Ionization (1959)

[Music] when we investigate the behavior of solution we find that solutions of some substances the non-electrolytes will not conduct an electric current while solutions of others the electrolytes will when the bulb lights we know that the circuit is complete and that electricity is flowing through the solution michael faraday an english chemist and physicist of the 19th century performed many experiments concerning the electrical conductivity of solutions he theorized that a current flowed through a solution because molecules of the solute had been split by the electric potential into charged particles he called ions svonte arrhenius a swedish chemist in 1887 put forward his theory of electrolytic dissociation arrhenius believed an electric current was not necessary for the formation of ions are in uses theory though it has since been modified provides the foundation for our present understanding of ions and ionization pure water will not appreciably conduct electricity but if we dissolve hydrogen chloride gas in the water a current will flow watch the bulb light because the current is flowing through the solution we know that ions must be present molecules of the gas hydrogen chloride each contain one atom of hydrogen and one of chlorine a hydrogen atom consists of one proton as a nucleus and one orbiting electron actually if we think of the proton as the size of a pinhead the electron would be about 40 feet away a chlorine atom has 17 protons in its nucleus and 17 electrons in orbit there are seven electrons in the outermost ring which can hold eight when the hydrogen atom and the chlorine atom form the hydrogen chloride molecule they combine in such a way that the hydrogen and chlorine atoms share a pair of electrons such a compound is called covalent we might also represent the hydrogen chloride molecule this way three-dimensionally water is a covalent compound too here is a three-dimensional representation of the water molecule with its two hydrogen atoms and one oxygen atom the hydrogen end of the molecule tends to be positive and the oxygen end tends to be negative such a molecule with two poles is said to be a dipole or a polar molecule polarity is caused by non-uniform electron distribution the hydrogen chloride molecule is polar too the hydrogen end is positive and the chlorine end negative when hydrogen chloride is dissolved in water the attraction of the polar water molecules is strong enough to break the covalent bonds of the polar hydrogen chloride molecules in a dilute solution about 95 percent of the hydrogen chloride molecules are broken up in this way the proton nucleus of the hydrogen atom separates from the chlorine atom leaving behind both shared electrons changing the number of electrons in the outer shell of an atom also changes its properties therefore the properties of the hydrogen and chlorine atoms have changed and we can call them ions negative ions are larger than the parent atom positive ions are smaller the hydrogen atom single electron has now completed the outer electron shell of the chlorine atom the chlorine atom has 17 positively charged protons in its nucleus when it has 17 negatively charged electrons it is electrically balanced but now with the additional single electron that once was part of the hydrogen atom the chlorine is electrically unbalanced it has a charge of minus one and is called the chloride ion the hydrogen atom consists of one positively charged proton as a nucleus when it has its one electron the atom is electrically balanced but with the negative electron gone the hydrogen is left electrically unbalanced with a positive charge of one it is a proton now or a hydrogen ion whenever neutral atoms gain or lose electrons ionization has taken place and ions are formed the number of positive or negative charges on the ion is its valence hydrogen chloride in water solution is called hydrochloric acid all acids ionize and are electrolytes we can express the ionization of hydrogen chloride this way one molecule of hydrogen chloride yields a hydrogen ion with a single positive charge and a chloride ion with a single negative charge the hydrogen ion is a proton however hydrogen ions actually do not exist free in water solution but are hydrated attached to a water molecule covalent compounds such as hydrogen chloride are not ionized in every solvent benzene does not conduct electricity when hydrogen chloride is added to it electricity still does not flow benzene molecules are not polar and so they cannot exert the attractive force necessary to break the covalent bonds of the hydrogen chloride molecule no reaction occurs between the benzene and the hydrogen chloride no ions are formed no current flows we've seen as in the case of the hydrogen chloride molecule that ionization is the process whereby a neutral atom gains or loses electrons we can expand that definition when hydrogen sulfate is added to water the current flows so we know that ions are now present the solution is called sulfuric acid we can represent what is happening this way here is a hydrogen sulfate molecule the two hydrogen atoms on the left are each sharing a pair of electrons with an atom of oxygen the ionization of h2so4 in water takes place in two steps the one atom of sulfur and four atoms of oxygen have separated as a unit called a radical the radical takes with it the single electron from each of the hydrogen atoms the hydrogen atoms have become ions or protons each with a single positive charge the one atom of sulfur and four of oxygen called the sulfate radical now have become an ion with a negative charge of minus two this charge comes from the two electrons gained from the two hydrogen atoms so now we can say that ionization is the process whereby a neutral atom or group of atoms gains or loses electrons notice that when h2so4 ionizes three particles are formed two hydrogen ions and one sulfate ion we have seen that acids are electrolytes let's see if solutions of bases conduct electricity sodium hydroxide is a base its water solution does conduct electricity therefore we know that ions are present we can represent the ions of sodium hydroxide in water this way the sodium ion has a valence of plus one and the hydroxide ion which is a radical has a valence of minus one all bases ionize in water and are electrolytes what about salts when we test dry sodium chloride for conductivity it is clear that no current flows apparently there are no ions now while we keep a potential between the electrodes let's heat the sodium chloride to its melting point several minutes later when the temperature reaches about 800 degrees centigrade the salt will begin to liquefy at the bottom of the crucible watch the bulb now it is clear that current is flowing through the molten salt so ions must now be present actually ions were present in the sodium chloride crystals from the start solid sodium chloride is one hundred percent ionized sodium chloride is called an electrovalent compound when it is formed there is no sharing of electrons instead the single electron in the outer shell of the sodium atom is transferred to the outer shell of the chlorine atom so ionization has taken place the sodium ion has a charge of plus one and the chloride ion a charge of minus one this relationship is called electro valence raising sodium chloride to its melting point gives the ions mobility this same mobility of ions is achieved by dissolving sodium chloride crystals in water the separation of ions from the crystals of electrovalent compounds during the solution process is called electrolytic dissociation in general most salts are in the ionic form as the crystals of sodium chloride dissolve sodium and chloride ions are surrounded by the polar water molecules the negative oxygen end of the water dipole is turned toward the positive sodium ion and the positive hydrogen end of the water dipole is turned toward the negative chloride ion when there is an electric potential between electrodes in the solution the positive ions tend to move toward the negative pole picking up electrons there and the negative ions move toward the positive pole releasing electrons there this action is fundamental to the flow of electricity in a solution it also explains the process of electrolysis in which a compound either the solute or solvent is separated into simpler substances by an electric current pure water ionizes slightly even though it does not appreciably conduct an electric current only about one water molecule out of every 10 million will ionize the process probably occurs this way a hydrogen bond forms between two water molecules under just the right conditions this bond might be stronger than the normal covalent bond of one of the molecules and so ionization takes place the result is a positively charged hydrated proton which we call the hydronium ion and a hydroxide ion with a single negative charge these few ions have an important bearing in the electrolysis of water and in hydrolysis ions affect the freezing and boiling points of solutions here is a four molal aqueous solution of an electrolyte calcium chloride in the test tube is a four molal solution of a non-electrolyte sugar solutions of non-volatile substances have higher boiling points than pure water but watch as we heat the two solutions in a few minutes the sugar solution in the test tube has begun to boil before the salt solution in the beaker salt raises the boiling point of water more than sugar does careful experiments show that a one molal solution of sugar raises the boiling point of water 52 hundredths of a degree centigrade a one molal solution of calcium chloride raises the boiling point one and forty six hundredths degrees centigrade calcium chloride raises the boiling point about three times as much as sugar does the freezing point of water is similarly affected the freezing point of pure water is zero degrees centigrade a molal sugar solution lowers it one and eighty six hundredths degrees centigrade a molal solution of calcium chloride lowers at five and 2100 degrees centigrade again the effect of the calcium chloride is almost three times that of the sugar the abnormal effect on freezing and boiling points by electrolytes can be explained by ionization all molal solutions contain the same number of molecules when solutions of sugar and calcium chloride are prepared for each molecule of sugar there is only one particle in solution but for each so-called molecule of calcium chloride three particles are actually present since the boiling point and freezing point of a solution depend on the number of particles of solute present the effect of calcium chloride should be three times the effect of the sugar it is actually a little less than that because of inter-ionic attraction which causes some of the calcium and chloride ions to clump together while our understanding of ionization arose mainly from a study of electrolytes the theory of ionization has many applications in science certain vacuum tubes called thyrotrons contain a rarefied gas such as argon neon or mercury vapor when streams of electrons emitted from the glowing filament acquire the necessary speed and energy a point called the critical grid voltage is reached the gas in the tube suddenly ionizes and a large current flows this trigger characteristic of thyrotron tubes makes them useful in switching and relay applications pyrotrons are especially useful in lighting controls geiger counters and other types of radiation counting instruments also depend on ionization for their operation radiation ionizes molecules of a gas within the geiger muller tube each time molecules of the gas ionize energy is conducted in small bursts between two electrodes in the tube at high voltage these bursts of energy are mechanically counted giving us a measure of the radiation being emitted another application of ionization can be found in the process of electroplating in this process ions of a metal whose salt is in solution are ions of a metal whose salt is in solution are deposited as atoms on an object used as a cathode during electrolysis this electronics equipment is being cadmium plated the principles of ionization have been put to many practical uses but more important our understanding of ions and electrolytic dissociation is basic to our growing knowledge in the field of chemistry