Acids and Bases

Relationship between Ka and Kb

***Example: written in Kaplan Basically states that the product of an acid and its conjugate base Ka and Kb is 1.0x10^-14 = Kw, so you can basically find out the Ka or the Kb by just knowing one. The Ka and Kb are greater for strong acids and bases respectively and pKa and pKb are smaller for stronger acids and bases respectively.

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pH=

-log[H+]

Range of pH scale is

0-14. Some strong acids and bases extend past the range.

Stronger acid

1) High Ka, Ka>1 2) More stable (weak) conjugate base (large conjugate base) 3) 6 strong: HI, HBr, HCl, H2SO4, HClO4, HNO3

What is a salt?

A salt is an ionic compound consisting of a cation and an anion. In water, the salt dissociates into ions and depending on how those ions react with water, the resulting solution will either be acidic, basic, or pH neutral. Remember that one way to make a salt is a neutralization reaction which a reaction between an acid and a base that produces water and a salt. How a salt reacts with water upon dissociation is determined in 4 ways.

Example problem: Of the following, which acid has the weakest conjugate base? A) HClO4 B) HCOOH C) H3PO4 D) H2CO3

A) HClO4 Which of the following acids is the strongest? We already know that HClO4 is one of our 6 strong acids, so it will have the weakest conjugate base.

The conjugate base of a WEAK POLYPROTIC ACID is always ______ because it can either donate or accept another proton.

ALWAYS

Weak bases incude

AMMONIA (NH3) Amines And the conjugate bases of many weak acids

Acids and their conjugate bases

Acid → Conjugate Base HF → F- HBr → Br- HNO3 → NO3- H2SO4 → HSO4- H2O → OH-

Tip: 1) Compare it to its future self. Which one looks like a donor (more H+) and the other one would be an acceptor 2) B-L acid donates H+. The remaining is its conjugate base. 3) When a B-L base binds H+, this new species that is bound to H+ would be the conjugate acid of the base. 4) You have to consider the forward and reverse reactions separately...

Acid- conjugate base (no H+) Base- conjugate acid (now bound to H+) In the end, the acid is always an acid and the base is always a base.

Acids and bases are _____ related.

Acids and bases are inversely related. The greater the concentration of H+ ions, the lower the concentration of OH- ions and visa versa.

Bronsted-Lowry Acids - Proton DONORS Must have an H+ to donate. HF can act as a Bronsted-Lowry Acid, but not as a lewis acid which accepts an electron pair.

Acids are proton donors H2CO3 + H2O → H3O+ + HCO3- H2CO3 and H3O+ are proton donors, or produce H+ so they are acids while the other two are bases since they are proton acceptors (OH- is a type of proton acceptor)

Arrhenius Acid

Acids ionize in water to produce hydrogen ions H+

Water is an

Amphoteric substance. It can react with itself with one molecule acting as an acid and the other acting as a base. H2O(l) + H2O(l) ⇌ H3O+(aq) + OH-(aq) This is called the self ionization of water.

Example problem: Of the following anions, which is the strongest base? A) I- B) CN- C) Br- D) NO3-

B) CN- Look at the strength of the conjugate acid. HI is a very strong acid, as is HBr, and HNO3. HCN is not one of the strong acids in the list of 6, therefore it is a weak acid and thus makes a stronger base. The others have conjugate bases with no basic properties in water.

The conjugate base of a strong acid has no _____ properties in water.

BASIC. The conjugate base of a strong acid has no basic properties. Example: HCl(aq) + H2O(l) → Cl-(aq) + H3O+(aq) We know HCl is a very strong acid...thus Cl-, the conjugate base, has no basic properties in water. Strong acid= non-basic conjugate base

Bronsted-Lowry Bases - Proton ACCEPTORS

Bases are proton acceptors

Arrhenius Bases

Bases ionize in water to produce hydroxide ions OH-

why do we use the pH scale?

Because H+ concentrations can vary enormously and are super small. pH scale gives us more convenient numbers to work with. This is true for pKa and pKb. Lets think back. What was Ka and Kb? They were the acid dissociation or acid ionization constant. Where the larger the Ka value, the stronger the acid. When Ka>1, we had a very strong acid.

A strong acid is one that dissociates_________ in water

Completely. Example is HCl + H2O → H3O+ + Cl- This reaction essentially goes to completion. A strong aid is one that dissociates COMPLETELY in water.

When a Bronsted-Lowry base bonds with an H+ in solution, this new species is called ______

Conjugate acid of the base (now bound to H+)

When a Bronsted-Lowry ACID (one that donates protons) donates an H+, the REMAINING structure is called the _____ of the acid.

Conjugate base of the acid.

B(aq) + H2O(l) ⇌ HB+(aq) + OH-(aq)

Dissolution of base in water.

Kb= [HB+][OH-]/[B]

Equilibrium expression for a base dissociation reaction. Kb= base ionization or base dissociation constant

Polyprotic

Ex: Carbonic acid has more than one proton to donate.

Of the following, which statement best explains why HF is a weak acid, but HCl, HBr, and HI are strong acids?

F- has a smaller radius than Cl-, Br-, and I-. F is smaller than Cl, Br, or I. This we know. Atomic radius increases SouthWest. Ionization energy is associated with forming a cation from a neutral atom (which has no bearing here on strong acids or weak acids)

Example of how the weaker the acid (smaller Ka and dissociates less), the more the reactants are favored and the more basic the conjugate base is.

For example in comparing HF to HCN: HF~ Ka= 7 x 10^-4 HCN~ Ka= 5 x 10^-10 which is much smaller than HF Therefore, the conjugate base of HCN which is CN- is a stronger base than if F-. Therefore, CN- is a stronger base than F-! We can make this conclusion since its conjugate acid is weaker.

pH calculations: for strong and weak acids

For strong acids= since strong acids dissociate completely, the hydrogen ion concentration will be the same as the concentration of the acid. That means I can calculate pH directly from the molarity of the solution. 0.01M HCl will have [H+]=0.01M and thus, a pH of 2. For weak acids= weak acids do not dissociate completely. instead, they come to EQUILIBRIUM with their (few/some) dissociated ions. in fact, for a weak acid at equilibrium, as we saw with HCN or HF, the concentration of undissociated acid will be much greater than the concentration of hydrogen ions. To get the pH of a weak acid solution, you will need to use the EQUILIBRIUM EXPRESSION. Lets say you add 0.2 mol HCN to water and want to know the pH of this 1-liter solution. When you first add HCN, it will be completely undissociated so it will be present as 0.2M and the ions, H+ ad CN- will be present as 0M. Then at eqilibrium, we will say that X number of H+ and an X number of CN- have dissociated which would leave 0.2-X number of of HCN which has not dissociated. We will set up the Ka expression such that Ka= [H+][CN-]/[HCN]= X^2/0.2-X. IMPORTANT NOTE: Whenever the given Ka is smaller than 10^-4, then we can assume that X is negligible and we will write: X^2/0.2=Ka of HCN= 4.9*10^-10. And we will get the result as a pH of 5.

Really good example problem:

Given that the self-ionization of water is endothermic, what is the value of the sum of pH+pOH at 50 degrees C. ***Whenever you see the words endothermic, add heat to the reactants of the equation. When you add heat as a reactant to the equation, using Le Chatelier's principle, we know that the reaction will go right towards the products and thus increase the concentrations of [H3O+] and [OH-] which would reduce the pH and pOH to below 7, thus the sum of the two mat 50 degrees would be less than 14.

Which two cations dont react with water?

Group 1 cations and larger group 2 cations

Strong bases to know for the MCAT

Group 1 hydroxides (NaOH) Group 1 oxides (Li2O) Some Group 2 hydroxides (Ba(OH)2, Sr(OH)2, Ca(OH)2) Metal Amides NaNH2

Example problem: A 10M solution of HCl will full dissociate into H+ and Cl-. The pH of HCl is....

H+=[10M] pH= -log[H+] -pH=log(base 10)[10M] 10^(-pH)=10M pH=-1

Note: Ever time a polyprotic acid donates a proton, the resulting species will be a weaker acid than its predecessor.

H2CO3(aq) + H2O(l) ⇌ H30+(aq) +HCO3-(aq) HCO3-(aq)+ H2O(l) ⇌ H30+(aq) +CO3 2-(aq) Ka of H2CO3(aq) = 4.5 x 10^-7 Ka of HCO3-(aq)= 4.8 x 10^-11 [the only reason we can use this again as an acid is because it has an ionizable proton, furthermore, carbonic acid whic his H2CO3 is said to be polyprotic because it has more than one proton to donate)!

Question: in a 1 M aqueous solution of boric acid (H3BO3, Ka= 5.8x 10^-10), which of the following species will be present in solution in the greatest quantity?

H3BO3 (aq) This is because H3BO3 + H2O → H3O+ + H2BO3- we know that boric acid is not one of the strong 6 acids, so it is a weak acid, so the equilibrium lies to the left. Also, the Ka value is less than 1 which means reactants are favored. So we would have very few products present and lots of H3BO3 present.

Strong acid to memorize: Anything not on this list is a weak acid

HI HBr HCl HClO4 H2SO4 ← Multiprotic HNO3

Low pH means high [H+] which means the solution is acidic

High pH means low [H+] which means the solution is basic.

The STRENGTH of an acid is DIRECTLY RELATED to

How much the PRODUCTS are favored over the reactants. Remember that strong acids go to completion while weak acids remain in the most part in their dissociated form in water. HCl vs. HF.

Ionization energy has to deal with forming a cation from a neutral atom and has no bearing on why HF is a weak acid. It has nothing to do with weak acids and bases.

Ionization energy has nothing to do with acidity.

We can rank the relative strengths of acids by comparing their _______ values.

Ka values. I can rank the relative strength of the acid by comparing the relative Ka values. The higher the Ka value, the stronger the acid.

We can rank the relative Strength of bases by comparing the _______ value.

Kb value

The equilibrium expression for the autoionization of water is Kw

Kw=[H3O+][OH-]=1.0 x 10^-14 Only a very small fraction of the water molecules will undergo this reaction (as seen by the small Kw showing that reactants are favored) This occurs at 25 degrees C.

_______ anions are better able to spread out their negative charge, making them more stable.

Larger anions are better able to spread out their negative charge, making them more stable. More stable anions (the conjugate bases that loose the H and become anions) make more a stronger acid HF is the weakest of the H-X acids because it has the least stable conjugate base due to its size. F- has a smaller radius than Cl-, Br-, and I-, thus it is not able to spread out the negative charge.

Lewis Acids

Lewis acids are electron pair acceptors Lewis acid/base reactions frequently result in the formation of coordinate covalent bonds. Water acts as a lewis base since it donates both of the electrons involved n the coordinate covalent bond between OH- and AlCl3. AlCl3 acts as a lewis acid, since it accepts the electrons involved in this bond. AlCl3 + H2O → (AlCl3OH)- + H+

Lewis Bases

Lewis bases are electron pair donors

The pH scale measure the concentration of H+ or H3O+ ions in solution.

Logarithmic. pH=-log[H+]

If Ka>1

Meaning more products than reactants, then at equilibrium, products are favored. If products are favored, we have a STRONG ACID

Stronger acids than water include: remember that an acid is a proton DONOR, these want to get rid of a positive charge and become their conjugate base.

NH4+ Be2+

If the introduction of an acid increases the concentration of H3O+ ions, then the equilibrium is disturbed and the ______ reaction is favored.

REVERSE reaction is favored. This will result in decreasing concentration of OH- ions since we are running the reaction towards making H2O, so we will decrease in the OH - concentration, however the product of the concentrations will still always equal Kw.

Example: If we dissolve NaCl in water what happens? What is the final solution? Acidic, basic or pH neutral?

Since Na+ and Cl- ions go into solution, we know that Na+ ions are group 1 ions and thus are unreactive in water. Cl- is the conjugate base of a super strong acid HCl, thus it, also, is unreactive in water. All that happens is the ions become hydrated and surrounded by water molecules. Therefore, the solution will be pH neutral.

Example: Suppose we add 0.002 moles of HCl to water to create a 1-liter solution. Since the dissociation of HCl goes to completion, since its a strong acid, it will create 0.002 moles of H3O+ ions, so [H3O+]=0.002 M. Why H3O+? Because it is the acid in the reverse reaction.

So this means that since the concentration of H3O+ has increased, we expect the OH- concentration to decrease, which it does: [OH-]=Kw/[H3O+]= (1x10^-14)/(2x10^-3)= 5x10^-12M

Thus Ka>1

Strong acid

Amphoteric substances

Substances that can act as either an acid or a base. The conjugate base of a WEAK polyprotic acid is ALWAYS amphoteric because it can either donate or accept another proton.

Like all other equilibrium constants, Kw varies with temperature. Kw increases as temperature________.

The Kw will increase as the temperature INCREASES.

The difference between a Bronsted Lowry base and its conjugate acid is that....

The acid has an extra H+

Since the number of H3O+ ions in pure water will be equal to the number of OH- ions, then we can say that Kw=x^2, and that x= 1 x 10^-7.

The concentration of both types of ions in pure water is 1 x 10^7 M.

3 things: The conjugate base of a strong acid is not basic at all The conjugate base of a weak acid is a weak base The weaker the conjugate acid, the stronger the conjugate base.

The conjugate acid of strong base has no acidic properties in water. Ex: LiOH is one of the memorized strong bases, whose conjugate acid is Li+ which does not act as an acid in water. the conjugate acid of a weak base is a weak acid (and the weaker the base, the stronger the conjugate acid) for example, the conjugate acid of NH3 is NH1+ which is a weak acid.

The difference between a Bronsted Lowry acid and its conjugate base is that....

The conjugate base is missing an H+ simply remove the H+

The larger the Ka value, the _____ the acid.

The larger the Ka value, the stronger the acid. The smaller the Ka value, the weaker the acid.

The ________ the Kb value, the stronger the base, and the smaller the Kb value, the weaker the base

The larger the Kb value, the stronger the base, and the smaller the Kb value, the weaker the base. This is consistent with Ka for acids.

Since "p" means to take the negative log of, the lower the pKa value, the ______ the acid.

The lower the pH, the stronger the acid, the higher the [H+] The lower the pKa, the stronger the acid. The larger the Ka value, the stronger the acid. Same thing applies to pKb, the lower the pKb, the stronger the base. And remember that the strength of a base is directly related to how much the products are favored over the reactants: [HB+][OH-]/[B].

The more ______ an acid's conjugate base is, the stronger the acid.

The more STABLE an acid's conjugate base is, the stronger the acid. Larger anions are better able to spread out their negative charge, making them more stable.

The strength of the base is directly related to how much the __________ are favored over the __________ .

The strength of the base is directly related to how much the products are favored over the reactants.

If [H+] is less than 10^-7

The the pH will be more than 7, basic or alkaline

The weaker the acid, the ______ the reverse reaction is favored and the ______ its conjugate base.

The weaker the acid, the MORE the reverse reaction is favored, and the STRONGER its conjugate base.

If [H+] is greater than 10^-7

Then the pH will be less than 7, acidic

If Ka<1

Then the reactants are favored and the acid is weak.

the conjugate bases of weak acids DO react with water

They are a stronger base in water

The conjugate base of a weak acid is a ______base!

WEAK! The conjugate base of a weak acid is a weak base. HF(aq) + H2O(l) ⇌ H3O+(aq) + F-(aq) HF as we know is a weak acid since its dissociation is not complete! Since the reverse reaction DOES take place to a significant extent, the conjugate base of HF, the fluoride ion F-, DOES have some tendency to accept a proton, and so it behaves as a weak base. Weak acid= weak conjugate base

Hydrofluoric acid HF is an example of a ______ acid.

Weak acid.

Weak acids do not go to _____; most of the HF remains _______.

Weak acids do not go to completion, most of the HF remains undissociated. HF is a Bronsted Lowry acid, but not a lewis acid. For the most part it remains undissociated.

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

Where HA is used to denote a generic acid Acid dissociation in water. Remember, that whether an acid is strong or weak depends on how completely it ionizes in water. The stronger the acid, the more complete its ionization.

Essentially, there are two possibilities for both the cation and an anion in a salt that dissociates into water.

Whether a salt solution will be acidic, basic, or pH neutral depends on which combination of possibilities (4 total) apply. The diagram for this is drawn in Kaplan.

In pure water, [H+]=

[H+]= 10^-7 M, so the pH of pure water must be 7. At 25 degrees, this define a pH neutral solution

The equilibrium expression for this reaction is: Ka= The reaction of acid dissociating in water (which is directly related to strength of an acid in observing how much products are favored over reactants)

[H3O+][A-]/[HA] This is written as Ka rather than Keq to emphasize that this is the EQUILIBRIUM expression for an ACID dissociation reaction. The numerical value of Ka is used to predict the extent of acid dissociation. ***The acid that would dissociate to the greatest extent would have the greatest Ka value.

Neutralization reactions are reactions in which an acid and a base are combined to form...

a salt and water. HCl+ NaOH→ H2O+ NaCl real life application of the neutralization reaction: is when you take an antacid to relieve excess stomach acid, the antacid is a weak base, usually carbonate, that reacts within the stomach to neutralize acids. When a strong acid and a strong base react together, as shown above, you get a neutral pH in the products. However if the reaction involves a weak acid or weak base, the resulting solution will generally not be pH neutral.

Bronsted-Lowry acids can be placed into two big categories: Strong and weak. It depends on....

how completely it ionizes in water. (To what degree does it ionize in water?)

Since [H+][OH-]=10^-14 at 25 degrees celcius

pH+ pOH = 14 So, for example if you know one, you know the other. If you know the pOH, you can find the pH. If you know that the pH is 5, then the pOH must be 9.

An alternate measurement expresses the acidity and basicity in terms of the hydroxide ion concentration, using pOH

pOH=-log[OH-] [OH-]=10^-pH

Hydrolysis reaction

reaction of a substance (such as a salt or ion) with eater is called a hydrolysis reaction.

Remember that when the Ka<1 that means that it is a

weak acid. And that the weaker the acid, the stronger the conjugate base.