Endothermic and Exothermic Reactions

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Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0). In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).

Examples of Endothermic and Exothermic Processes

Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:

sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)

An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced:

Na(s) + 0.5Cl2(s) = NaCl(s)

How do Enzymes Catalyze Reactions

Written by 631 days ago
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Enzymes

  • Enzymes are proteins that catalyze, or affect the rate, of chemical reactions without themselves being altered in the process. Specific enzymes catalyze each cellular reaction.
    The main role of enzymes during the respiration reaction is to assist in transferring electrons from one molecule to another. These transfers are called “redox” reactions, where the loss of electrons from one molecule (oxidation) must coincide with the addition of electrons to another substance (reduction).

    • Glycolysis

  • This first step of the respiration reaction takes place in the cytoplasm, or fluid, of the cell. Glycolysis consists of nine separate chemical reactions, each catalyzed by a specific enzyme.
    The key players in glycolysis are the enzyme dehydrodgenase and a coenzyme (non-protein helper) called NAD+. Dehydrodgenase oxidizes glucose by stripping two electrons from it and transferring them to NAD+. In the process glucose is “split” into two molecules of pyruvate, which continue the reaction.

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