9.5.4 Exothermic and Endothermic Reactions

This is the fourth lecture from Chapter 5: “Energetics” in the new Class 9 Chemistry book (Punjab Board – PCTB). It discusses two types of reactions: exothermic reactions and endothermic reactions. The lecture also includes a multiple-choice quiz, short question and long question notes.

MCQs Based Quiz

9.5.4 Exothermic and Endothermic Reactions

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Most of the physical and chemical changes are accompanied with:

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Chemical reactions in which heat energy is evolved (released) are called:

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Chemical reactions in which heat energy is absorbed are called:

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$\mathrm{2H_{2\,(g)} + O_{2\,(g)} \longrightarrow 2H_2O_{(l)}}\  \mathrm{+\ 571.6\ kJ}$ is an:

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$ \mathrm{C_{\ (s)}\ +\ O_{2\ (g)}} \longrightarrow \mathrm{CO_{2\ (g)}}\ \mathrm{+\ 393.5\ kJ} $ is an:

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$\mathrm{H_{2(g)} + I_{2(s)} \longrightarrow 2HI_{(g)}}\ \mathrm{-\ 53.08\ kJ} $ is an:

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$\mathrm{N_{2(g)} + O_{2(g)} \longrightarrow 2NO_{(g)}}\ \mathrm{-\ 180.6\ kJ} $ is an:

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Which of the following happens during the burning of fuel?

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Food provide use with energy due to conversion of:

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If a chemical reaction involves the breakage of weaker bonds and formation of stronger bonds, it will be classified as:

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If a chemical reaction involves the breakage of stronger bonds and formation of weaker bonds, it will be classified as:

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From where does an endothermic reaction absorb energy from?

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Which of the following is a biological fuel for human body?

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Which of the following is a biological fuel for human body?

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Which of the following converts chemical energy into heat energy in our bodies?

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Fuel in the engine of vehicles produce heat as a result of:

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Fireworks are a result of ___ reaction:

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During combustion ___ react with oxidizing agents to produce bright colours.

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Short Questions

Q1. What is meant by heat of a reaction?

The amount of heat that is absorbed or evolved (released) during a chemical reaction is called the heat of that reaction.

Q2. What is an endothermic reaction?

A type of reaction that involves the absorption of energy (heat) is called an endothermic reaction.

For example:

$\mathrm{
H_{2\ (g)}\ +\ I_{2\ (g)} \longrightarrow 2HI_{\ (g)}\ -\  53.08\ kJ
}$

Q3. What is an exothermic reaction?

A type of reaction that involves the evolution (release) of energy (heat) is called an exothermic reaction.

$\mathrm{
2H_{2\ (g)}\ +\ O_{2\ (g)} \longrightarrow 2H_2O_{\ (l)}\ +\  571.6\ kJ
}$

Q4. What happens to the heat that is evolved during an exothermic reaction?

The heat that is evolved during an exothermic reaction is absorbed by the surroundings, i.e., by the container in which the reaction is being carried out.

Q5. From where does an endothermic reaction absorb energy?

An endothermic reaction absorbs energy from its surroundings, which decreases the temperature of the container in which the reaction is happening.

Q6. How can the temperature of a container help you determine whether a reaction is endothermic or exothermic?

  1. If a reaction is endothermic, it will absorb heat from the surroundings. This will decrease the temperature of the container.
  2. If a reaction is exothermic, it will release heat into the surroundings. This will increase the temperature of the container.

Q7. If the formation of water from H₂ and O₂ releases 571.6 kJ of heat, what kind of reaction would the decomposition of water be: endothermic or exothermic?

$\mathrm{
2H_{2\ (g)}\ +\ O_{2\ (g)} \longrightarrow 2H_2O_{\ (g)}\ +\  571.6\ kJ
}$

If formation of water is an exothermic reaction, its decomposition would be an endothermic process absorbing the same amount of heat that was released during its formation.

$\mathrm{
2H_2O_{\ (g)} \longrightarrow 2H_{2\ (g)}\ +\ O_{2\ (g)}\ -\  571.6\ kJ
}$

Q8. If the formation of hydrogen iodide (HI) from hydrogen (H₂) and iodine (I₂) absorbs 53.08 kJ of heat, what kind of reaction would the decomposition of hydrogen iodide be: endothermic or exothermic?

$\mathrm{
H_{2(g)}\ +\ I_{2(s)}\ \longrightarrow\  2HI_{(g)}\ -\ 53.08\ kJ
}$

If formation of hydrogen iodide is an endothermic reaction, its decomposition would be an exothermic process releasing the same amount of heat that was absorbed during its formation.

$\mathrm{
2HI_{(g)}\ \longrightarrow\ H_{2(g)}\ +\ I_{2(s)}\ +\ 53.08\ kJ
}$

Q9. Write three applications of an exothermic reaction.

  1. Fuel in our vehicles provides energy through exothermic combustion reaction.
  2. Metabolism that converts food into energy for our body is also an exothermic reaction.
  3. In thermal power plants, chemical energy from fuel is converted into electrical energy.

Q10. How do self-heating and self-cooling packs work?

Self-cooling and self-heating packs contain reactants that undergo exothermic (in self-heating) or endothermic (in self-cooling) reactions, resulting in an increase or decrease in the temperature of the pack.

Q11. Why does chemical reaction between sodium metal and water proceeds violently?

The reaction of sodium metal with water is an exothermic reaction. The hydrogen produced during this reaction catches fire due to the excessive amount of heat released, making the reaction very violent.

$\mathrm{
2Na_{(s)}\ +\ 2H_2O_{(l)}\ \longrightarrow\ 2NaOH_{(aq)}\ +\ H_{2\ (g)}\ +\ 368kJ/mol
}$

Q12. Is melting of ice an exothermic change or endothermic change?

Melting of ice is an endothermic change because in this process heat from the surrounding is absorbed to break the hydrogen bonding between water molecules in ice.

Q13. Can exothermic reaction be reversed?

Yes, theoretically, it is possible for every exothermic reaction to be reversed. For example, combustion of methane (natural gas) can be reversed through an endothermic reaction.

$\mathrm{ C_{(s)}\ +\ O_{2(g)}\ \longrightarrow \ CO_{2(g)}\ +\ 393.5kJ\ \quad \boxed{\text{Exothermic}} }$

$\mathrm{ CO_{2(g)} \longrightarrow \ C_{(s)}\ +\ O_{2(g)}\ -\ 393.5kJ\ \quad \boxed{\text{Endothermic}} }$

But in reality, not every exothermic reaction can be reversed. Only moderately or less exothermic reactions can be reversed, while highly exothermic reactions tend to be irreversible.

Q14. What happens to energy when chemical bonds are formed or broken?

The formation of a bond between two atoms releases energy, and the breaking of a bond between two atoms absorbs energy

Q15. How can you estimate an exothermic reaction by comparing the strength of chemical bonds in reactants and products?

If the reactants have weaker bonds as compared to the products, then it would be an exothermic reaction. This is because less energy would be consumed to break weaker bonds of reactant, and more energy would be released during the formation of stronger bonds in products.

Q16. How can you estimate an endothermic reaction by comparing the strength of chemical bonds in reactants and products?

If the reactants have stronger bonds as compared to the products, then it would be an endothermic reaction. This is because more energy would be consumed to break stronger bonds of reactants, and less energy would be released during the formation of weaker chemical bonds in products.

Q17. The total bond dissociation energy of H₂ and O₂ in the reaction below is 1368 kJ, while the bond formation energy of H₂O molecules is 1936 kJ. Based on this date, can you determine whether the reaction is endothermic or exothermic?

$\mathrm{2H_{2\,(g)} + O_{2\,(g)} \longrightarrow 2H_2O_{(g)}}$

According to the provided data, the energy released during the formation of bonds in products is more than the energy absorbed during the bond dissociation of reactants. Therefore, the reaction is considered exothermic because it releases this excess energy.

Q18. Calculate the enthalpy change for the formation of one mole of liquid water.

Formation of one mole of liquid water involves two steps:

  1. Reaction between hydrogen and oxygen which releases 284 kJ of energy.

$\mathrm{H_{2\,(g)} + \frac{1}{2}O_{2\,(g)} \longrightarrow H_2O_{(g)}\ +\ 284\ kJ/mol}$

  1. The water produced in first step is in gaseous (steam) phase. To convert it into liquid water, some more energy (46 kJ/mol) needs to be released.

$\mathrm{ H_2O_{(g)}\ \longrightarrow\ H_2O_{(l)}\ +\ 46kJ/mol}$

So total amount of energy released during this process is $\mathrm{284 + 46 = 330 kJ/mol}$.

Q19. How fireworks produce vivid and bright colours?

Fireworks are a result of combustion reactions that produces heat, light and sound. Different metal salts and oxidizing agents produce variety of colors when burnt.

Descriptive Questions

Q1. Explain the exothermic and endothermic reactions with one example of each. How can the type of reaction be identified by observing changes in the temperature of the container?

Chemical reactions based on energy changes are classified into two categories:

  • Exothermic Reactions
  • Endothermic Reactions

Exothermic Reactions:

A type of reaction that involves the evolution (release) of energy (heat) is called an exothermic reaction.

The heat that is evolved during an exothermic reaction is absorbed by the surroundings, i.e., by the container in which the reaction is taking place.

Example: Hydrogen and oxygen gas react to produce water in an exothermic reaction.

$\mathrm{
2H_{2\ (g)}\ +\ O_{2\ (g)} \longrightarrow 2H_2O_{\ (g)}\ + 568\ kJ
}$

This reaction releases $\mathrm{568 kJ}$ of heat which can be shown separately as change in enthalpy, $\Delta \mathrm{H=-568 kJ}$. If this reaction goes in backward direction and water decomposes into hydrogen and oxygen, same amount of energy will be absorbed.

Example: Carbon burns in oxygen gas to produce carbon dioxide gas.

$\mathrm{
C_{(s)}\ +\ O_{2\ (g)}\ \longrightarrow\ CO_{2\ (g)}\ +\  393.5\ kJ
}$

It is an exothermic reaction and releases $\mathrm{393.5kJ}$ of energy (heat) which can be shown separately as change in enthalpy $\Delta \mathrm{H=-393.5 kJ}$. If this reaction goes in backward direction and carbon dioxide decomposes into carbon and oxygen, the same amount of energy ($\mathrm{393.5kJ}$) will be absorbed.

Endothermic Reaction:

A type of reaction that involves the absorption of energy (heat) is called an endothermic reaction.

An endothermic reaction absorbs energy from its surroundings, which decreases the temperature of the container in which the reaction is taking place.

Example: Hydrogen and iodine react to produce hydrogen iodide.

$\mathrm{
H_{2\ (g)}\ +\ I_{2\ (g)} \longrightarrow 2HI_{\ (g)}\ -\  53.08\ kJ
}$

It is an endothermic reaction and absorbs $\mathrm{53.08kJ}$ which can be shown separately as change in enthalpy $\Delta \mathrm{H=+53.08 kJ}$. If this reaction goes in backward direction and hydrogen iodide decomposes into hydrogen and iodine, the same amount of energy ($\mathrm{53.08kJ}$) will be released.

Example: Nitrogen and oxygen react to produce nitric oxide.

$\mathrm{
N_{2\ (g)}\ +\ I_{2\ (g)} \longrightarrow 2NO_{\ (g)}\ -\  180.6\ kJ
}$

It is an endothermic reaction and absorbs $\mathrm{180.6kJ}$ which can be shown separately as change in enthalpy $\Delta \mathrm{H=+180.6 kJ}$. This reaction is carried out in clouds due to energy provided by lightning. If this reaction goes in backward direction, the same amount of energy ($\mathrm{180.6kJ}$) will be released.

Temperature of the Container:

Hot container: If a reaction is exothermic, it will release heat into the surroundings, which will increase the temperature of the container.

Cold Container: If a reaction is endothermic, it will absorb heat from the surroundings, which will decrease the temperature of the container.

Q2. How do our lives depend on exothermic reactions?

Our lives depend on exothermic reactions because they fulfill our energy requirements in various forms. This is because exothermic reactions convert chemical energy into heat energy which can be used for different purposes.

Food:

Foods such as fats and carbohydrates are biological fuels. During metabolism the chemical energy from this food is converted to heat. This energy is used to keep our bodies warm.

Households:

The heat from fuels like coal, oil and natural gas is used to cook food and for other household purposes. In this process, substances present in the fuel react with oxygen from the air to produce heat.

Electricity:

Fuels such as coal, natural gas and oil are also used in power stations to produce electricity. In these power stations chemical energy from fuel is converted into heat through combustion. This heat is then used to produce steam at high pressure which is then used to run turbines and generate electricity.

Transportation:

In vehicles, the combustion of petrol and diesel converts their chemical energy into heat energy that drives them forward.

Fireworks:

In fireworks, metal salts react with oxidizing agents to undergo combustion reaction which produce heat, light and sound. Different metals salts are used to produce variety of colors.

Q3. How does the strength of chemical bonds in the reactants and products help determine whether a reaction is exothermic or not?

Chemical Bonding and Energy Change:

Bond Formation: Bond formation releases energy. Therefore, it is an exothermic process.

Bond Breaking: Bond breaking (dissociation) absorbs energy. Therefore, it is an endothermic process.

Chemical Reactions and Chemical Bonding:

During a chemical reaction:

  1. Chemical bonds of reactants are broken which absorbs energy (endothermic).
  2. Chemical bonds of products are formed which releases energy (exothermic).

Total Energy Change and Classification of Reaction:

  1. If bond formation releases more energy than bond breaking consumes, the reaction is exothermic.
  2. If bond formation releases less energy than bond breaking consumes, the reaction is endothermic. 

Example of Water Formation:

$\mathrm{
2H_{2\ (g)}\ +\ O_{2\ (g)} \longrightarrow 2H_2O_{\ (g)}
}$

Water formation from hydrogen and oxygen is an exothermic reaction. Let’s understand why.

Formation of water involves two processes:

  1. Bonds dissociation (breaking) of reactants (H2 and O2).
  2. Bonds formation in product (H2O).

Bonds Dissociation in Reactants:

Breaking the bonds in one mole of hydrogen (H2) absorbs $\mathrm{435\ kJ/mol}$ energy.

$\mathrm{
H_{2\ (g)}\ \longrightarrow 2H_{\ (g)}\ + 435\ kJ/mol \quad \boxed{\text{Endothermic}}
}$

Breaking the bonds in two moles of hydrogen will require twice that amount.

$\mathrm{
\mathrm{2H_{2\,(g)} \longrightarrow 4H_{(g)} + 870\,\text{kJ} \quad \boxed{\text{Endothermic}}}
}$

Breaking the bonds in one mole of oxygen (O2) absorbs $\mathrm{498\ kJ/mol}$ energy.

$\mathrm{
\mathrm{O_{2\,(g)} \longrightarrow 2O_{(g)} +\ 498\,\text{kJ/mol}} \quad \boxed{\text{Endothermic}}
}$

The total energy absorbed in bond breaking is:

$$\begin{gathered}
\mathrm{2H_{2\,(g)} \longrightarrow 4H_{(g)} + 870\,\text{kJ}}\\
\mathrm{O_{2\,(g)} \longrightarrow 2O_{(g)} + 498\,\text{kJ/mol}}\\
\rule[0.5ex]{20em}{0.4pt}\\
\mathrm{2H_{2\,(g)} + O_{2\,(g)} \longrightarrow 4H_{(g)} + 2O_{(g)} + 1368\,\text{kJ}}
\end{gathered}
$$

Bond Formation in Products:

Water (H–O–H) is formed by the formation of O–H bonds, which releases $\mathrm{484\ kJ/mol}$ of energy. 

$\mathrm{
H\  +\  O\  \longrightarrow \ce{H-O}\ -\  484\ kJ/mol \quad \boxed{\text{Exothermic}}
}$

The formation of O–H bonds in two moles will release four times that amount of energy.

$\mathrm{
4H_{(g)} + 2O_{(g)} \longrightarrow 2H_2O_{\ (g)}\ -\  1936\ kJ \quad \boxed{\text{Exothermic}}
}$

Total Energy Change During the Formation of Water:

Total energy change during the overall reaction can be calculated as followed:

$\mathrm{-1936kJ\ +\ 1368kJ\ =\ -568kJ}$

This means that formation of two moles of water molecules will produce (release) $\mathrm{568\ kJ}$ of energy. You can also say that the change in enthalpy in the formation of two moles of water is $\mathrm{\Delta H=-568kJ}$.

$\mathrm{
2H_{2\ (g)}\ +\ O_{2\ (g)} \longrightarrow 2H_2O_{\ (g)}\ + 568\ kJ \quad\ \boxed{\mathrm{ \Delta H\ =\ -658kJ}}
}$

The enthalpy change for one mole of water molecules is:

$$\mathrm{\frac{-568kJ}{2}\ =\ -284kJ/mol}$$

The experimental value of formation of gaseous water is $\mathrm{-284.3kJ}$ which is very close to calculated value.

Q4. Calculate the enthalpy change of the following chemical reaction.

$\mathrm{H_{2(g)}\ +\ I_{2(g)}\ \longrightarrow\ 2HI_{(g)} }$

Bond energies of $\mathrm{H_2}$, $\mathrm{I_2}$ and $\mathrm{HI}$ are $\mathrm{435}$, $\mathrm{151}$ and $\mathrm{-299kJ/mol}$ respectively.

Solution:

$$\begin{aligned}
\mathrm{Bond\ dissociation\ energy\ (BDE)\ of\ H_2}\ &=\ \mathrm{436kJ/mol} \\
\mathrm{ Bond\ dissociation\ energy\ (BDE)\ of\ I_2}\ &=\ \mathrm{151kJ/mol } \\
\mathrm{ Bond\ formation\ energy\ (BFE)\ of\ HI}\ &=\ \mathrm{-299kJ/mol } \\
\\
\mathrm{Enthalpy\ Change\ \Delta H}\ &=\ \mathrm{Total\ BDE\ of\ Reactants\ +\ Total\ BFE\ of\ Products}\\
&=\ \mathrm{(436+151)\ +\ (-299 \times 2) } \\
&=\ \mathrm{587 – 598}\\
&=\ \mathrm{-11kJ/mol}
\end{aligned}$$

The enthalpy change of the reaction is -11 kJ/mol indicating that this is an exothermic reaction.