Difference Between Endothermic and Exothermic Reactions

Main Difference – Endothermic vs Exothermic Reactions

Chemical reactions can be divided into two groups as endothermic reactions and exothermic reactions according to the energy transfer between the surrounding and the system where the reaction is taking place. In order to categorize a particular chemical reaction as endothermic or exothermic, we can calculate the enthalpy change between reactants and products. If not, we can observe the change in temperature of the reaction mixture. The main difference between endothermic and exothermic reactions is that endothermic reactions absorb energy from the surrounding whereas exothermic reactions release energy to the surrounding.

Key Areas Covered

1. What are Endothermic Reactions
      – Definition, Properties, Examples
2. What are Exothermic Reactions
      – Definition, Properties, Examples
3. What is the Difference Between Endothermic and Exothermic Reactions
      – Comparison of Key Differences

Key Terms: Combustion, Endothermic, Enthalpy, Exothermic, Heat, Internal Energy

What are Endothermic Reactions

Endothermic reactions are chemical reactions that absorb heat energy from the surrounding. This means energy should be given from outside for the initiation and progression of an endothermic reaction. As a result, the temperature of the system decreases.

As the system absorbs energy from the outside, the enthalpy change of the reaction takes a positive value. Enthalpy is the sum of the internal energy of a system and the energy required to maintain the volume and pressure of that system in that environment. In the beginning, the enthalpy of the system equals the sum of the enthalpies of the reactants. At the end of the endothermic reaction, the enthalpy or the energy of the products is higher due to the absorption of energy. This can be explained as below.

A     +     B        →       C    +    D

ΔH    =     {H_C  +  H_D}   –   {H_A  +  H_B}

ΔH    =     (H_products)   –   (H_reactants)      =    A positive value

Where,

ΔH is the enthalpy change that is occurred after the reaction,
HC and HD are the enthalpies of products C and D respectively
HA and HB are the enthalpies of reactants A and B respectively.

Figure 1: Enthalpies of reactants and products of an endothermic reaction

The endothermic reaction can easily be recognized by touching the beaker or the test tube where the reaction is taking place. The beaker will feel colder than before. This is because it absorbs energy from the outside.

Examples of Endothermic Reactions

• Dissolving ammonium chloride solid in water:

                     NH₄Cl_(s)   +   H₂O_(l)    +   heat    →     NH₄Cl_(aq) 

• Mixing water with potassium chloride:

                      KCl_(s)     +   H₂O_(l)   +   heat   →     KCl_(aq)

• Reacting Ethanoic acid with sodium carbonate:

                     CH₃COOH_(aq)     +    Na₂CO_3(s)     +    heat    →      CH₃COO^–Na⁺_(aq)     +    H⁺_(aq)    +    CO₃^2-_(aq)

Note that the “heat” is included in the right side of the reaction equation. This is to indicate the absorption of heat by the system.

What are Exothermic Reactions

Exothermic reactions are chemical reactions that release heat energy to the surrounding. This means the energy is released to the outside when the chemical reaction progresses. Since the internal energy is released from the system, the enthalpy of the products is lower than the enthalpy of reactants. This can be explained as below.

P    +     Q       →     R    +    S

ΔH    =     {H_R  +  H_S}   –   {H_P  +  H_Q}

ΔH    =     (H_products)   –   (H_reactants)      =    A negative value

The change in the enthalpy is a negative value now since the internal energy of reactants is less due to the release of energy. The temperature of the system will increase as the exothermic reaction proceeds. Therefore, one can guess whether a particular chemical reaction is endothermic or exothermic just by touching the wall of the container where the reaction is taking place.  In an exothermic reaction, the container will get warmer.

Figure 2: Enthalpies of reactants and products of an exothermic reaction

Examples of Exothermic Reactions

• Burning of Hydrogen gas:

                     2H_2(g)     +      O_2(g)       →         2H₂O_(l)     +   heat

• Combustion of ethanol (complete combustion):

                      CH₃CH₂OH_(l)     +      3O_2(g)       →       2CO_2(g)     +      3H₂O_(l)

Difference Between Endothermic and Exothermic Reactions

Definition

Endothermic Reactions: Endothermic reactions are chemical reactions that absorb heat energy from the surrounding.

Exothermic Reactions: Exothermic reactions are chemical reactions that release heat energy to the surrounding.

Temperature

Endothermic Reactions: The temperature decrease with the progression of endothermic reactions.

Exothermic Reactions: The temperature increase with the progression of exothermic reactions.

Enthalpy

Endothermic Reactions: The enthalpy of reactants is lower than that of products in endothermic reactions.

Exothermic Reactions: The enthalpy of reactants is higher than that of products in exothermic reactions.

Enthalpy Change

Endothermic Reactions: The change in enthalpy (ΔH) is a positive value for endothermic reactions.

Exothermic Reactions: The change in enthalpy (ΔH) is a negative value for exothermic reactions.

Energy

Endothermic Reactions: Energy should be given to the system in endothermic reactions.

Exothermic Reactions: Energy is released from the system in endothermic reactions.

Conclusion

Chemical reactions are categorized as endothermic and exothermic reactions according to the energy transfer between the system and the surrounding. The main difference between endothermic and exothermic reactions is that endothermic reactions absorb energy from the surrounding whereas exothermic reactions release energy to the surrounding. Each and every chemical reaction can be grouped into these two categories by calculating the enthalpy change in the reaction.

References:

1.”Endothermic Reactions.” Endothermic Reactions, Examples of Endothermic Reactions | [email protected]. N.p., n.d. Web. Available here. 21 July 2017. 
2.”Exothermic vs. Endothermic and K.” Chemistry LibreTexts. Libretexts, 08 Mar. 2017. Web. Available here. 21 July 2017. 

Image Courtesy:

1. “Endothermic Reaction” By Brazosport College – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Exothermic Reaction” By Brazosport College – Own work (CC BY-SA 3.0) via Commons Wikimedia