2.3 Chemical Reactions

Energy

(en = in; ergy = work) is the capacity to do work

Does a catalyst change the potential energies of the products and reactants?

A catalyst does not alter the difference in potential energy between the reactants and the products. Rather, it lowers the amount of energy needed to start the reaction. For chemical reactions to occur, some particles of matter—especially large molecules—not only must collide with sufficient force, but they must hit one another at precise spots. A catalyst helps to properly orient the colliding particles. Thus, they interact at the spots that make the reaction happen. Although the action of a catalyst helps to speed up a chemical reaction, the catalyst itself is unchanged at the end of the reaction. A single catalyst molecule can assist one chemical reaction after another.

Chemical Reactions

A chemical reaction occurs when new bonds form or old bonds break between atoms. Chemical reactions are the foundation of all life processes, and as we have seen, the interactions of valence electrons are the basis of all chemical reactions. Consider how hydrogen and oxygen molecules react to form water molecules (Figure 2.7). The starting substances—two H2 and one O2—are known as the reactants. The ending substances—two molecules of H2O—are the products. The arrow in the figure indicates the direction in which the reaction proceeds. In a chemical reaction, the total mass of the reactants equals the total mass of the products. Thus, the number of atoms of each element is the same before and after the reaction. However, because the atoms are rearranged, the reactants and products have different chemical properties. Through thousands of different chemical reactions, body structures are built and body functions are carried out. The term metabolism refers to all the chemical reactions occurring in the body.

Chemical energy

A form of potential energy that is stored in the bonds of compounds and molecules.

Catalysts

As we have seen, chemical reactions occur when chemical bonds break or form after atoms, ions, or molecules collide with one another. Body temperature and the concentrations of molecules in body fluids, however, are far too low for most chemical reactions to occur rapidly enough to maintain life. Raising the temperature and the number of reacting particles of matter in the body could increase the frequency of collisions and thus increase the rate of chemical reactions, but doing so could also damage or kill the body's cells. Substances called catalysts solve this problem. Catalysts are chemical compounds that speed up chemical reactions by lowering the activation energy needed for a reaction to occur (Figure 2.9). The most important catalysts in the body are enzymes, which we will discuss later in this chapter. Figure 2.9 Comparison of energy needed for a chemical reaction to proceed with a catalyst (blue curve) and without a catalyst (red curve) Catalysts speed up chemical reactions by lowering the activation energy.

Activation Energy

Because particles of matter such as atoms, ions, and molecules have kinetic energy, they are continuously moving and colliding with one another. A sufficiently forceful collision can disrupt the movement of valence electrons, causing an existing chemical bond to break or a new one to form. The collision energy needed to break the chemical bonds of the reactants is called the activation energy of the reaction (Figure 2.8). This initial energy "investment" is needed to start a reaction. The reactants must absorb enough energy for their chemical bonds to become unstable and their valence electrons to form new combinations. Then, as new bonds form, energy is released to the surroundings. Activation energy is the energy needed to break chemical bonds in the reactant molecules so a reaction can start.

Energy Transfer in Chemical Reactions

Chemical bonds represent stored chemical energy, and chemical reactions occur when new bonds are formed or old bonds are broken between atoms. The overall reaction may either release energy or absorb energy. Exergonic reactions (ex = out) release more energy than they absorb. By contrast, endergonic reactions (end = within) absorb more energy than they release. A key feature of the body's metabolism is the coupling of exergonic reactions and endergonic reactions. Energy released from an exergonic reaction often is used to drive an endergonic one. In general, exergonic reactions occur as nutrients, such as glucose, are broken down. Some of the energy released may be trapped in the covalent bonds of adenosine triphosphate (ATP), which we describe more fully later in this chapter. If a molecule of glucose is completely broken down, the chemical energy in its bonds can be used to produce as many as 32 molecules of ATP. The energy transferred to the ATP molecules is then used to drive endergonic reactions needed to build body structures, such as muscles and bones. The energy in ATP is also used to do the mechanical work involved in the contraction of muscle or the movement of substances into or out of cells.

Decomposition Reactions—Catabolism

Decomposition reactions split up large molecules into smaller atoms, ions, or molecules. A decomposition reaction is expressed as follows: For example, under proper conditions, a methane molecule can decompose into one carbon atom and two hydrogen molecules: The decomposition reactions that occur in your body are collectively referred to as catabolism (kaTABōlizm). Overall, catabolic reactions are usually exergonic because they release more energy than they absorb. For instance, the series of reactions that break down glucose to pyruvic acid, with the net production of two molecules of ATP, are important catabolic reactions in the body. These reactions will be discussed in Chapter 25.

Kinetic Energy

Energy associated with matter in motion. When the gates of the dam are opened or the person jumps, potential energy is converted into kinetic energy.

Potential Energy

Energy stored by matter due to its position. For example, the energy stored in water behind a dam or in a person poised to jump down some steps is potential energy.

Exchange Reactions

Many reactions in the body are exchange reactions; they consist of both synthesis and decomposition reactions. One type of exchange reaction works like this: AB + CD --> AD + BC The bonds between A and B and between C and D break (decomposition), and new bonds then form (synthesis) between A and D and between B and C. Notice that the ions in both compounds have "switched partners": The hydrogen ion (H+) from HCl has combined with the bicarbonate ion from NaHCO3, and the sodium ion (Na+) from NaHCO3 has combined with the chloride ion (Cl−) from HCl.

Reversible Reactions

Some chemical reactions proceed in only one direction, from reactants to products, as previously indicated by the single arrows. Other chemical reactions may be reversible. In a reversible reaction, the products can revert to the original reactants. A reversible reaction is indicated by two half-arrows pointing in opposite directions: Some reactions are reversible only under special conditions: water, heat In that case, whatever is written above or below the arrows indicates the condition needed for the reaction to occur. In these reactions, AB breaks down into A and B only when water is added, and A and B react to produce AB only when heat is applied. Many reversible reactions in the body require catalysts called enzymes. Often, different enzymes guide the reactions in opposite directions.

Law of Conservation of Energy

The total amount of energy present at the beginning and end of a chemical reaction is the same. Although energy can be neither created nor destroyed, it may be converted from one form to another. For example, some of the chemical energy in the foods we eat is eventually converted into various forms of kinetic energy, such as mechanical energy used to walk and talk. Conversion of energy from one form to another generally releases heat, some of which is used to maintain normal body temperature.

Synthesis Reactions—Anabolism

When two or more atoms, ions, or molecules combine to form new and larger molecules, the processes are called synthesis reactions. The word synthesis means "to put together." A synthesis reaction can be expressed as follows: One example of a synthesis reaction is the reaction between two hydrogen molecules and one oxygen molecule to form two molecules of water (see Figure 2.7). All of the synthesis reactions that occur in your body are collectively referred to as anabolism (aNABōlizm). Overall, anabolic reactions are usually endergonic because they absorb more energy than they release. Combining simple molecules like amino acids (discussed shortly) to form large molecules such as proteins is an example of anabolism.

Oxidation-Reduction Reactions

You will learn in Chapter 25 that chemical reactions called oxidation-reduction reactions are essential to life, since they are the reactions that break down food molecules to produce energy. These reactions are concerned with the transfer of electrons between atoms and molecules. Oxidation refers to the loss of electrons; in the process the oxidized substance releases energy. Reduction refers to the gain of electrons; in the process the reduced substance gains energy. Oxidation-reduction reactions are always parallel; when one substance is oxidized, another is reduced at the same time. When a food molecule, such as glucose, is oxidized, the energy produced is used by a cell to carry out its various functions.

What influences the chance that a collision will occur and cause a chemical reaction?

•Concentration: The more particles of matter present in a confined space, the greater the chance that they will collide (think of people crowding into a subway car at rush hour). The concentration of particles increases when more are added to a given space or when the pressure on the space increases, which forces the particles closer together so that they collide more often. •Temperature: As temperature rises, particles of matter move about more rapidly. Thus, the higher the temperature of matter, the more forcefully particles will collide, and the greater the chance that a collision will produce a reaction.