Energy from Chemicals | Enthalpy Change | Difference Between Endothermic / Exothermic Reactions | Energy Profile Diagrams | Combustion of Fuel | Hydrogen Fuel Cell | Petroleum | Photosynthesis
Topics:
- Types of reactions
- Enthalpy Change
- Difference between Endothermic and Exothermic
- Energy Profile diagrams
- Combustion of fuel
- Hydrogen Fuel Cell
- Petroleum
- Photosynthesis
Types of Reactions:
There are two types of chemical reactions:
- Exothermic
- Endothermic
Exothermic Reactions:
- Exothermic chemical reactions are reactions in which heat is produced or given out.
- When a product is formed where bonds are made, energy is released and hence it is called bond making/ formation.
Endothermic Reactions:
- Endothermic chemical reactions are reactions in which heat is absorbed.
- When a reactant is involved in the breaking of chemical bonds, energy is absorbed in order to break the bonds. Hence, this is bond breaking.
Enthalpy change:
Enthalpy change shows the difference in energy of the reactants of a reaction in the beginning and end. If energy is taken in during a reaction, then the enthalpy change will be positive and if energy is given out in a reaction, then the enthalpy change will be negative.
Difference between Exothermic and Endothermic Reactions:
| Exothermic Reactions | Endothermic Reactions |
| Temperature of surrounding rises | Temperature of surrounding decreases |
| Container with reactant gets hot | Container with reactants get cold |
| Products have low energy | Products have high energy |
| Reactants initially have high energy | Reactants initially have low energy |
| Enthalpy change is negative | Enthalpy change is positive |
| Heat given out | Heat absorbed |
| Bond made | Bonds broken |
| ∆H = -ve | ∆H= +ve |
| Less activation energy | High activation energy |
Energy Profile diagrams:
- Energy profile diagrams are used to represent the energy pathway from reactants to products in a reaction.
- The energy profile diagram for exothermic reaction show that the reactants have higher initial energy and products have lower energy since the energy has been given out during the reaction.
- The energy profile diagram for endothermic reactions show that the reactants have lower energy and since the products form by gaining energy, they have higher energy at the end of the reaction.
- The bump at the top is the activation energy that is required for the reaction to start. Without this energy, there will be no reaction.
- ∆H represents enthalpy.
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Combustion of Fuels:
When fuels e.g. wood, coal, oil, natural gas and hydrogen are burned in oxygen, the reaction is exothermic since it gives out heat. This heat is what we utilize in various applications such as cooking, or heating water in power stations to turn it to steam.
Hydrogen Fuel Cell:
- Hydrogen is a fuel that can be obtained from water or hydrocarbons (discussed in organic chemistry).
- Since burning fuels is dangerous for the surrounding as it causes pollution, hydrogen fuel cells are being used. In hydrogen fuel cells, hydrogen and oxygen are utilized in the process that creates electricity. As a result, only non-pollutants are released. (details of the construction and operation of a fuel cell are not required).
Advantages/ Disadvantages of hydrogen fuel cells:
| Advantages | Disadvantages |
| Non-pollutant/ environment-friendly | expensive |
| More fuel efficient (more electricity per unit) | Difficult to store |
| Readily available | Highly flammable |
| renewable |
Petroleum:
- Petroleum is a mixture of various hydrocarbons. (more details about hydrocarbons in organic chemistry).
- Petroleum has many different useful hydrocarbon components that are separated using fractional distillation, based on the difference in their boiling points.
Petroleum Components:
The major petroleum components together with their uses are listed below:
| Component | Use |
| petrol (gasoline) | fuel in cars |
| naphtha | feedstock for the chemical industry |
| paraffin (kerosene) | a fuel for heating and cooking and for aircraft engines |
| diesel | fuel for diesel engines |
| lubricating oils | as lubricants and as a source of polishes and waxes |
| bitumen | for making road surfaces |
Photosynthesis:
Photosynthesis is a reaction between carbon dioxide and water in the presence of chlorophyll. The reaction uses sunlight (energy) to produce glucose. Photosynthesis is used by plants to create their own food in the daytime. Photosynthesis can provide a renewable energy resource since carbon dioxide and sunlight readily available and renewable.
Lesson Tags
Energy from Chemicals | Enthalpy Change | Detailed Notes For Preparation & Revision | O Level Chemistry 5070 and IGCSE Chemistry 0620
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