Presentation on theme: "Modern Theories of Acids & Bases"— Presentation transcript:
1Modern Theories of Acids & Bases The ArrheniusandBronsted-Lowry Theories
2Acids & BasesAcids and bases are special kinds of electrolytes. Like all electrolytes they break up into charged particles.What sets them apart from each other, and other electrolytes is the way that they break up.
3Arrhenius AcidsSvente Arrhenius (who first proposed the theory of ionization) identified acids as substances that ionize in water to produce hydronium ion.For example:HCl + H2O H3O+ + Cl-Any substance that ionizes in water to produce hydronium ion can be classified as an acid.
4H3O+ (aka H+)The hydronium ion is also known as a hydrogen ion.This allows us to shorten the ionization reactions for acids. The following equations represent the same chemical change:HCl + H2O H3O+ + Cl-andHCl H Cl-However, it should be understood that H+ is an abbreviation for the hydronium ion. H+ ions DO NOT exist in water solution but are snatched up by water molecules to form hydronium ions.
5Arrhenius BaseSvente Arrhenius also identified bases as substances that ionize in water to produce hydroxide ion.For example:NaOH Na+ + OH-Any substance that ionizes in water to produce hydroxide ion can be classified as a base.
6SaltsIonic substances that break up in solution to produce ions other than hydronium and hydroxide ions.NaCl (s) Na+(aq) + Cl- (aq)KNO3(s) K+ (aq) NO3- (aq)Li2SO4 (s) 2 Li+ (aq) + SO42-(aq)Salts are made up of positive (metal) and negative (non-metallic or polyatomic) ions. The more familiar you become with Table E, the easier it will be for you to identify salts.
7SaltsSalts are generally defined as ionic substances that PRIMARILY produce positive and negative ions other than hydronium or hydroxide when they dissolve in water.
8PracticeIdentify each of the following as acids/bases/salts and show how they ionize:HC2H3O2K2SO4KOHLiOHHNO3Acid HC2H3O2 H+ + C2H3O2-Salt K2SO4 2K+ + SO42-Base KOH K OH-Base LiOH Li OH-Acid HNO3 H NO3-
9Properties of Acids Why? All acids have the following properties: Neutralize bases to form a salt and waterHave a sour taste (example: citric acid, vinegar)React with active metals to produce a salt plus hydrogen gas.Have pH’s less than 7Affect indicatorsWhy?Because all acids have H3O+ ions present!
10Properties of Bases Why? All bases have the following properties: Neutralize acids to form a salt and waterHave a bitter taste (example: unsweetened chocolate, heroin)Are slippery. React with fats/oils to form soap (saponification)Have pH’s greater than 7 / Affect indicatorsWhy?Because all bases have OH- ions present!
11Bronsted-Lowry Acids & Bases Another theory of acids & bases
12Not everyone was happy with Arrhenius’s definition A major problem with the Arrhenius definition of acids and bases is that it limits acids and bases to water (aqueous) solutions.Since an acid requires water to ionize and form hydronium ions, there can be no Arrhenius acids unless water is involved as the solvent.
13Relationship between the two models Bronsted-Lowry Acids/Bases can exist when no water is presentArrhenius Acids/Bases only exist in water solutions.All Arrhenius acids and bases can also be classified as Bronsted-Lowry acids and bases.
14Bronsted-Lowry Definitions Bronsted and Lowry felt that this was too limiting, since there are many non-aqueous systems (no water is present).They came up with the following definitions for acids and bases.An acid is a proton (H+ ion) donorA base is a proton acceptor
15An exampleIn the reaction below there are no Arrhenius acids or bases present (because no hydronium ions or hydroxide ions are formed).However, the HCl is acting as a Bronsted-Lowry acid because it is giving a H+ ion to the NH3 (which is acting as a H+ ion acceptor - a base)
16Acid & Bases are RolesIn the Bronsted-Lowry definition, substances are classified as acids or bases depending on how they behave in a given situation.This means that the same substance can act as a acid in one reaction (by donating a proton) while acting as a base in another reaction
17HCl as a proton donor Consider the following reaction: Since the HCl gives up a H+ ion to the water it is acting as a Bronsted-Lowry acid. In the process of donating the proton it also forms a hydronium ion, and that makes it an Arrhenius acid as well.
18Water as a base But what does that make the water molecule? Since the water molecule is accepting the H+ ion, it is acting as a Bronsted-Lowry base. Since there is no hydroxide ion (OH-) formed, the water is not acting as an Arrhenius base in this reaction.
19Ammonia as a base Let’s look at another example: Here the ammonia molecule is accepting a H+ ion and therefore is acting as an Bronsted-Lowry base. However, in the process of reacting with the water it is also forming a hydroxide ion. That makes the ammonia an Arrhenius base as well.
20But what about the water? Since the water is giving up a H+ ion, it is acting as a Bronsted-Lowry acid. Since it does not form hydronium ions, it is NOT acting an Arrhenius acid.
21So is water an acid or a base? In one example, we said that water was acting as a base, and in another example we said that it was acting as an acid.Some of you may be confused by this because you are thinking of acids and bases as being like boys and girls. Boys are boys and girls are girls, and they can’t switch back and forth. However, acids and bases are NOT like this.
22Teacher-student model Teachers give off information (like acids give off protons)Students accept information (like bases accept protons)Sometimes teachers are students, and sometimes students are teachersTeacher and students are roles that individuals play depending on the situation.Acid and base are roles that molecules play in a particular chemical reaction. In different reactions they may play different roles.
23Amphoteric/Amphiprotic Sometimes a molecule can donate a proton (act as an acid) and sometimes it can accept a proton (act as a base).Molecules that have this ability to act as both an acid and a base are called amphoteric or amphiprotic.Water is the most common example of an amphoteric substance.
24Reality checkFor each of the following reactions identify any Bronsted-Lowry acids and bases.HNO H2O H3O NO3-HNO NH3 NH NO3-S H2O HS OH-HS OH- S H-OHHS HCl H2S Cl-Are any of the substances above amphoteric?acidbaseacidbasebaseacidacidbasebaseacid
26Strong/Weak AcidsAcids can be either strong electrolytes or weak electrolytes.Strong acids (such as HCl) completely break up into their ions:HCl (aq) H+(aq) Cl-(aq)Weak acids (such as HC2H3O2) only partially break up into their ions:HC2H3O2 H+ (aq) + C2H3O2-(aq)Weak acids don’t completely break up because they go to equilibrium!
27Acid-base equilibrium Many acid base reactions go to equilibrium, that is they have both a forward and reverse reactionsFor instance, acetic acid (HC2H3O2) reacts with water to form hydronium ion and acetate ion.
28The reverse reactionHowever, the acetic acid only partially ionizes because a reverse reaction takes place preventing the forward reaction from reaching completion.In the reverse reaction, the Hydronium ion acts as an acid (a proton donor) while the acetate ion acts as the base.
29Every acid has a conjugative base Conjugate Acid-BasesWhen a substance donates a proton, the substance that is left is its conjugative base:Notice that the conjugative base is accepting a proton in the reverse reaction.Every acid has a conjugative base
30Conjugative Acid-Base Pairs 2 When a substance accepts a proton, the substance that is formed is its conjugative acid:Notice that the conjugative acid is donating a proton in the reverse reaction.Every base has a conjugative acid
31Strong/Weak BasesBases can be either strong electrolytes or weak electrolytes.Strong bases (such as NaOH) completely break up into their ions:NaOH (aq) Na+(aq) OH-(aq)Weak bases (such as NH3) only partially break up into their ions:NH3 (aq) + H2 O NH4+ (aq) + OH-(aq)Weak bases don’t completely break up because they go to equilibrium!
32Identify the amphoteric substances in this chart.
34Salts can be classified as being: Types of SaltsSalts can be classified as being:neutralacidicBasicHow a salt is classified depends upon whether secondary reactions between the ions making up the salt and water form either hydronium or hydroxide ions.
35Determining the type of salt The type of salt for a particular salt can be determined experimentally by testing the salt solution with universal indicator paper.neutral salts will have a pH of 7acidic salts will have a pH of less than 7basic salts will have a pH of greater than 7Typically, the pH values of salt solutions will be closer to 7 than that of acids or bases.
36Predicting the type of salt All salts can be considered to be formed from their “parent” acid and base by means of the neutralization reaction.Acid Base Salt + WaterThe type of salt can be theoretically predicted based on the properties of the “parent” acid and the “parent” base.
37Neutral SaltsNeutral salts are formed from a reaction between a strong acid and a strong base.(Remember neutralization reactions are double replacement reactions.)For instance,HCl NaOH NaCl HOH(since HCl is a strong acid and NaOH is a strong base, NaCl is a neutral salt.)
38Acidic SaltsAcidic salts are formed from a strong acid and a weak base.For instance,HCl NH4OH NH4Cl HOH(since HCl is a strong acid and NH4OH is a weak base, NH4Cl is an acidic salt.)
39Basic Salts Basic salts are formed from a weak acid and a strong base. For instance,HC2H3O NaOH NaC2H3O HOH(since HC2H3O2 is a weak acid and NaOH is a strong base, NaC2H3O2 is a basic salt.)
40Strong and weak acidsIn predicting the type of salt it is helpful to remember the three strong acids:HCl – hydrochloric acidHNO3 – nitric acidH2SO4 – sulfuric acidMost other acids are weak. Acetic acid (HC2H3O2) is the most common weak acid discussed in Regents chemistry.
41Strong and Weak BasesFor bases, the Group I hydroxides are all strong bases.LiOH – lithium hydroxideNaOH – sodium hydroxideKOH – potassium hydroxideMost other hydroxides can be considered as weak bases. Ammonia or Ammonium hydroxide (NH3 or NH4OH) is the most common weak base discussed in Regents chemistry.
42Try theseIdentify the parent acid and base for the following salts, identify them as strong or weak and predict the type of salt formed.