Modern Theories of Acids & Bases
Embed Size (px)
Transcript of Modern Theories of Acids & Bases
Modern Theories of Acids & BasesThe Arrhenius and Bronsted-Lowry Theories
Acids & 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.
Arrhenius 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.
H3O+ (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- and HCl 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.
Arrhenius 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.
SaltsIonic 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.
SaltsSalts are generally defined as ionic substances that PRIMARILY produce positive and negative ions other than hydronium or hydroxide when they dissolve in water.
PracticeIdentify each of the following as acids/bases/salts and show how they ionize:HC2H3O2K2SO4KOHLiOHHNO3Acid HC2H3O2 H+ + C2H3O2-Acid HNO3 H+ + NO3-Base KOH K+ + OH- Base LiOH Li+ + OH-Salt K2SO4 2K+ + SO42-
Properties of AcidsNeutralize 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 pHs less than 7 Affect indicatorsWhy? All acids have the following properties:Because all acids have H3O+ ions present!
Properties of BasesNeutralize 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 pHs greater than 7 / Affect indicatorsWhy? All bases have the following properties:Because all bases have OH- ions present!
Bronsted-Lowry Acids & BasesAnother theory of acids & bases
Not everyone was happy with Arrheniuss definitionA 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.
Relationship between the two modelsArrhenius Acids/Bases only exist in water solutions.Bronsted-Lowry Acids/Bases can exist when no water is presentAll Arrhenius acids and bases can also be classified as Bronsted-Lowry acids and bases.
Bronsted-Lowry DefinitionsBronsted 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) donor
A base is a proton acceptor
An 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)
Acid & 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
HCl as a proton donorConsider 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.
Water as a baseBut 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.
Ammonia as a baseLets 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.
But 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.
So 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 cant switch back and forth. However, acids and bases are NOT like this.
Teacher-student modelTeachers give off information (like acids give off protons)
Students accept information (like bases accept protons)
Sometimes teachers are students, and sometimes students are teachers
Teacher 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.
Amphoteric/AmphiproticSometimes 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.
Reality checkFor each of the following reactions identify any Bronsted-Lowry acids and bases.
HNO3 + H2O H3O+ + NO3-HNO3 + NH3 NH4+ + NO3-S2- + H2O HS- + OH-HS- + OH- S2- + H-OHHS- + HCl H2S + Cl-
Are any of the substances above amphoteric?acidacidacidacidacidbasebasebasebasebase
Go to pH- Indicator PowerPoint
Strong/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 dont completely break up because they go to equilibrium!
Acid-base equilibriumMany acid base reactions go to equilibrium, that is they have both a forward and reverse reactions
For instance, acetic acid (HC2H3O2) reacts with water to form hydronium ion and acetate ion.
The 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.
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
Conjugative Acid-Base Pairs 2When 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
Strong/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 dont completely break up because they go to equilibrium!
Identify the amphoteric substances in this chart.
Types of SaltsSalts can be classified as being:neutralacidicBasic
How a salt is classified depends upon whether secondary reactions between the ions making up the salt and water form either hydronium or hydroxide ions.
Determining the type of saltThe 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 7
Typically, the pH values of salt solutions will be closer to 7 than that of acids or bases.
Predicting the type of saltAll salts can be considered to be formed from their parent acid and base by means of the neutralization reaction.
Acid + Base Salt + Water
The type of salt can be theoretically predicted based on the properties of the parent aci