Module 6: Acid/Base Reactions Flashcards
Common Acids and Bases and strengths
HNO3
H2SO4
CH3COOH
(COOH)2
H3PO4
KOH
NH3
HNO3 - Nitric acid (strong acid)
H2SO4 - Sulfuric acid (strong acid)
CH3COOH - Acetic acid (weak acid)
(COOH)2 - Oxalic acid (weak acid)
H3PO4 - phosphoric acid (weak acid)
KOH - Potassium hydroxide (strong base)
NH3 - Ammonia (weak base)
Acid-Hydroxide reaction?
Acid + Hydroxide –> Salt + Water
Acid-Carbonate reaction?
Acid + Carbonate –> Salt + Water + Carbon Dioxide
Acid-Metal?
Acid + Metal –> Salt + H2
Bromothymol Blue
Methyl Orange
Phenolphtalein
BB: yellow below 6, green at 6-7.6, blue greater than 7.6
Methyl: red below 3.1, orange at 3.1-4.4, yellow greater than 4.4
PP: Colourless below 8.3, light pink at 8.3-10, pink greater than 10
Bronsted-Lowry theory?
Acid-base reactions involve the transfer of a proton from an acid to base. Acids are proton donors, while bases are proton acceptors.
What was Arrhenius’ Theory?
Acids produce hydrogen ions in aqeuous solutions, while bases ionise to form hydroxide ions in aqeuous solutions.
What do conjugate acid-base pairs differ by?
One proton/Hydrogen ion
Compare Arrhenius and Bronsted-Lowry and which is better
Arrhenius’ Theory is about ionisation, how acids and bases ionise in aqeuous solutions to produce hydrogen and hydroxide ions respectively, which is also stated in Bronsted-Lowry Theory. However, Bronsted-Lowry expands on this idea, with the concept of acids as proton donors, and bases being proton acceptors. This meant that acids and bases do still ionise to produce hydrogen and hydroxide ions respectively, but it also accounts for acid-base reactions that are not in solution, as there is no mention of aqeuous solutions, as well as for bases that do not contain hydroxide in their chemical formula, such as carbonates.
What does monoprotic/polyprotic mean? How do you write out their reactions?
Tells you the number of acidic hydrogens an acid can donate. Stepwise fashion for each acidic hydrogen.
What does the strength of acids indicate?
How readily the acid will donate protons to water. Strong acids will fully dissociate.
Which molecules can act as an acid and a base?
Amphiprotic species have the ability to accept or donate their protons.
What does the strength of bases indicate?
Ability of form hydroxide ions in solution. Strong bases will fully dissociate in water.
How can you tell whether an acid is strong or weak?
Forwards arrow or equilibrium arrow
Limitation of Bronsted-Lowry?
Does not explain the reactions between acidic oxides and basic oxides, such as the acidic oxide SO3 and the basic oxide CaO. There is no proton transfer, and is thus not a Bronsted-Lowry reaction.
Which will be more acidic in solution, 0.1M hydrochloric acid or 0.1M sulfuric acid
Sulfuric acid because it is diprotic.
What is the Ionic product of water? (at 25 degrees)
1.0 x 10^-14 = [Hydronium ions][Hydroxide ions]
Helps us convert from hydroxide concentration to hydrogen concentration.
How do you link pH and pOH?
pH + pOH = 14
How do you perform neutralisation pH calculations?
- Write out balanced chem equation
- Find the limiting reagent
- The excess acid/base will contribute to the pH of the solution
- Makes sure to include the entire volume of the solution in final calculations
What is the acid Dissociation constant?
Tells us the strength of an acid in quantitative terms. It has the same formula as the equilibrium constant, except that its an acid dissociating in water.
What is pKa?
A more convenient way of representing the strength of an acid: -log10(Ka).
The lower the pKa, the stronger the acid.
How do you link pH and Ka?
- Write out the acid dissociation equation
- Use a RICE table
What is the relationship between Ka and Kb?
Their product is the ionic product of water (1.0 x 10^-14 at 25 degrees) if we are given a conjugate acid base pair.
What is the purpose of titration?
To calculate the unknown concentration of an acid or base.
Justify the use of a primary standard solution in titration?
Primary Standard solution is a very accurately known concentration made of a known mass of primary standard and distilled water. The purity, stability and high MM of the substances make them preferable to other substances in titration as their use improves the reliability, accuracy and validity of titration.
What is a titration curve?
Plot of how the pH of the analyte changes over the course of a titration
Shape of Strong acid - Strong base pH curve
Equivalence point (middle of the vertical section) is at pH = 7
Shape of Strong acid - Weak base pH curve
Equivalence point is at pH = 5.5
Shape of Weak acid - Strong base pH curve
Has a buffer region before the vertical section and equivalence point is at pH = 9
Shape of Weak acid - Weak base pH curve
Does not have a vertical section. Has a buffer region.
How do you choose an appropriate indicator for titration?
Indicator’s end point must be within the steep section of the pH graph.
What do conductivity graphs do?
Conductivity graphs reveal the equivalence point without an indicator. It is indicated by a sudden change in the solutions conductivity.
When is conductometric titration useful?
When the acid or base is coloured or turbid or really dilute.
How does conductometry work?
As ions are formed in solution, the conductivity will change accordingly as different ions have different conductivity levels.
Ion Conductivity
H+ ions are most conductive, with OH- being slightly less. Conductivity is never zero.
Conductivity curve - Strong Acid - Strong Base
Conductivity curve - Strong Acid - Weak Base
Due to common ion effect, the weak base will ionise at dynamic equilibrium, and thus, LCP will keep the additional base from ionising further.
Conductivity curve - Weak Acid - Strong Base
Conductivity curve - Weak Acid - Weak Base
Due to common ion effect, the weak base will ionise at dynamic equilibrium, and thus, LCP will keep the additional base from ionising further.
What is a back titration?
Back titration reacts a reactant with unknown concentration with a standard solution in excess. This solution is then titrated with another standard solution to find the excess of the first reaction and thus the number of moles that reacted.
Why Back Titration?
If a substance of unknown concentration is:
- Unreactive
- Low Concentration
- Weak acid/base
- Volatile
- Insoluble in water but soluble in acid/base
What is a blank titration?
A titration where the analyte is not present, and a titration is done to find dissolution of different reactants.
Good properties for standard solution?
High purity
Have a known formula
Not react with air or water from the atmosphere
Have high molecular weight
Be stable and of low reactivity
What is a buffer?
Buffers are able to maintain the pH of a solution even if extra acid or base is added.
What is buffer capacity and its factors?
The capacity for a buffer to resist changes in pH.
Capacity depends on:
- Higher concentrations of conjugate pair = better capacity
- Conjugate pair should have the same molar ratios
How do buffers work?
Buffers work as weak acids/bases exist in equilbrium in solution. If acid is added to an acidic buffer, the equilibrium will shift to the left. If base is added, the OH- ions will react with the H3O+, shifting equilibrium to the right. Just think as adding acids or bases as adding H+ ions or OH- ions.
Relationship between pH and pKa
Because a buffer works best when its conjugate acid base pair are in equal amounts,
this means that a buffer works best when its pH = pKa.
If the pH is within 1 unit of the optimal pH(or the pKa value), it is good.
How do indicators work?
They are weak acids or bases, which have distinct colour changes when their equilibrium is disturbed.
All the ionic equations (molecular, complete, net)
Also don’t split up insoluble molecules.
Enthalpy calculations?
q = mCAT
q is the heat of the solution in joules, mass is in grams, C is 4.18 x 10^3
AH = -q/n
where n is moles of water produced
What are spectator ions?
Ions that do not participate in the reaction, and so remain in the same state on both sides of equation.
What is the Bronsted-Lowry example
Where do acidic hydrogens normally occur?
At the ends of carboxyl groups, carboxylic acids.
