11 Endothermic Reaction Example: Detailed Explanations

In this article, “endothermic reaction examples” different examples and some numerical problems on endothermic reaction is discussed briefly.

The examples are-

  1. Melting of Ice to Form Water
  2. Sublimation of Solid Carbon Dioxide
  3. Thermal Decomposition of Calcium Carbonate
  4. Photosynthesis
  5. Evaporation of Water
  6. Partial Oxidation of Natural Gas
  7. Formation of Nitric Oxide
  8. Dissolving Ammonium Chloride in Water
  9. Separation of Ion Pair
  10. Melting of Solid Salts
  11. Reaction of Thionyl Chloride with Cobalt (II)
  12. Formation of Cation in Gas Phase

What is an Endothermic Reaction?

In Chemistry, endothermic reaction is defined as one type of reaction in which any system absorbs energy in form of heat, light from surroundings.

Enthalpy change for an endothermic reaction is always positive (ΔH>0).

endothermic reaction examples
Energy Diagram of an Endothermic Reaction.
Image Credit: Wikimedia Commons

To know more please follow: Stereoselective vs Stereospecific: Detailed Insights and Facts

Melting of Ice to form water

Melting of ice to form water is an example of phase change reaction and it proceeds by endothermic pathway. Ice melts at 273 K or above 273K temperature. For melting of ice at 273K temperature, heat absorbed by system is equal to the latent heat (80cal/g) and for melting ice above 273K temperature heat absorbed by the system is more than this latent heat.

Sublimation of Solid Carbon Dioxide

Sublimation is a phase change process in which solid form is directly changed to vapour state without changing the phase from solid to liquid state. When solid CO2 known as dry ice is sublimed from its solid state to vapour state (gaseous Carbon dioxide), the system absorbs a large amount of heat from surroundings. Thus sublimation of solid CO2 is an example of endothermic process.

To know more please go through : Peptide Bond vs Disulfide Bond: Comparative Analysis and Facts

Thermal decomposition of Calcium Carbonate

Thermal decomposition is one type of decomposition reaction that takes place using thermal energy.

Calcium carbonate can be prepared by the reaction between calcium hydroxide and carbon dioxide.

Ca(OH)2 + CO2 CaCO3 + H2O

 When calcium carbonate is decomposed in presence of heat it produces calcium oxide (CaO) and CO2.

CaCO3 → CaO + CO2


Photosynthesis, an endothermic reaction, proceeds by absorbing sunlight (light energy). During photosynthesis, chlorophyll absorbs sunlight and carbon dioxide is reduced in presence of water to form glucose molecule.

CO2 + H2O → C6H12O6 + 6O2

Image Credit: Wikimedia Commons

To know more please check: Peptide Bond vs Ester Bond: Comparative Analysis And Facts

Evaporation of Water

Evaporation of water needs energy in form of heat to form vapour. Phase change (liq=vap) takes place through the evaporation of water. Evaporation of water takes place at 373K or above 373K. When water evaporated at 373K (1000C) the energy absorbed is equal to latent heat of evaporation (540cal/g) and for above 373K heat will be absorbed more than this latent heat of evaporation.

Partial Oxidation of Natural Gas

Partial oxidation of natural gas is definitely an endothermic reaction at it takes place under a very high temperature (1200-15000C). Natural gas contains methane (CH4) and it  undergoes oxidation in presence of steam (H2O). Hydrogen and carbon monoxide gas are obtained as the product of this partial oxidation process.

CH4 (g) + H2O (g) → CO (g) + 3H2

Formation of Nitric Oxide

In formation of nitric oxide heat energy is absorbed and thus del H is positive for this reaction. Almost 181 KJ amount of energy is absorbed during this reaction when dinitrogen and dioxygen reacts with each other.

N2 + O2 2NO

Dissolving Ammonium Chloride in Water

 Ammonium chloride (NH4Cl), a solid crystalline compound, is a product of ammonia and chlorine. In water it is dissociated into its two constituent atoms, ammonium cation (NH4) and chloride anion (Cl).

NH4Cl (s)→ NH4+ (aq) + Cl (aq)

NH4+ (aq) + H2O (liq) → NH3 (aq) + H3O+ (aq)

H3O+ + OH 2H2O (reversible reaction)

This dissolution proceeds towards forward direction by absorbing heat. Thus enthalpy change will be always positive.

To know more please follow: CH2CL2 Lewis Structure Why, How, When And Detailed Facts

Separation of Ion Pair

Ion pairs are formed mainly in solution due to electrostatic attraction force between positively and negatively charged ions. Formation of ion pairs proceeds through releasing of energy (exothermic process). Separation of ion pair occurs when this distinct chemical entity containing two positively charged ions gets separated and form two ions. Absorbing thermal energy is the main determining factor to proceed in the forward direction. So, it is the reverse process of formation of ion pair, and it is an endothermic process.

Melting of Solid Salts

Salt is one type of crystalline compound having very high melting point. But this solid salt is melted at standard temperature and pressure. Regular table salt (NaCl) has a melting point 8000C and heat of fusion (ΔH(fusion)) is 520 Joule per gram. Melting of solid salt requires high thermal energy and high positive enthalpy change.

Reaction of Thionyl Chloride with Cobalt (II)

Reaction between cobalt chloride hexahydrate with thionyl chloride give hydrochloric acid, cobalt chloride and sulfur dioxide as products. This is an endothermic process and takes place by absorbing heat from the surroundings. Temperature of the reaction medium is decreased from 160C to 5.90C and change of enthalpy is positive.

CoCl2. 6H2O + 6SOCl2 CoCl2 + 12HCl + 6SO2

Formation of a Cation in Gas Phase

Formation process of cation in gas phase requires thermal energy. To form a cation, energy equals to ionisation energy is needed to remove electrons from valence shell of an atom.

This ionisation energy depends on the electronic configuration of the respective atom. Thus formation of cation is definitely an endothermic process. Whereas formation of anion is an example of exothermic process because after adding electron on valence shell, some energy will be released.

Some numerical problems with answers on endothermic process is discussed below-

Calculate del H for the process- N2 (g) +2O2 (g) = 2NO2 (g) The enthalpy change for the given reactions are-

N2 (g) + O2 (g) = 2NO   ΔH = 180.5 KJ NO (g) + (1/2) O2 = NO2 (g) ΔH = -57.06 KJ

Answer:               N2 (g) + O2 (g)   2NO (g)                                                                                                           (2nd reaction× 2)  NO (g) + (1/2) O2 NO2 (g)

Resultant equation will be = N2 (g) + 2O2 (g) 2NO2 (g) Thus, enthalpy change of this reaction is = {180.5 +2×(-57.06)} KJ = 66.38 KJ.

This is an endothermic reaction as the change of enthalpy is positive.

Calculate the change of enthalpy for the following reaction : Hg2Cl2 (s) = 2Hg (l) + Cl2 (g) Enthalpy change for the given reactions areHg (liq) + Cl2 (g) = HgCl2 (s)   ΔH= -224KJ Hg (liq) + HgCl2 (s) = Hg2Cl2 (s)   ΔH = -41.2 KJ

Answer:    The above given reactions can be written as-

HgCl2 = Hg (liq) + Cl2 (g) (s)       ΔH= 224KJ                                    Hg2Cl2 (s)= Hg (liq) + HgCl2 (s)  ΔH = 41.2 KJ

Resultant equation will be:  Hg2Cl2 (s) = 2Hg (l) + Cl2 (g)

Thus, change of enthalpy is = (224 + 41.2) KJ                                                         = 265.2 KJ.

Calculate the change of enthalpy for the following reaction – CO2 (g) + H2O (liq) = CH4 (g) + O2 (g) Given enthalpy change for CH4, H2O and CO2 are -74.8, -285.8 and -393.5 KJ/mol respectively.

Answer: change of enthalpy = enthalpy of products – enthalpy of reactants.

Del Hf for oxygen is 0.

The balanced equation is- CO2 (g) + 2H2O (liq) = CH4 (g) + O2 (g) ΔH = {(-74.8) – 2×(-285.8) – (-393.5)} KJ/mol =890.3 KJ/mol

Frequently Asked Questions (FAQ)

How can the rate of an endothermic reaction be increased?

Answer: An endothermic reaction depends on temperature of reaction medium. Decreasing the temperature of reaction medium increases the extent of reaction towards forward direction.

What is the change of entropy for an endothermic reaction?

Answer: Change of entropy for an endothermic reaction is always negative an energy is absorbed from surroundings to system

State a reaction which will always be an endothermic reaction?

Answer: Thermal decomposition is one type of reaction that will always be an example of endothermic reaction.

Aditi Roy

Hi..I am Aditi Roy, I have completed my graduation from University of Calcutta and am pursuing M.Sc in Chemistry from Techno India University. My area of specialization is on Inorganic Chemistry.

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