Nitric Acid(1962)

a Nitric acid molecule as shown by this model consists of an atom of nitrogen surrounded by three oxygen atoms with a hydrogen atom attached to one of the oxygens nitric acid is a versatile substance it can acts as an acid as in the manufacturer of chemical fertilizers it can act as a base as in the manufacturer of nitro compounds which are used in dyes fuels and drugs it can act as an oxidizing agent as in many explosives most chemical fertilizers contain nitrogen as an essential component one might therefore think that concentrated nitric acid which contains 15% nitrogen would make an effective chemical fertilizer but it is corrosive and damages plants perhaps the acidity of nitric acid causes the damage let's investigate this acidity using an acid base indicator here we have a set of tubes containing Solutions of varying acidity each containing methyl violet as an indicator note that the color varies from green to Violet as the hydrogen ion concentration changes from 10 m to 1 1000 mol when methyl violet is placed in 16 M nitric acid the color indicates a very high hydrogen ion concentration at least 10 m thus nitric acid is a very strong acid it readily donates a proton to a base this model shows a nitric acid molecule donating a proton to a water molecule a nitrate ion and a hydronium ion form in concentrated nitric acid the protons move back and forth from one molecule to another and even to and from the nitrate ion since neither water nor nitrate ion holds protons firmly the solution is strongly acidic if we are to reduce the acidity we need to have present a stronger proton acceptor that is a stronger base than water or nitrate ion this chart shows the relative strengths of certain acids and bases we see that nitrate ion and water are among the weak bases that is poor proton acceptors looking down the list for a stronger proton acceptor preferably a base containing a high percentage of nitrogen we find NH3 ammonia if we add ammonia gas to concentrated nitric acid the acidity should decrease the safety trap would catch any backup of nitric acid the ice bath absorbs most of the heat of the reaction a glass tube with a porous tip is used to introduce Tiny Bubbles of ammonia gas into the nitric acid the white smoke solid ammonium nitrate forms when gaseous ammonia and gaseous nitric acid react above the solution the net reaction is ammonia plus nitric acid gives ammonium nitrate but most of the reaction occurs in the solution forming aquous ammonium ions and nitrate ions as more and more ammonia is introduced the acidity continues to drop the methyl violet changes through the intermediate colors finally becoming violet let's compare our product with the known indicator colors we see that the final solution ammonium nitrate has a hydrogen ion concentration of 1 1,000 mol or less few loosely held protons are present the acidity is low let's review the reaction in animation the nitric acid is a proton donor this time the proton acceptor is an ammonia molecule proton transfer yields a nitrate ion and an ammonium ion aquous ammonium nitrate the ammonium ion does not readily donate a proton to water the proton is tightly held and the acidity is low repeating the neutralization in the absence of methyl V gives a colorless solution we use a hot plate to evaporate the water pure solid ammonium nitrate remains in the beaker ammonium nitrate is a very useful fertilizer its 35% nitrogen content is more than twice that of nitric acid and when properly applied it does not damage plants so in making fertilizers nitric acid acts as a strong acid a good proton donor but nitric acid can also act as a base accept a proton split out water and form nitronium ion NO2 plus used in making Nitro compounds nitronium ion can react with many substances to form compounds such as nitrobenzene nitromethane and TR Nitro taline in making Nitro compounds such as nitrobenzene we start with a carefully measured amount of concentrated nitric acid we then add a measured amount of concentrated sulfuric acid The Marked rise in temperature indicates a reaction is occurring reactions producing Nitro compounds are extremely dangerous they should be carried out only under carefully controlled conditions and at low temperatures next we add a measured amount of benzene to the mixture Benzene is only slightly soluble in the acids we must shake to mix the two yellow Nitro Benzene the desired product collects in the upper layer let's review what has taken place in the preparation of nitrobenzene we use animated models to follow the molecular behavior in the first step nitric acid acts as a base accepting a proton from sulfuric acid the resulting complex splits out water in the solution we now have two ions a hydrogen sulfate ion hso4 minus a nitronium ion no 2 plus the third reactant Benzene consists of a ring of six carbons each bonded to a hydrogen atom when a nitronium ion and a Benzene Collide a proton transfers to a hydrogen sulfate ion this regenerates sulfuric acid the sulfuric acid is a catalyst in this reaction the product nitrobenzene is used to make dyes and other organic compounds we have seen nitric acid can act as an acid and we have seen it can act as a base now let's see how nitric acid acts as an oxidizing agent photographic etching is done by slowly oxidizing metal plates with concentrated nitric acid yet the same substance concentrated nitric acid is often used to to oxidize the fuel in Rockets rapidly and vigorously let's investigate the oxidizing power of nitric acid which acts in such different ways nitric acid in water solution exists largely as hydronium and nitrate ions in both nitric acid and the nitrate ion the nitrogen atom has an oxidation state of plus five it can be reduced to a number of products such as NO2 hno2 n o n2o N2 NH H4 plus and many others the nitrogen in these products ranges in oxidation state from plus4 to minus 3 in most nitric acid reactions several of these products form but usually one reaction predominates how do we predict which reaction will predominate and which produ products will result perhaps EZ values give us a clue the formation of nitrogen gas has the greatest energy potential the largest EZ therefore we might expect it to predominate even in concentrated nitric acid Solutions we can compare EZ values since the relative potentials do not vary much with concentration now let's try a couple of reactions with concentrated n acid to see what products we do get if we add the reducing agent copper to nitric acid we might expect to get colorless nitrogen gas actually we get brown NO2 gas yet this reaction has the smallest EZ value suppose we use a stronger reducing agent zinc again Brown NO2 gas is obtained though this reaction is more vigorous instead of producing nitrogen these reactions have produced the product with the smallest energy potential how can we interpret this the relation between energy potential and the reaction which dominates the system can be examined with this analogy of marbles resting on a plateau below are various levels to which the marbles could fall in this case when they fall the marbles come to rest at the lowest level in this analogy the marbles on the plateau represent nitrate ions the falling marbles represent the reduction of the ions to products of different stability the distance of fall is proportional to the EZ value the greater the fall or ezo value the more stable the product nitrogen gas is the most stable product its formation involves the greatest ezo value just as the marbles fell to the most stable resting place one might predict that nitrate ions would be reduced to nitrogen gas the most stable product but our experiments produced nitrogen dioxide gas rather than nitrogen so it is evident that while EZ values can tell us which reactions are possible they do not enable us to predict which products will result what other fact must be considered nitrate ions cannot be reduced to nitrogen gas without crossing the potential energy barrier to the reaction experiments show this barrier is much higher than the barrier to the reaction producing nitrogen dioxide thus at low temperatures many reactants will have enough energy to go over the lower barrier but few will be able to cross the higher barrier similarly though it is possible to reduce nitrogen dioxide gas to nitrogen gas there is also a high barrier to this reaction therefore nitrate ions will be more often reduced to nitrogen dioxide gas even though its formation has less energy potential than the formation of nitrogen gas the EZ values tell us that both these reactions are possible but the availability of energy in the system that is the temperature plays a major role in determining which reaction predominates we have observed that a low temperature favors the product nitrogen dioxide gas if we want to favor nitrogen gas as a product a high temperature is required let's consider the reactions which go on in the rocket rocket ignition supplies enough activation energy for some reactants to cross the high barrier after crossing the barrier a large additional amount of energy is released the rocket chamber retains much of the energy the temperature rises more react are activated and an extremely fast high temperature reaction occurs great quantities of hot expanding nitrogen and other gases result in a powerful thrust in accord with our energy barrier and energy potential theories we have seen that nitric acid makes possible a wide variety of products how is nitric acid produced in manufacturing nitric acid ammonia gas and air are piped into this plant the ammonia and air enter this reactor where they pass over a platinum Catalyst the heat of reaction keeps the Catalyst red hot here gaseous ammonia reacts with oxygen to form gaseous water and ano the products from the catalytic chamber pass into these pipes as they are cooled no gas reacts with oxygen to form NO2 gas high pressures and cooling are used to increase the yield finally the nitrogen dioxide is sent to this absorption column where nitric acid forms let's study what is happening inside the absorption column the NO2 gas reacts with excess oxygen and water to form nitric acid with a further release of energy theater principle is applied to achieve a maximum yield of nitric acid at equilibrium since the reaction gives off heat the formation of nitric acid is favored by removing heat cooling coils surround the column also the reactants are kept at high concentration by adding excess Air at the bottom of the column and water at the top and finally since we are forming a liquid from gases the operation takes place at higher than atmospheric pressure to review We oxidized ammonia through no and NO2 to nitric acid in stages we oxidize the nitrogen atom in ammonia from -3 to +2 to plus4 and finally to plus five forming nitric acid we have seen that nitric acid can act either as an acid or as a base acting as an acid nitric acid donates its proton to a base and forms the nitrate ion acting as a base nitric acid accepts a proton from a stronger acid splits out water and forms the nitronium ion as an oxidizing agent the nitrate ion can be used in a wide variety of oxidations these vary considerably in reaction rate though all have large EZ values controlling the temp temperature helps control the product the application of Leader's principle allows us to control the equilibrium yield thus through a knowledge of fundamental principles we can understand and control the complex processes in which we produce and use nitric acid