كتاب الطالب Chemistry الصف 12

كتاب الطالب Chemistry الصف 12

NATURE OF SCIENCE

Better Eating Through Chemistry

Everyone loves the sight and smell of freshly baked bread. Did you know that a chemical reaction called the Maillard reaction is the cause of bread’s pleasant effects on our senses ? This reaction is what gives foods their distinctive tastes and colors. The Maillard reaction is actually many reactions that occur in different patterns, producing many different chemical compounds. The reactions include the oxidation-reduction. or redox, reaction that occurs when people cook food

The “browning” reaction

The Maillard reaction. which occurssimple sugars and proteins. was first identified in 1912 by French physician and biochemist Louis-Camille Maillard. It is also called the “browning” reaction because of the color produced in foods—such as meats. coffee chocolate. and potato chips—that have undergone the
High temperatures and low-moisture conditions produce the Maillard reaction most rapidly. including the cooking techniques of browning. frying. baking. grilling, and roasting

However, the Maillard reaction also occurs slowly to some foods, in the absence Of a heat source. Slow Maillard reactions that

Electrochemical cells

The device shown in Figure 2 is a type of electr€xhemical cell called a cell. An ekctrochemkal cell is an apparatus that uses a redox reaction to prcxiuce electrical enerw or electrical enenw to cause a chemical reaction. A vdtaic cell is a type of electrcxhemical cell that converts chemical enerw to electrical by a sB»ntaneous redox reaction. The voltaic cell is named for Alessandro Volta (1745—1827), the Italian physicist who is credited with its invention in INN). A replica of one of the first cells he made is shown in Figure 3

Chemistry of Voltaic Cells

An electrcxhemical cell consists of two parts called half-cells, in which the separate oxidation and reduction reactions take place. Each half-cell contains an electrode and a q»lution containing ions. An electrode is an electrically conductive material, usuallv a metallic strip or graphite, that conducts electrons into and out of the solution in the half cell. In Figure 2, the beaker with the zinc electrcxie is where the oxidation half of the redox reaction takes place. The beaker with the electrcxle is where the reduction half of the reaction takes place. The reaction that takes place in each half cell is called a half-cell reaction. The electrode where oxidation takes place is called the ancxie. •nte where reduction takes place is called the cathcxle

Voltaic cells and energy

Recall that an object’s potential enerw is due to its m»sition or comB»sition. In electro- chemistrv, electric votential energv is a measure of the amount of current that can generated from a voltaic cell to do work. Electric charge can flow between tv,o only when a difference in electric B)tential energv exists the two points. In an electr€xhemical cell, these two are the electrodes. Electrons at the ancxle, the site of oxidation, are thought to be pushed or driven toward the cathexie by the electromotive force (EMF). This force is due to the difference in electric potential energv the two It is referred to as the cell A volt is a unit to measure cell potential. electric vx»tential difference of a voltaic cell is an indication of the ener&’ available to move electrons from the to the

Figure a The coaster at the top Of the track has tugh energy rel•tive to the track below because ot the difference in height. Similarly, an celi has potential energy to prcxiuce a current because there is a difference in the ability of the electrodes to move electrons trom tne to eth cathode

Consider the ana10R’ illustrated in Figure 4. roller coaster is almost stationary at the top Of the trxk. •men, it plummets from its high Bhition because of the difference in gravita- tional Bitential ener»• (PE) the top and of the track. Ihe kinetic energ (KE) attained by the roller coaster is determined by the difference in height (potential enew) the top and lx»ttom parts of the triKk. Similarly, the enerw of the el«trons flowtng from the to the cathcxie in a voltak- cell is determined by the difference in eJ«tric
B)tential erwrw the elex-tr€xles. In redox terms, the voltage of a cell is deter-
mine«i by comparing the difference in the tendencv of the two electrcxle materials to accept el«trons. lhe greater difference, the greater the B’tential energy’ difference the electrcxies and the larger the volta» of the cell will

The force of gravity always causes a diver to fall downward to a lower enerw state, never upward to a higher energy state. When a diver steps off a diving lx»ard, his or her svxmtaneous motion is always downward. Similarly, in the zinc-copper cell, under standard conditions, copvvr(ll) ions at the cath«xie accept electrons more readily than the zinc ions at the anode. Thus, the redox reaction ‘X-curs sB»ntaneously only when the electrons flow from the zinc to the 

Calculating Electrochemical Cell Potentials

Recall that gaining electrons is called r«duction. Building on this fact, the tendency of a substance to gain electrons is its ptential. •me reduction B)tential of an electr€xle cannot be determined directly because the reduction half-reaction must coupled with an oxidation half-reaction. When two half-reactions are the voltage corresponds to the difference in B’tential between the half-reactions The electrical B)tential difference between two points is expressed in volts (V)

Preventing corrosion

Corrosion of cars, ships, the structures of buildings, and other metallic Objects causes more than SI(K) billion in damage a year in the United States. For this reason, several means to minimize corrosion have Eu•en devised. One example is to apply a coat of paint to seal out both air and moisture, but because paint deteriorates over time, objects such as the bridge shown in Figure 15 must be repainted often

The steel hulls of ships are constantly in contact with salt water. so the prevention of corrosion is vital. Although the hull can be painted. another method is used to mini- mize corrosion. Blocks of metals, such as magnesium. aluminum, or titanium, are placed in contact with the steel hull. These blcxks oxidize more easily than iron and become the anode of the corrosion cell. Thev are called sacrificial anenies because they are corr•tx.ied, while the iron in the hull is spared. A similar technique is used to protect underground iron Magnestum bars are attached to the by wires, and these bars corrcxle instead of the pivu•, as shown in Figure 16

Another approach to preventing corrosion is to coat iron with another metal that is more resistant to corrosion. In the galvanization prcxess, iron is coated with a layer of zinc by either dipping the object into molten zinc or by el«troplating the zinc onto it. Although. zinc is more remiily oxidized than iron. it is one of the self-protecting metals, a group that also includes aluminum and chromium. When to air, these metals oxidize at the surface, creating a thin metal-oxide coating that seals the metal from further oxidation