2.4 the Breakdown Chemical Reactions Bonds Break and Form During Chemical Reactions

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including speed skating. energy for all activities, of glucose provides chemical FIGURE 4.1 endothermic exothermic activation energy equilibrium bond energy product reactant reaction chemical VOCABULARY Unit 1:Introducing Biology 2.4 The breakdown Chemical Reactions Bonds break and form during chemical reactions. chemical reactions inyour cells. but most ofthecarbon dioxide andwater vapor that you breathe outare made by of liquidsbubblinginbeakers. You probably donot thinkoftheairinyour breath, When you hear theterm chemical reaction, what comes to mind?Maybe you think Connect to Your World Key C

6O Products Your oxygen the cells need that molecules you breathe in.Oxygen (O Reactants, P reactions inliving things. ecules by bonding amino acids together. are These just afew of chemical the cells break down sugars to get usable energy. cells build And protein all mol- Plant cells make by cellulose simple linking sugars together. Plant and animal glucose andglucose results incarbon dioxide (CO changes form, matter both and are inachemical energy reaction. conserved substances by breaking and forming chemical bonds. Although matter the process that changes oxygen and into glucose carbon dioxide and water is are to show used what happens during areaction. The overall equation for the Oxygen and are glucose reactants. the break down simple the sugar (C glucose reactions,These which chapter areinthe indetail described and Cells Energy, of inaseries chemicalpart reactions that provides usable for energy your cells. when new bonds new when form incarbon dioxide and water? causes bonds inoxygen and to molecules glucose break? And what happens must broken be reactants, in the and bonds must form products. inthe What that you breathe out. But for carbon dioxide and water made, to be bonds cellular respiration,which is called makes carbon the dioxide and water vapor rightthe side. The arrow shows direction the of reaction. the This process, during achemical reaction. Carbon dioxide and water are products. the MAIN MAIN

oncept The reactants are on side left of the equation, the and products the are on Chemical reactions release orabsorb energy. Bonds break andform duringchemical reactions. ID 2 ID Reactants Direction +C are substances the made by achemical reaction. equations Chemical E E A AS

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), water (H + 6H O change substances into different 6 ). The process oxygen uses and are substances the changed 2 O 2 O), and usable energy. 2 ) plays a

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QUICKLAB Modeling Chemical Bonding You use energy to put things together, but chemical bonding is dif- Materials ferent. Energy is added to break bonds, and energy is released when 2 flat magnets bonds form.

Problem How is chemical bonding similar to the interaction between two magnets? Procedure 1. Bring the magnets close to each other until they snap together. 2. Pull the magnets away from each other. Analyze and Conclude 1. Infer How is bond formation represented by the snapping sound? 2. Apply How is bond energy related to the separation of the magnets?

First, energy is added to break bonds in molecules of oxygen and glucose. Bond energy is the amount of energy that will break a bond between two atoms. Bonds between different types of atoms have different bond energies. Energy is released when bonds form, such as when molecules of water and carbon dioxide are made. When a bond forms, the amount of energy released is equal to the amount of energy that breaks the same bond. For example, energy is released when hydrogen and oxygen atoms bond to form a water molecule. The same amount of energy is needed to break apart a water molecule. Chemical Equilibrium Some reactions go from reactants to products until the reactants are used up. However, many reactions in living things are reversible. They move in both directions at the same time. These reactions tend to go in one direction or the other depending on the concentrations of the reactants and products. One such reaction lets blood, shown in Figure 4.2, carry carbon dioxide. Carbon dioxide reacts with water in blood to form a compound called carbonic acid

(H2CO3). Your body needs this reaction to get rid of carbon dioxide waste from your cells.

CO2 + H2O H2CO3 The arrows in the equation above show that the reaction goes in both directions. When the carbon dioxide concentration is high, as it is around your cells, the reaction moves toward the right and carbonic acid forms. In your lungs, the carbon dioxide concentration is low. The reaction goes in the other direction, and carbonic acid breaks down. When a reaction takes place at an equal rate in both directions, the reac- FIGURE 4.2 Blood cells and tant and product concentrations stay the same. This state is called equilib- plasma transport materials rium. Equilibrium (ee-kwuh-lihb-ree-uhm) is reached when both the throughout the body. Carbonic acid dissolves in the blood so that reactants and products are made at the same rate. carbon dioxide can be trans- Apply Explain why concentration is important in a chemical reaction. ported to the lungs. (composite colored SEM; magnification 10003) (tr) © HMH; (br) ©Rapho Agence de Presse/Phototake (tr) © HMH; (br) ©Rapho

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FIGURE 4.3 energy and Chemical Reactions Biology energy is required to break bonds in reactants, and energy is released HMDScience.com when bonds form in products. Overall, a chemical reaction either GO ONLINE absorbs or releases energy. Energy and Chemical Reactions

ACTIVATION ENERGY activation energy When enough activation energy is added to the reactants, bonds in the reactants break and the reaction begins. energy in Energy reactants activation energy activationReaction energy progress

energy in Energy reactants

Reaction progress

EXOTHERMIC REACTION Energy Released ENDOTHERMIC REACTION Energy Absorbed

total energy total energy added added total energy total energy released released

reactants products reactants dierence in energy products Energy Energy dierence in energy (released) (absorbed)

Reaction progress Reaction progress

The products in an exothermic reaction have a higher bond The products in an endothermic reaction have a lower bond energy than the reactants, and the difference in energy energy than the reactants, and the difference in energy is is released to the surroundings. absorbed from the surroundings.

CRITICAL Is the amount of activation energy related to whether a reaction VIEWING is exothermic or endothermic? Why or why not?

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