car’s engine is one exothermic reaction. This particular form of exothermic reaction is recognized as a combustion reaction. A burning reaction occurs as soon as a compound, such as the hydrocarbons that comprise fuel, react through oxygen to type a new product and produce heat. 

Endothermic reactions are the opposite of exothermic reactions. They absorb heat energy from your surroundings. This means that the next site of endothermic reaction are colder as a an outcome of the reaction. Melting ice cream is an example of this type of reaction.

How carry out you know what type of thermodynamic reaction is happening?

One method to perform this is by looking in ~ the system and also surroundings of a reaction. The system is wherein the reaction take away place, and also the surroundings are the area approximately the system. 

To identify if a reaction is exothermic or endothermic, friend could:

measure the temperature readjust of the device or the surroundings, orcalculate the energy of the system. 

Of these two methods, measure the temperature readjust is easier. To execute this, you merely measure the temperature the a reaction before and after that is completed. Since it can sometimes be difficult to measure up the temperature within the mechanism of a reaction, scientists regularly measure the temperature of the next site instead.


It is possible to predict whether a reaction will certainly be endothermic or exothermic by doing a small math. Because that this, it helps to understand a bit about chemical reactions and also chemical bonds.

There room two sides to any chemical reaction. ~ above one side are the reactants. A reactant is the substance (or substances) the you start with. Top top the various other side are the products. A product is the substance, or substances, that you end up v after the reaction happens. 

In a chemical reaction, the chemical bonds in the reactant molecules are broken. Brand-new bonds are formed in the product molecules. An example would it is in the combustion reaction between methane (CH4) oxygen (O2) (the reactants) that produces carbon dioxide (CO2) and also water (H20) molecules (the products). Bonds are broken in the methane and oxygen molecules. Bond are developed in the carbon dioxide and water molecules.


What is important to understand is that energy is needed to both make and also to rest bonds. To identify if a reaction is exothermic or endothermic, you need to compare the lot of power needed to rest the bond of the reactants to the quantity of energy released when brand-new bonds room made. If the amount of energy released as soon as the brand-new bonds are created in the products is greater, climate it is an exothermic reaction. If the quantity of energy needed to break the bond of the reactants is greater, then it is one endothermic reaction. 

One means to display this is utilizing an energy diagram. Energy diagrams present the power levels of reactants and also products in a reaction.


You deserve to see indigenous the diagram above that the power level the the products of one exothermic reaction is lower than the power level that the reactants. The difference between the power levels the reactants and also products is referred to as the enthalpy readjust (ΔH). In one exothermic reaction, the ΔH is NEGATIVE. In one endothermic reaction, the ΔH is POSITIVE.

Did you know?

Scientists have the right to measure power in foods by measuring just how much heat the food releases as soon as it is burned. They measure this using a tool dubbed a bomb calorimeter.

It is feasible to calculate ΔH without even doing an experiment! scientists have identified experimentally the energies required to make and also break details molecular bonds. These energies are recognized as average bond energies.

Examples of mean Bond Energies
O=O119 kcal/mol
C-H99 kcal/mol
O-H111 kcal/mol
C=O (in CO2)192 kcal/mol


Using the methane burning example again, the mathematics works choose this:

ΔH = -

= < 4 C-H bonds + 2 O=O bonds> - <4 O-H bond + 2 C=O bonds>

= <(4 x 99 kcal/mol) + (2 x 119 kcal/mol)> - <(4 x 111 kcal/mol) + (2 x 192 kcal/mol)>

= <396 + 238> - <444 + 384>

= 634 - 828

= - 194 kcal/mol

Since the enthalpy change is negative, we recognize that the reaction will be exothermic.

See more: How Many Potatoes In A Lb - How Many Potatoes In A Pound


Now, let’s go ago to our immediate cold pack. An prompt cold fill is the perfect example of one endothermic reaction. There are many feasible ingredients in an immediate cold pack, however they frequently contain solid ammonium nitrate and water. 

Did you know? 

Ammonium nitrate is a nitrate salt. That is greatly used in farming as a fertilizer. It is likewise used together an explosive in the mining industry.

The ammonium nitrate is save in a sealed plastic bag the is surrounded by water. As soon as you pop the bag, the ammonium nitrate comes into call with water and dissolves

Dissolving an ionic compound, favor table salt or ammonium nitrate, involves energy. Favor other species of reactions, heat power may be given off or bring away in once the product dissolves. This energy is referred to as the energy that solution and have the right to be composed as ΔHsoln

ΔHsoln = ∑ΔH - ∑ΔH

Rather than working out the ΔH because that the reactants and also products utilizing bond energies, scientists regularly use pre-calculated worths on Standard Enthalpy of Formation (ΔH°ftables. From together a table we learn that:

Enthalpy of development TableΔH°fkJ/mol

Let’s execute the math to calculation the power of solution

ΔHsoln = ∑ΔH - ∑ΔH

= < mol (NH4+(aq)) + mol (NO3-(aq)) > - < mol (NH4NO3(s)) >

= < (1 mol)(-132.8 kJ/mol) + (1 mol)(-206.6 kJ/mol) > - < (1 mol)(-365.1 kJ/mol) >

= - 339.4 + 365.1

= 25.7 kJ

Remember at the beginning we stated that if ΔH is NEGATIVE the reaction is exothermic and that if ΔH is POSITIVE the reaction is endothermic? Well, that likewise applies to power of solution problems. Since we calculated that the ΔHsoln was hopeful (25.7 kJ), the reaction have to be endothermic. We recognize this to it is in true because the cold pack made the surroundings an extremely cold!