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Thermal Energy in Chemical Reactions

Endothermic vs. Exothermic

All chemical reactions involve some change in energy. This occurs when the atoms in reactants recombine to form new products. Depending on the reaction, the energy change may be very small or very large. Each reaction is different, and each reaction can be defined by a specific energy change associated with it. Thermochemistry is the study of energy changes that occur in chemical systems.

To analyze this more closely, consider a set of reactants before a reaction occurs. This set of reactants can be thought of as a system. Everything else is considered the surroundings.

If the reaction is initiated between the reactants, a change in the energy of the system occurs. For any given reaction, the energy of the system (Esystem) either increases or decreases.

What do burning a candle and making ice have in common? You can tell just by looking at the burning candle that it is releasing heat and light energy into the surrounding air. However, what is happening to the ice cube may not be so obvious until you think about what makes water freeze. To make something cold, you have to remove heat. As the water in the freezer releases heat that is removed by the refrigerator, the water temperature drops to freezing. Both the candle and the ice cube release thermal energy into their surroundings, so both processes are considered exothermic. When energy is absorbed, such as when baking cookies, the process is considered endothermic.

exothermic – a process that releases heat

endothermic – a process that absorbs heat

If the energy in the system decreases, energy is released (exothermic) into the surroundings (see the blue arrow in the diagram above). If the energy in the system increases, energy is absorbed (endothermic) from the surroundings (see the red arrow in the diagram above).

thermal energy – the total kinetic (motion) energy of tiny particles that make up matter; the faster the particles move, the warmer the matter becomes

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Thermal Energy in Chemical Reactions

All chemical reactions begin with reactants that have a certain amount of energy associated with them. When these reactants interact to create products, a change in energy occurs. The change can be either positive or negative, depending on whether the products have more or less energy than the reactants. Look at the graphs below to visualize these two possibilities.

In the graph on the left, the reactants have less energy than the products. In this case, energy must be absorbed from the surroundings to produce the products. In the graph on the right, the reactants have more energy than the products. When the reaction occurs, energy is released to the surroundings.

Based on these two types of changes, chemists classify reactions as either exothermic or endothermic.

In an exothermic reaction, energy flows from the system to the surroundings, generally in the form of heat.

In an endothermic reaction, energy flows into the system from the surroundings, generally in the form of heat.

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Thermal Energy in Chemical Reactions

During photosynthesis, energy from the sun interacts with matter on Earth.Most plants require light to live. A few can live without light, but most plants will die if they do not receive adequate light. Light is a form of energy that plants take in and use to carry out photosynthesis. Photosynthesis is a series of chemical reactions that produces glucose, a compound the plant uses as food. In this way, plants use light from the sun to make their own food. Radiant energy from the sun is changed to chemical energy in glucose molecules. This is considered an endothermic reaction.

Plants differ from animals by using radiant energy in this way. Animals cannot use radiant energy from the sun to make glucose; they must obtain energy from the environment in the form of food, which contains chemical energy. Animals either eat plants to obtain chemical energy in the form of glucose or eat other animals that eat plants. This type of reaction is considered an exothermic reaction. Discover Science: Using Exothermic Reactions as a Heat

Source

Soldiers serving in remote locations for long periods of time need sufficient food. To provide this, the military has developed packaged foods abbreviated with the initials MRE. These letters stand for “meals ready to eat.” Some of this food was developed to be eaten just as it comes out of the package. However, some of it was developed to be heated before eating. Soldiers in the field may not have access to standard heating equipment. To heat meals under these conditions, the military developed a flameless ration heater. The heat produced by a flameless ration heater comes from a series of exothermic reactions.

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Thermal Energy in Chemical Reactions

Look at the two photographs below. Which photograph shows an exothermic reaction? Which shows an endothermic reaction? (Hint: Think about whether heat is released or absorbed during each reaction shown.)

Muffins baking: _____________________ Fireworks exploding: _______________

Classify the following descriptions as endothermic (absorbs energy) or exothermic (releases energy). Write “ENDO” for endothermic and “EXO” for exothermic on the blank next to each description.

________ Chemical bonds form when heat is released.

________ Water vapor in the air releases heat and condenses into rain droplets.

________ Combustion reaction (burning) of a match gives off heat and light.

________ Fried eggs absorb heat from a pan.

________ Photosynthesis by a tree absorbs energy to break apart carbon dioxide and water to produce glucose.

________ Exit

________ Enter