Endothermic vs. Exothermic reactions

The reaction which requires heat from the surroundings to the system is called an endothermic reaction.

 

The reaction which releases heat from the system to surroundings is called an exothermic reaction.

 

In simple words in the endothermic process energy is transferred as heat from the surroundings and in an exothermic reaction, energy is transferred as heat to the surroundings. However, one must not forget that heat is a process, not an entity. 

 

When an endothermic process occurs in an adiabatic system, the temperature falls. If the process is exothermic, then the temperature rises. When an endothermic process occurs in a diathermic container energy enters as heat from the surrounding and both system and surrounding remain at the same temperature. If the process is exothermic, the energy leaves as heat, and the process is isothermal.

 

Endothermic reaction

Endo means ‘inside’, or ‘absorb’ and therm means ‘heat’. So endothermic reaction is a reaction in which heat is absorbed from the surroundings to the system. The reactions which require heat are called endothermic reactions. 

 

Figure 

 

Endothermic reactions are also called endergonic reactions because such reactions also absorb energy from the surrounding.

 

When the heat is absorbed from the surroundings to the system then the temperature of the surroundings decreases.

 

Examples 

 

Some examples of endothermic reactions are given below.

 

  • Melting ice cubes
  • Conversion of frost to water vapor
  • Evaporation of liquids
  • Formation of cation
  • Baking bread
  • Cooking an egg
  • Photosynthesis reaction
  • Separating ion pairs
  • Thermal cracking
  • Splitting gas molecules apart
  • Mixing water and ammonium nitrate
  • Making an anhydrous salt from a hydrate
  • Melting solid salts
  • The reaction of barium hydroxide octahydrate crystals with dry ammonium chloride 

 

Enthalpy of reaction

Enthalpy of reaction tells us how much heat is absorbed or released during the reaction. Enthalpy of reaction can be calculated as

 

Hreaction=nHproductmHreactant

 

Enthalpy of reaction is the difference between the molar enthalpy of product and molar enthalpy of reactant. Since in endothermic reaction, the energy of the product is greater than the energy of reactant that’s why enthalpy of reaction H is positive.

 

Graphically

 For an endothermic reaction when a graph is plotted between reaction coordinate and energy it can be clearly seen that the energy of the product is greater than that of reactants. So the reaction favors forward direction.

 

Explanation of daily life examples 

 

  • Photosynthesis 

 

In photosynthesis, reaction plants absorb energy from sunlight and convert carbon dioxide and water into glucose and oxygen.

 

6CO2+6H2O+heat→C6H12O6+6O2

 

Figure 

 

  • Cooking an egg

 

In cooking eggs, eff absorbs heat energy from the pan to cook.

 

Figure

 

Exothermic reaction

 

Exo means ‘outside’, or ‘release’, and therm means ‘heat’. So exothermic reaction is a reaction in which heat is released from the system to the surrounding. The reactions which release heat are called exothermic reactions. 

 

Figure 

 

Exothermic reactions are also called exergonic reactions because such reactions also release energy to the surrounding.

 

When the heat is released to the surroundings from the system then the temperature of the surroundings increases.

 

Examples 

 

Some examples of exothermic reactions are given below.

 

  • Making ice cubes
  • Formation of snow in clouds
  • Condensation of rain from water vapors
  • A candle flame
  • Rusting of iron
  • Burning sugar
  • Mixing sodium sulfite and bleach
  • Forming ion pairs
  • Combining atoms to make molecules in the gas phase
  • Mixing water and strong acids
  • Mixing water with an anhydrous salt
  • Crystallizing liquid salts
  • Nuclear fission
  • Neutralization reaction
  • Respiration reaction
  • Combustion 

 

Enthalpy of reaction

Enthalpy of reaction tells us how much heat is absorbed or released during the reaction. Enthalpy of reaction can be calculated as

 

Hreaction=nHproductmHreactant

 

Enthalpy of reaction is the difference between the molar enthalpy of product and molar enthalpy of reactant. Since in exothermic reaction, the energy of reactant is greater than the energy of product that’s why enthalpy of reaction H is negative.

 

Graphically

 For an exothermic reaction when a graph is plotted between reaction coordinate and energy it can be clearly seen that the energy of the reactant is greater than that of products. So the reaction favors the reverse direction.

 

Example of endothermic and exothermic reaction

 

Melting of ice→ endothermic reaction

Freezing of water →endothermic reaction

Evaporation of liquid→ endothermic reaction

Condensation→exothermic reaction

 

Figure 

 

Why is the heat absorbed or released during a reaction?

 

When a chemical reaction takes place some bonds are broken and new bonds are formed. Since the breaking of bonds requires energy, it is an endothermic reaction. The formation of bonds releases energy so it is an exothermic reaction.

 

Endothermic Vs Exothermic

 

Sr. noEndothermicExothermic
Endo means inside.Exo means outside. 
The reaction which absorbs heat from the surrounding is called an endothermic reaction.The reaction which releases heat to the surrounding is called an exothermic reaction.
The temperature of the surrounding area decreases.The temperature of the surrounding area increases.
The energy of the product is greater than the energy of the reactants.The energy of the product is less than the energy of reactants.
The product is less stable than the reactant.The product is more stable than the reactant.
Breakage of bond is endothermic.The formation of bonds is exothermic.
H is positive.H is negative.
S(surrounding) is negative.S(surrounding) is positive.
It is non-spontaneous.It is spontaneous.
All endergonic reactions are endothermic.All exergonic reactions are exothermic.
Examples of endothermic reactions are melting of ice, photosynthesis, cooking an egg, evaporation of liquid and thermal cracking, etc. Examples of exothermic reactions are freezing of water, combustion, condensation, rusting of iron, fission, and fusion, and neutralization reactions.

 

Key Takeaway(s)

Exothermic vs Endothermic reactions

Concepts Berg

How do you know what type of thermodynamic reaction is happening?

 

What are the two main types of thermodynamic reactions?

 

What are some non-spontaneous reactions?

The reaction which takes place in the presence of some external agents is called non-spontaneous reaction.

Some examples of non-spontaneous reaction are 

  • Melting of ice
  • Evaporation of water
  • Burning of halogens
  • Photosynthesis
  • Oxidation of gold and mercury

What reaction takes place in the dissolution of ammonium nitrate (salt)?

Dissolution of ammonium nitrate is an endothermic reaction. Since energy is required to break the lattice interaction between ammonium nitrate and between the water molecules so this is an endothermic reaction.

 

How do I know if a reaction is exothermic?

The reaction which releases heat is called an exothermic reaction. If the energy of reactants is greater than the energy of the product then it will be an exothermic reaction.

How could you physically tell if a reaction is endothermic vs exothermic?

In an exothermic reaction, the heat is absorbed from the surrounding to the system and in an exothermic reaction the heat is released from the system to the surrounding. If the temperature of the surrounding area decreases it will be endothermic and if the temperature of the surrounding increases it will be exothermic.

What is the endothermic and exothermic reaction with an example?

The reaction which requires heat is called an endothermic reaction. For example melting ice, cooking an egg, evaporation of liquid and thermal cracking etc.

The reaction which releases heat is called an exothermic reaction. For example freezing of water, combustion, condensation, and rusting of iron etc.

How are exothermic and endothermic reactions different?

In an endothermic reaction the energy of a product is greater than the energy of a reactant while in an exothermic the energy of reactant is greater than energy of product. 

H is positive for endothermic reaction and negative for exothermic.

Endothermic reactions absorb heat so the temperature of the surrounding area decreases. While exothermic reactions release heat so the temperature of the surrounding area increases.

Why do exothermic and endothermic reactions happen?

When a chemical reaction takes place some bonds are broken and some bonds are formed. Endothermic reaction happens because breakage of bonds requires energy so it absorbs it from the surrounding. 

Exothermic reaction happens because new bonds formed release energy to the surrounding.

Are endothermic reactions faster than exothermic?

Exothermic is spontaneous while endothermic is non-spontaneous reaction. Since exothermic reaction does not require any input energy to cross the energy barrier and form products  rather it takes place by its own. So it is faster than an endothermic reaction which requires input energy to cross the energy barrier.

Why are elimination reactions endothermic?

Elimination reactions are endothermic because they require energy. In elimination reactions two steps are involved. First formation of carbocation and in second step deprotonation take place. These processes need energy so they are endothermic.

Are plants endothermic or exothermic?

The reaction in plants i.e photosynthesis is endothermic because plants absorb energy from the sun.They utilize this energy to convert carbon dioxide and water into glucose and oxygen.

Why are humans endothermic and not exothermic?

 

Is hydrolysis exothermic or endothermic?

The reaction in which salt and water react to form acid and base is called hydrolysis. In this reaction energy is required to break the water molecules. Products are more stable than reactants meaning they are lower in energy than reactants. So hydrolysis is an endothermic reaction.

Can an exothermic reaction be endergonic?

 

Is the formation of glucose an endothermic or exothermic reaction?

The glucose is formed by the process of photosynthesis. Since the plants always need energy to convert carbon dioxide and water into glucose and oxygen. So the formation of glucose is an endothermic reaction.

Why are single displacement reactions exothermic?

In a single displacement reaction one element is replaced by another. The energy is released in doing so. So they are exothermic.

Is CO2 exothermic or endothermic?

The formation of carbon dioxide releases energy so it is an exothermic reaction. 

Are there any non-combustion exothermic reactions?

 

What is the activation energy of an endothermic reaction?

Activation energy is the amount of energy required to convert reactants into products. It is the difference of energy between energy of transition state and energy of reactants. In an endothermic reaction, the energy of the product is greater than the energy of reactants therefore the activation energy of the endothermic reaction is greater than the exothermic reaction.

Why is the reaction of acids and alkalis exothermic?

Acid and alkalis react with each other to form water and salt. Since these are stable products means they are in lower energy than reactants. Therefore reactions release energy and it is exothermic.

Is an exergonic chemical reaction always exothermic?

 

Which reaction is more exothermic, adding calcium oxide to water or the thermite reaction?

As calcium oxide in water releases heat -63.7kJ/mol and thermite processes release heat -849kJ/mol. So the thermite process is more exothermic.

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