Science 10: ChemistryEnergy Change

Endothermic and Exothermic ReactionsChemical reactions transfer energy to, or from, the surroundings. They often cause a temperature change. For example, when a bonfire burns, it transfers heat energy to the surroundings. Objects near a bonfire become warmer. The temperature rise can be measured with a thermometer.During a chemical reactions:

Bonds in the reactants are broken New bonds are made in the products

Exothermic reactionsThese are reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions would beburning (combustion), neutralization reactions between acids and alkalis and explosions.

Endothermic reactionsThese are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer. Some examples of endothermic reactions are electrolysis, photosynthesis.

Progression of reaction Progression of reactionExothermic Reactions: Heat is released.

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System Surroundings

Change

Exothermic Endothermic

Heat releasing Heat absorbing

Macroscopic

Temperature

Science 10: ChemistryExamples:1) Combustion: The burning of carbon-containing compounds uses oxygen, from air, and produces carbon dioxide, water, and lots of heat. For example, combustion of methane (CH4), can be represented as follows:

CH4 + 2O2 CO2 + 2H2O + heat

2) Rain: Condensation of water vapor into rain releasing energy in the form of heat is an example of an exothermic process.

Endothermic Reactions: Heat is absorbed.Examples:1) Photosynthesis: Plants absorb heat energy from sunlight to convert carbon dioxide and water into glucose and oxygen:

6CO2 + 6H2O + heat C6H12O6 + 6O2

2) Cooking an egg: Heat energy is absorbed from the pan to cook the egg.

Enthalpy:Enthalpy of a reaction is defined as the heat energy change (ΔH) that takes place when reactants transform into products. If heat is absorbed during the reaction, ΔH is positive= Endothermic Reaction. If heat is released, then ΔH is negative= Exothermic Reaction.

Activation Energy:The reaction path (path from reactants to forming products) is NOT going to be a straight line. In chemical reactions, we have to put a certain amount of energy in order for the reaction to occur (reactions will not spontaneously occur!) Ex. AB + CD AC + BD

We have to break bonds in order to make bonds (A is bonded to B and C is bonded to D) There is a transition state, where there is a weakening of the bonds (The bonds between

AB and the bonds between CD) Once those bonds are broken, new bonds can now form, to produce the new products (Ie.

A now bonded to C and B now bonded to D.)

EA= Activation Energy

Energy DiagramsEnergy Diagrams chart potential energy of chemicals as they become products.

Reactants start with a certain amount of energy that is added to start the reaction which is then either released or absorbed as the reaction proceeds.

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Science 10: Chemistry The amounts of energy determine whether the reaction will be exothermic or endothermic

The reactants are at a higher energy level as compared to the products, as shown in the energy diagram. The products are more stable than the reactants. Overall ΔH for the reaction is negative i.e., energy is released in the form of heat.

The reactants are at a lower energy level compared to the products—as shown in the energy diagram

Products are less stable than the reactants. Since we are forcing the reaction in the forward direction towards more unstable entities, overall ΔH for the reaction is positive, i.e., energy is absorbed from the surroundings.

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