Endothermic and exothermic reactions can be thought of as having energy as either a reactant of the reaction or a product. Endothermic reactions require energy, so energy is a reactant. Heat flows from the surroundings to the system (reaction mixture) and the enthalpy of the system increases (\(\Delta H\) is positive). As discussed in the previous section, heat is released (considered a product) in an exothermic reaction, and the enthalpy of the system decreases (\(\Delta H\) is negative).
In the course of an endothermic process, the system gains heat from the surroundings and so the temperature of the surroundings decreases (gets cold). A chemical reaction is exothermic if heat is released by the system into the surroundings. Because the surroundings is gaining heat from the system, the temperature of the surroundings increases. See Figure \(\PageIndex{1}\).
Endothermic Reaction: When \(1 \: \text{mol}\) of calcium carbonate decomposes into \(1 \: \text{mol}\) of calcium oxide and \(1 \: \text{mol}\) of carbon dioxide, \(177.8 \: \text{kJ}\) of heat is absorbed. Because the heat is absorbed by the system, the \(177.8 \: \text{kJ}\) is written as a reactant. The \(\Delta H\) is positive for an endothermic reaction.
\[\ce{CaCO_3} \left( s \right) \rightarrow \ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right) \: \: \: \: \: \Delta H = +177.8 \: \text{kJ}\]
Exothermic Reaction: When methane gas is combusted, heat is released, making the reaction exothermic. Specifically, the combustion of \(1 \: \text{mol}\) of methane releases 890.4 kilojoules of heat energy. This information can be shown as part of the balanced equation in two ways. First, the amount of heat released can be written in the product side of the reaction. Another way is to write the \(\Delta H\) information with a negative sign, \(-890.4 \: \text{kJ}\).
\[\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + 2 \ce{H_2O} \left( l \right) \: \: \: \: \: \Delta H = -890.4 \: \text{kJ}\]
Is each chemical reaction exothermic or endothermic?
- CH4(g) + 2O2(g) → CO2(g) + 2H2O(ℓ) + 213 kcal
- N2(g) + O2(g) + 45 kcal → 2NO(g)
- Because energy (213 kcal) is a product, energy is given off by the reaction. Therefore, this reaction is exothermic.
- Because energy (45 kcal) is a reactant, energy is absorbed by the reaction. Therefore, this reaction is endothermic.
Is each chemical reaction exothermic or endothermic?
- H2(g) + F2(g) → 2HF (g) + 130 kcal
- 2C(s) + H2(g) + 5.3 kcal → C2H2(g)
a. The energy (130 kcal) is produced, hence the reaction is exothermic
b. The energy (5.3 kcal) is supplied or absorbed to react, hence, the reaction is endothermic
Endothermic and exothermic reactions can be visually represented by energy-level diagrams like the ones in Figure \(\PageIndex{2}\). In endothermic reactions, the reactants have higher bond energy (stronger bonds) than the products. Strong bonds have lower potential energy than weak bonds. Hence, the energy of the reactants is lower than that of the products. This type of reaction is represented by an "uphill" energy-level diagram shown in Figure \(\PageIndex{2A}\). For an endothermic chemical reaction to proceed, the reactants must absorb energy from their environment to be converted to products.
In an exothermic reaction, the bonds in the product have higher bond energy (stronger bonds) than the reactants. In other words, the energy of the products is lower than the energy of the reactants, hence is energetically downhill, shown in Figure \(\PageIndex{2B}\). Energy is given off as reactants are converted to products. The energy given off is usually in the form of heat (although a few reactions give off energy as light). In the course of an exothermic reaction, heat flows from the system to its surroundings, and thus, gets warm.
| Heat is absorbed by reactants to form products. | Heat is released. |
| Heat is absorbed from the surroundings; as a result, the surroundings get cold. | Heat is released by the reaction to surroundings; surroundings feel hot. |
| \(\Delta H_{rxn}\) is positive | \(\Delta H_{rxn}\) is negative |
| The bonds broken in the reactants are stronger than the bonds formed in the products. | The bonds formed in the products are stronger than the bonds broken in the reactants. |
| The reactants are lower in energy than the products. | The products are lower in energy than the reactants. |
| Represented by an "uphill" energy diagram. | Represented by an "downhill" energy diagram |
- Chemical bonds have a certain energy that is dependent on the elements in the bond and the number of bonds between the atoms.
- Energy changes because bonds rearrange to make new bonds with different energies.
- Reaction A is exothermic because heat is leaving the system making the test tube feel hot. Reaction B is endothermic because heat is being absorbed by the system making the test tube feel cold.
- "Burning paper" is exothermic because burning (also known as combustion) releases heat
Updated April 25, 2017
By Ariel Balter, Ph.D.
A calorimeter is a device that carefully measures the temperature of an isolated system both before and after a reaction takes place. The change in temperature tells us whether thermal energy was absorbed or released, and how much. This gives us important information about the products, reactants and the nature of the reaction.
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An endothermic process absorbs heat from the surroundings, while an exothermic process releases heat into the surroundings. Adding heat helps sugar and salt dissolve in water. That reaction is endothermic: Reactants + Energy → Products. The chemical reactions in a candle flame give off heat. These are exothermic: Reactants → Product + Energy.
Calorimetry experiments measure the amount of thermal energy gained or lost during a reaction by measuring the temperature before and after. Based on the temperature change, the masses of the substances and equipment, and another property called the heat capacity (which may be different for each component), one calculates the change in thermal energy that occurred during the reaction. If the change is positive, then thermal energy was released, and the process is exothermic. If the change is negative, then thermal energy was absorbed, and the process is endothermic.
A calorimeter is a closed, insulated container in which the chemical reaction proceeds in an isolated environment. The calorimeter also includes a way to measure the temperature before and after the reaction. There are two main types of calorimeters: constant pressure calorimeters and constant volume calorimeters. A Styrofoam cup with a lid and a thermometer makes a basic constant pressure calorimeter useful for home experiments. The reaction is always at atmospheric pressure. A constant volume bomb calorimeter is more complicated. The reaction takes place in a thick-walled, sealed container that is immersed in an insulated water bath.
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The calories in foods can be determined by burning them in a bomb calorimeter. The food sample to be measured is placed in the inner chamber, which is filled with oxygen and has a heating element that will ignite the sample. Because we use foods to get energy, the process of burning them must release energy -- they are exothermic. Consequently, the temperature will be higher afterwards than beforehand. Another example of an exothermic reaction is what takes place in an instant hot pack.
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Many people have performed the experiment where you mix baking soda and vinegar together and get an exciting reaction. This reaction is endothermic. It would not be hard to test this in a simple home calorimeter. The opposite of instant hot packs are instant cold packs, which are often found in first-aid kits and use endothermic reactions.
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The word “reaction” in this article should really be thought of more generally. Phase changes, such as water freezing or boiling, are physical processes, not chemical reactions. The heat needed to be added or removed to make these phase changes happen give us an important physical constant called the heat of transformation. One can use a calorimeter to measure this.