6.02 Endothermic and Exothermic

Review Examples:

-Water must absorb energy before it will boil. Therefore, boiling water absorbs energy. -For water to freeze, it must lose energy. Therefore, freezing water releases energy. -When wood burn, it releases heat and light energy. Therefore, burning wood releases energy. -Plant cells, like all cells, must use energy in order to grow. Therefore they absorb energy. -The chemical reaction inside the ice pack requires energy to proceed. By absorbing energy from its surroundings, the surroundings (the water inside the pack) release heat and feel colder. -As the cookies sit on the rack, they lose heat to the environment thus releasing energy.

Endothermic

In an endothermic reaction, the products end up with more stored potential energy than the reactants. This is why energy must be absorbed by the reaction to provide enough energy to form the products.

Exothermic

Scientists often use a potential energy diagram to represent the general change in energy over the course of a reaction. By understanding what the different components of the diagram represent, you will be able to draw and interpret potential energy diagrams for a variety of chemical reactions. A potential energy diagram tracks the potential energy of a system (vertical) over the course of a reaction or process (horizontal) Notice that the y-axis shows a measure of potential energy, usually in the unit kilojoules, and the x-axis shows the progress of a reaction. In an exothermic reaction, the products have less potential energy than the reactants had at the beginning of the reaction. This is why the potential energy diagram for an exothermic reaction starts at a higher energy value and ends at a lower energy value. The law of conservation of energy is obeyed because the extra energy that does not end up stored in the final products is released to the surroundings in the form of heat, light, or other kinetic energy.

Compare

The first step of most chemical reactions involves breaking chemical bonds in the reactants, whether the reaction is endothermic or exothermic. Because it always requires energy to break bonds, even exothermic reactions need to take in energy to get the reaction started. Activation energy is the minimum amount of energy required to initiate a chemical reaction. In a potential energy diagram, the activation energy is represented by the positive slope, or hill, that follows the reactants. For a reaction to occur, enough energy must be provided to the system to meet the activation energy requirement. If a system does not absorb enough energy to meet the activation-energy requirement, the reaction will not proceed. Some reactions require more activation energy than others. You probably know that when gasoline comes in contact with oxygen, you do not want to expose it to a spark or open flame because even the smallest amount of energy can get that combustion reaction going. Other reactions require a lot more activation energy. The reaction that occurs when you bake a cake requires much more than a small spark; the cake must be heated continually for a long time for the chemical change to be completed. Potential energy diagrams show the difference in potential energy of reactants and products, which shows the amount of energy absorbed or given off by a reaction. The same diagram also shows the amount of activation energy required to get a reaction started.

Exothermic Reactions

The prefix exo- means "out of," which relates to words like exit and exoskeleton. In an exothermic reaction, energy that was once in the form of stored potential energy is released as kinetic energy in fast-moving particles, heat, and/or electromagnetic radiation. The amount of energy released by an exothermic reaction depends on the amount of reactants as well as the difference in the potential energy of the reactants and products. In an exothermic reaction, the products have a lower total potential energy than the reactants. ~Examples~ -Shuttle Launch: The reaction of large amounts of hydrogen and oxygen, along with some other reactions, releases enough energy to lift the space shuttle into orbit. -Hot Pack: Hot packs often contain a capsule of calcium chloride (CaCl2) surrounded by water. When you break the capsule, the calcium chloride dissolves in the water. This dissolving process is exothermic, which results in the temperature of the water (surroundings) increasing as heat is released. -Ice Tray: When you place water in your freezer to make ice cubes, the water undergoes the exothermic process of freezing. Heat is removed from the water and transferred outside your freezer, allowing the liquid water to freeze into ice.

Endothermic Reactions

We can use the temperature of an object to determine whether heat has moved into or out of the object. When heat flows out of an object or substance, the temperature of that object decreases. When heat flows into a sample of matter, the temperature of the sample of matter will increase. Evaporating and boiling are both endothermic processes because both require energy to be absorbed from the surroundings for the process to occur. When rubbing alcohol or sweat evaporates from the surface of your skin, your skin feels cooler. This is because thermal energy from your skin flows into the liquid, raising the temperature of the liquid until it evaporates. Even though the liquid is increasing in temperature, the loss of thermal energy from your skin makes your skin feel cooler. Photosynthesis is another example of an endothermic process because it involves a series of endothermic reactions that require energy from the sun.

endothermic

reactions that gain energy absorbed from the surroundings, resulting in a net increase in energy

exothermic

reactions that release energy to the surroundings, resulting in a net decrease