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Unit 7: Acids, Bases, & Buffers, Study Guides, Projects, Research of Chemistry

Chemistry

The concepts of acids, bases, and buffers. It describes the strength of acids and bases, the comparison of strengths of acids and bases, and the factors that affect the strength of acids and bases. The document also explains the concept of pH and how it changes with the strength of acids and bases. It is a useful resource for students studying chemistry and related fields.

Typology: Study Guides, Projects, Research

2021/2022

Available from 04/26/2023

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Unit 7: Acids, Bases, & Bu๏ฌ€ers
โ˜…Acid: Proton (H+) donor
โ—‹pH < 7
โ˜…Base: Proton (H+) acceptor
โ—‹pH > 7
Strength of Acids and Bases:
โ˜…Strong: completely ionizes in aqueous solutions
โ—‹No reversible dissociation (direction)
โ—‹Even though itโ€™s covalent
โ– The electrons are uneven due to
electronegativities, the partial parts
of the molecule are attracted to
different parts of water molecules,
the attractions are so strong that
A&Bs just separate
โ—‹Acids: HCl, HBr, HI, H2SO4, HNO3, HCLO4
โ—‹Bases: Group I and Group II hydroxides
โ˜…Weak: partially ionizes in
aqueous solutions
โ—‹Reversible
dissociation
(direction)
โ– Equilibrium
and K values
โ– Ka and Kb
โ—‹If it can, will form no
matter what
โ˜…Comparing Strengths of A&Bs:
โ—‹Stronger acids dissociate more, thus produce more product in their dissociation
equations
โ– Higher Ka or Kb means stronger
โ—‹HA + B โ‡ŒA-+ HB+
โ– If the K of an acid-base reaction is > 1
โ—Forward reaction is favorable, More product was formed, so the
reactants were doing more proton donating/receiving
โ—‹Reactants are stronger Acids and Bases
โ– B is stronger than A-, HA is stronger than HB+
โ– If the K of an acid-base reaction is < 1
โ—Backward reaction is favorable, More reactant was formed/stayed,
so the products were doing more proton donating/receiving
โ—‹Products are stronger Acids and Bases
pf3
pf4
pf5

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Unit 7: Acids, Bases, & Buffers

โ˜… Acid: Proton (H+) donor

โ—‹ pH < 7

โ˜… Base: Proton (H+) acceptor

โ—‹ pH > 7

Strength of Acids and Bases:

โ˜… Strong: completely ionizes in aqueous solutions

โ—‹ No reversible dissociation (direction)

โ—‹ Even though itโ€™s covalent

โ–  The electrons are uneven due to

electronegativities, the partial parts

of the molecule are attracted to

different parts of water molecules,

the attractions are so strong that

A&Bs just separate

โ—‹ Acids: HCl, HBr, HI, H 2 SO 4 , HNO 3 , HCLO 4

โ—‹ Bases: Group I and Group II hydroxides

โ˜… Weak: partially ionizes in

aqueous solutions

โ—‹ Reversible

dissociation

(direction)

โ–  Equilibrium

and K values

โ–  Ka and Kb

โ—‹ If it can, will form no

matter what

โ˜… Comparing Strengths of A&Bs:

โ—‹ Stronger acids dissociate more, thus produce more product in their dissociation

equations

โ–  Higher Ka or Kb means stronger

โ—‹ HA + B โ‡Œ A-^ + HB+

โ–  If the K of an acid-base reaction is > 1

โ— Forward reaction is favorable, More product was formed, so the

reactants were doing more proton donating/receiving

โ—‹ Reactants are stronger Acids and Bases

โ–  B is stronger than A-, HA is stronger than HB+

โ–  If the K of an acid-base reaction is < 1

โ— Backward reaction is favorable, More reactant was formed/stayed,

so the products were doing more proton donating/receiving

โ—‹ Products are stronger Acids and Bases

โ–  A- is stronger than B, HB+^ is stronger than HA

โ—‹ Based on Molecular Structure

โ–  If there are more oxygens, then the substance is a stronger acid

โ— When more oxygens are added to the central atom, the oxygens

draw electrons toward them from the central atom. So, the central

atom draws electrons away from the O-H bond. So the bond

becomes more polar (remember the O has the central atom e

s).

Therefore, it is easier for the H+^ to disassociate, making that

compound stronger.

โ–  If the same # of oxygens, the more electronegative central atom (HEcentralO),

a stronger acid

โ— E can draw away electrons from the O-H bond, so the O-H bond

becomes more polar. So, H+^ can dissociate more easily.

โ—‹ For Binary Substances: Bond Length/Bond Strength of A-H and B-H+

โ–  The shorter the bond length, the stronger the bond, thus the weaker

acid/base

โ— Less likely the H will dissociate

โ˜… When the pH changes by 1, the volume increases by x

Buffer: a solution containing large quantities of both components of a weak acid-base conjugate pair

โ— One component can be added as a salt, just the salt has to 100% disassociate โ—‹ Na+, K+, NH 4 +, NO 3 - โ— Function: Resist Change in pH โ—‹ How: Each component can neutralize modest amounts of added strong acid or base โ–  Add Strong Base > H 3 O+^ concentration decreases > The Weak Acid dissociates to increase the H 3 O+^ concentration โ–  Add Strong Acid > H 3 O+^ concentration increases > The Weak Base โ€œdissociatesโ€(forms acid) to decrease the H 3 O+^ concentration โ— Buffer Capacity: the amount of acid or base that can be added to a given volume of buffer solution before the pH changes significantly โ—‹ The measure of how well the Buffer can resist changes in pH under stress โ—‹ The more concentration of BOTH weak acid and base of the conjugate pair, the higher the capacity โ–  pH doesnโ€™t change when both are added โ— Ideal Buffer: Has equal amounts of both weak acid and base of the conjugate pair โ—‹ Can easily counteract an addition of strong acid OR base โ—‹ A buffer is not very useful when one component is significantly less than the other โ— Buffer to maintain pH greater than 7 (basic solution): more weak base than weak acid in the solution is better โ—‹ An acid added would decrease the pH, so to maintain a HIGH pH, more base is preferred โ—‹ If a base added, it would increase the pH and this buffer would not be very capable of decreasing the pH โ— Buffer to maintain pH less than 7 (acidic solution): more weak acid than weak base in the solution is better โ—‹ A base added would decrease the pH, so to maintain a HIGH pH, more base is preferred โ—‹ If an acid added, it would decrease the pH and this buffer would not be very capable of increasing the pH

โ— pH = pKa + log( ) [๐ดโˆ’] [๐ป๐ด] โ—‹ pKa = -log(Ka) โ—‹ Since an ideal buffer [A-] and [HA] would be making log ([A-]/[HA]) = 0, the pH = pKa โ˜… The pHโ€™s unit place will be 1 less than the exponent of Ka โ˜… pH > pKa: base form has a higher concentration than acid form โ˜… pH < pKa: acid form has a higher concentration than base form โ— When the a is so small, it indicates that the reactant barely dissociated (very weak). So the x in the RICE table is insignificant compared to the starting amount of the reactant. โ— To find the percent dissociated: 100 x ๐ถ ๐‘–๐‘› ๐‘…๐ผ๐ถ๐ธ ๐ผ ๐‘–๐‘› ๐‘…๐ผ๐ถ๐ธ โ— For strong acids/bases: โ—‹ pH is determined by the amount of strong substance left over after an acid-base reaction โ–  The [H 3 O+] can be found using RICE table ratios from the strong substanceโ€™s dissociation reaction (or OH-, then H 3 O+) โ— For weak acids/bases: โ—‹ pH is determined by the amount of the weak substance left over and its conjugate after an acid-base reaction โ–  Use Ka or Kb formula to determine [H 3 O+] formed โ— K is found from the dissociation reaction of the weak substance โ— Strong Base to Weak Acid, New pH? HA + OH- > A- + H 2 O โ—‹ Equimolar: use A-^ + H 2 O โ‡Œ HA + OH- > Use Ka expression or H-H equation โ—‹ More Acid: Buffer > Use Ka expression or H-H equation โ—‹ More Base: pOH > pH โ— Strong Acid with Weak Base: pH B + H 3 O+^ > HB+^ + H 2 O โ—‹ Equimolar: use HB+^ + H 2 O โ‡Œ B + H 3 O+^ > Use Ka expression or H-H equation โ—‹ More Base: Buffer > Use Ka expression or H-H equation โ—‹ More Acid: pH โ— Strong Base with Strong Acid: pH determined by the excess reactant โ— Calculating pH of buffer solution โ—‹ Determine concentrations through RICE, ratios, or dilutions โ—‹ pH = pKa + log etc