*Please click video title headings to play video if video does not load properly.
Endothermic reactions
The products of an endothermic reaction have more energy than the reactants because heat energy is gained/ absorbed from the surroundings.
Examples of endothermic reactions:
1. Photosynthesis
2. Thermal decomposition (e.g. using heat to break down compounds such as calcium carbonate to its products calcium oxide and carbon dioxide)
3. Dissolving some salts in water (e.g. ammonium chloride, potassium nitrate)
4. Removing water from hydrated copper (II) sulphate crystals, CuSO4.5H2O
5. Melting and boiling <Take note these two endothermic processes are physical change in states and NOT chemical reactions>
Examples of endothermic reactions:
1. Photosynthesis
2. Thermal decomposition (e.g. using heat to break down compounds such as calcium carbonate to its products calcium oxide and carbon dioxide)
3. Dissolving some salts in water (e.g. ammonium chloride, potassium nitrate)
4. Removing water from hydrated copper (II) sulphate crystals, CuSO4.5H2O
5. Melting and boiling <Take note these two endothermic processes are physical change in states and NOT chemical reactions>
Exothermic reactions
The products of an exothermic reaction have less energy than the reactants because heat energy is lost to the surroundings.
Examples of exothermic reactions:
1. Neutralisation reactions (e.g. Acid react with alkali to produce salt and water only)
2. Combustion reactions (e.g. burning of fuels in the presence of oxygen to produce carbon dioxide and water vapour in the case of complete combustion)
3. Reaction of metals with water (e.g. calcium reacts with water to form calcium hydroxide and hydrogen gas)
4. Dissolving some salts in water (e.g. calcium chloride)
5. Freezing and condensation <Take note these two exothermic processes are physical change in states and NOT chemical reactions>
Examples of exothermic reactions:
1. Neutralisation reactions (e.g. Acid react with alkali to produce salt and water only)
2. Combustion reactions (e.g. burning of fuels in the presence of oxygen to produce carbon dioxide and water vapour in the case of complete combustion)
3. Reaction of metals with water (e.g. calcium reacts with water to form calcium hydroxide and hydrogen gas)
4. Dissolving some salts in water (e.g. calcium chloride)
5. Freezing and condensation <Take note these two exothermic processes are physical change in states and NOT chemical reactions>
Endothermic and exothermic reactions
In this video, the example given for endothermic reaction is the electrolysis of water while the example given for exothermic reaction is the reaction of metal calcium with water. This video also shows a simple sketch of the energy level diagram for both types of reactions. Please refer to your school notes and textbook for a more complete energy level diagram for both types of reactions.
Energy level diagrams for exothermic and endothermic reactions
For exothermic reactions, since products have less energy than reactants as energy is lost to surroundings, the enthalpy change (also known as heat of reaction) is Δ H = -ve value. Negative value suggests that heat energy is lost to the surroundings.
For endothermic reactions, since products have more energy than reactants as energy is absorbed from surroundings, the enthalpy change (heat of reaction) is Δ H = +ve value. Positive value suggests that heat energy is absorbed from surroundings.
See following video for energy level diagrams for exothermic and endothermic reactions. The discussion of the effect of a catalyst is also given which is to lower the activation energy for the reaction to proceed (effect of catalyst is same as enzymes for those taking biology which is to lower the activation energy Ea).
For endothermic reactions, since products have more energy than reactants as energy is absorbed from surroundings, the enthalpy change (heat of reaction) is Δ H = +ve value. Positive value suggests that heat energy is absorbed from surroundings.
See following video for energy level diagrams for exothermic and endothermic reactions. The discussion of the effect of a catalyst is also given which is to lower the activation energy for the reaction to proceed (effect of catalyst is same as enzymes for those taking biology which is to lower the activation energy Ea).
Calculating enthalpy change (heat of reaction Δ H) from bond energies
For O level chemistry, calculation of enthalpy change from bond energies is not so critical to know. One just need a basic understanding of this as the questions asked if any during O level chemistry exam will be just simple calculations rather than complex ones (complex ones are given at A level chemistry). If such questions are even asked, the relevant types of bond energies required will also be given in the question. Calculations of enthalpy change from bond energies is more relevant to students taking Integrated Program (from schools like Raffles Instituition etc.).
For this video, the complete combustion of methane in oxygen to produce carbon dioxide and water vapour is given.
CH4 + 2O2 → CO2 + 2H2O
Step 1: Form a balanced equation.
Step 2: Draw the structural formulae of all the reactants and products so as to go to step 3 to determine the number and types of bonds involved (see video for the structural formulae of all the reactants and products in this combustion reaction).
Step 3: Determine the number of each types of bonds involved. Pay attention also to the number in front of the molecule, e.g. there are 2 molecules of O2 and 2 molecules of H2O. For the reactants on left side of this equation, there are a total of 4 C-H bonds and 2 O=O bonds to be broken. For the products on right side of equation, there are a total of 4 O-H bonds and 2 C=O bonds to be made.
Step 4: Refer to the given bond energies for each type of bonds and calculate their total energies involved (in this video, the total bond energies are already calculated for you based on a reference table that is not shown to you).
Step 5: Add up all the total bond energies involved in bond breaking of reactants on left side of equation. In this video, a total of 2640 KJ of energy is absorbed to break bonds of reactants.
Step 6: Add up all the total bond energies involved in bond making of products on right side of equation. In this video, a total of 3338 KJ of energy is released in bond making of products formed.
Step 7: Use the formula to calculate enthalpy change.
Enthalpy change Δ H = Energy absorbed in bond breaking - Energy released in bond making.
In this combustion reaction, Enthalpy change Δ H = 2640 - 3338 = -698 KJ. Since Δ H is a negative value, this combustion reaction is an example of exothermic reaction.
We must also understand that during this reaction, the energy released from bond making in products is more than energy absorbed in bond breaking in reactants. Thus, combustion reactions are exothermic in nature. Use a small amount of heat absorbed when burning the fuel (methane) in oxygen to break all their bonds in their molecules and when the products carbon dioxide and water vapour are formed, a larger amount of energy is released when new bonds are made in the carbon dioxide and water vapour molecules. Thus, there is a net release of energy to surroundings (Δ H is negative value) and we can make use of this net release of energy for our own use such as to run power stations to produce electricity or to drive car and aircraft engines or machineries from combustion reactions.
Lastly, we also arrive at the conclusion that bond breaking is endothermic process since energy is absorbed from surroundings to break bonds while bond making is exothermic process since energy is released to surroundings when bonds are formed.
For this video, the complete combustion of methane in oxygen to produce carbon dioxide and water vapour is given.
CH4 + 2O2 → CO2 + 2H2O
Step 1: Form a balanced equation.
Step 2: Draw the structural formulae of all the reactants and products so as to go to step 3 to determine the number and types of bonds involved (see video for the structural formulae of all the reactants and products in this combustion reaction).
Step 3: Determine the number of each types of bonds involved. Pay attention also to the number in front of the molecule, e.g. there are 2 molecules of O2 and 2 molecules of H2O. For the reactants on left side of this equation, there are a total of 4 C-H bonds and 2 O=O bonds to be broken. For the products on right side of equation, there are a total of 4 O-H bonds and 2 C=O bonds to be made.
Step 4: Refer to the given bond energies for each type of bonds and calculate their total energies involved (in this video, the total bond energies are already calculated for you based on a reference table that is not shown to you).
Step 5: Add up all the total bond energies involved in bond breaking of reactants on left side of equation. In this video, a total of 2640 KJ of energy is absorbed to break bonds of reactants.
Step 6: Add up all the total bond energies involved in bond making of products on right side of equation. In this video, a total of 3338 KJ of energy is released in bond making of products formed.
Step 7: Use the formula to calculate enthalpy change.
Enthalpy change Δ H = Energy absorbed in bond breaking - Energy released in bond making.
In this combustion reaction, Enthalpy change Δ H = 2640 - 3338 = -698 KJ. Since Δ H is a negative value, this combustion reaction is an example of exothermic reaction.
We must also understand that during this reaction, the energy released from bond making in products is more than energy absorbed in bond breaking in reactants. Thus, combustion reactions are exothermic in nature. Use a small amount of heat absorbed when burning the fuel (methane) in oxygen to break all their bonds in their molecules and when the products carbon dioxide and water vapour are formed, a larger amount of energy is released when new bonds are made in the carbon dioxide and water vapour molecules. Thus, there is a net release of energy to surroundings (Δ H is negative value) and we can make use of this net release of energy for our own use such as to run power stations to produce electricity or to drive car and aircraft engines or machineries from combustion reactions.
Lastly, we also arrive at the conclusion that bond breaking is endothermic process since energy is absorbed from surroundings to break bonds while bond making is exothermic process since energy is released to surroundings when bonds are formed.