Chemical Reactions

Chemical reactions involve (1) the breaking of chemical bonds in reactant molecules, followed by (2) the making of new chemical bonds to form the product molecules. In the chemical reaction in which carbonic acid is transformed into carbon dioxide and water, for example, two of the chemical bonds in carbonic acid are broken, and the product molecules are formed by establishing two new bonds between different pairs of atoms:

HtO-CtO-H t t Broken Broken


Carbonic acid

CO2 Carbon dioxide

Since the energy contents of the reactants and products are usually different, and because energy can neither be created nor destroyed, energy must either be added or released during most chemical reactions. For example, the breakdown of carbonic acid into carbon dioxide and water occurs with the release of 4 kcal of energy per mole of products formed since carbonic acid has a higher energy content (155 kcal/mol) than the sum of the energy contents of carbon dioxide and water (94 + 57 = 151 kcal/mol).

The energy that is released appears as heat, the energy of increased molecular motion, which is measured in units of calories. One calorie (1 cal) is the amount of heat required to raise the temperature of 1 g of water 1° on the Celsius scale. Energies associated

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Protein Activity and Cellular Metabolism CHAPTER FOUR

TABLE 4-2 Determinants of Chemical Reaction Rates

1. Reactant concentrations (higher concentrations: faster reaction rate)

2. Activation energy (higher activation energy: slower reaction rate)

3. Temperature (higher temperature: faster reaction rate)

4. Catalyst (increases reaction rate)

with most chemical reactions are several thousand calories per mole and are reported as kilocalories (1 kcal = 1000 cal).

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