Chem y12 > Mod 6 Topic 3 - Quantitative Analysis > Flashcards

Mod 6 Topic 3 - Quantitative Analysis Flashcards

(55 cards)

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1
Q

What is volumetric analysis and what is it used for

A

Analytical methods used to determine exact concentrations/amounts of chemical substances.

Used in industries including:
Food safety
Medicine production
Wine production
Chemical testing
Environmental testing

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2
Q

What is the most common method of volumetric analysis

A

titration where you measure the volume of a known substance that reacts with a volume of a substance you want to know information about (the “unknown”).

*We know the unknown element just not deets.

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3
Q

What is the equivalence point of a titration

A

the exact moment when the amount of added titrant (solution thats slowly added to analyte) has chemically and stoichiometrically reacted with all of the analyte (the substance being tested), meaning moles of reactants are perfectly balanced, resulting in complete neutralization.

Given in a pH value

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4
Q

What is the endpoint of a titration reaction

A

An endpoint in titration is the visual or instrumental signal (like a color change or pH shift) that indicates the titration reaction is complete, signaling when to stop adding the titrant and when measurements are made. Usually occurs after the addition of one extra drop of acid or base following equivalence point

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5
Q

What must we do with volumetric glassware to ensure accurate measurements

A

A specific rinsing procedure to ensure accurate concentrations/volumes of substance.

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6
Q

Why do we use volumetric glassware

A

Bc its designed to get very accurate results, especially in industry, so specialised glassware is used to ensure accurate measurements.

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7
Q

How accurately does a volumetric flask measure

A

It will accurately measure volumes to 0.1 mL of its stated volume. It is calibrated, meaning it only has one line at that certain volume to measure.

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8
Q

What is the use of a volumetric flask

A

To make up solutions of known concentration from a solid, or to dilute solutions of known concentration.

The end result is a solution of known concentration.

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9
Q

What is a burette

A

A graduated volumetric cylinder (volume markings along the length of the burette so you can measure any volume rather than just one specific volume like the volumetric flask)

It has a tap at the bottom that controls the release of the solution in the burette during a titration.

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10
Q

What does a burette do

A

It is designed to add a known volume of solution to another solution until the endpoint of the reaction is reached.

An initial volume reading is taken then a final volume reading when the endpoint is reached which allows titre volume to be measured

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11
Q

Titre Volume

A

Volume delivered from the burette.

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12
Q

What is a primary standard and what are some of its features

A

A chemical that allows an accurate concentration solution to be made

1) Stable over time – must not decompose or react with substances in the air, or water
2) Large molecular mass – results in a smaller percentage error if any errors in measurement are made
3) Cheap and readily available
4) High purity - so no side reactions occur
5) Dissolve in a solvent – usually water

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13
Q

To determine the most appropriate indicator to use in a titration, what is the order or operations.

A

1) Your combination of strong/weak acid/base
2) Whether the salt formed is acidic/basic/neutral
3) What pH range you expect the endpoint to occur
4) Which indicators change colour in the expected pH range

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14
Q

To make a solution of accurate concentration you need a chemical called a ______________

A

Primary standard

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15
Q

What are the two ways to rinse a burette

A

1) With DI water
2) With the solution to be delivered in titration

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16
Q

Two primary standard examples

A

Anhydrous sodium carbonate and sodium hydrogen carbonate.

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17
Q

If our type of titration is STRONG ACID-STRONG BASE, where would the predicted equivalence point be and what indicator should we use

A

Equivalence Point –> Neutral, thus indicator should be bromothymol blue

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18
Q

If our type of titration is WEAK ACID-STRONG BASE, where would the predicted equivalence point be and what indicator should we use

A

Equivalence Point –> Basic, thus indicator should be phenolphthalein

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19
Q

If our type of titration is STRONG ACID-WEAK BASE, where would the predicted equivalence point be and what indicator should we use

A

Equivalence Point –> Acidic, thus indicator should be methyl orange

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20
Q

If our type of titration is WEAK ACID-WEAK BASE, where would the predicted equivalence point be and what indicator should we use

A

Generally do not use direct titration, thus no endpoint + indicator

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21
Q

What is a problem using coloured solutions in titration

A

Indicators may be unable to demonstrate the endpoint of the titration.

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22
Q

How can pH meters be used during a titration experiment

A

To measure the pH in the conical flask as specific volumes of titrant (from the burette) are added. A titration graph is produced that shows how the pH changes.

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23
Q

What do the x and y axis represent in a titration curve

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A

x axis –> Volume of acid added
y axis –> pH

24
Q

What would a titration curve look like for a strong acid-strong base reaction

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A

There is a clear drop in pH and the equivalence point will be approximately neutral as the salt produced is neutral.

25
What would a titration curve look like for a weak acid-strong base reaction
Starts high and takes a drop, not as drastic as strong acid-strong base. The equivalence point is in the basic pH range as the ionic salt produced is basic
26
What would a titration curve look like for a strong acid-weak base reaction
Instant slight drop off, slow curved decline, sharp drop (not as drastic as weak acid-strong base) then slowly straightens out. The equivalence point is in the acidic pH range. as the produced ionic salt is acidic
27
What is the issue for weak acid-weak base titration curves
Titration curves are not very accurate for weak acid/weak base titrations as the equivalence point is not clear.
28
A solutions conductivity is proportional to:
The number of charge carrying ions present in the solution.
29
What is conductometric titration
A titration that measures the change in conductivity to determine the equivalence point of a reaction.
30
What is conductometric titration used for
- Very dilute solutions - When substances are present only at trace levels - With coloured or turbid solutions (turbid = contains suspended particles) - Incomplete chemical reactions - Acid-base, redox reactions (no acids/bases present), precipitation reactions (no acid/base) and non-aqueous titrations (using organic solvents rather than water
31
How are Conductivity graphs plotted
plotting change in conductance (y axis) against volume of titrant added (x axis)
32
As chemical reactions progress, what can happen to the ions
Case 1: They combine, meaning there is an overall decrease in the am. of ions Case 2: They are formed, meaning there is an overall increase in the am. of ions
33
What factors affect conductivity
- Strong/weak nature of acid/base - Concentration of solutions - Size of the ions – larger ions are less mobile and thus less conductive - Temperature – conductivity increases as temperature increases
34
What are the 3 situations in which two reactants cannot be directly titrated
1) Reaction is very slow (can take hours) 2) Chemical to be analysed is not soluble in water or other solvents 3) Chemical to be analysed is toxic, volatile (easily evaporated), gaseous or unreactive
35
What are buffers
Solutions of weak acid/conjugate base or weak base/conjugate acid that can resist a change in pH when an acid or base is added to a system.
36
Why are buffers necessary
In aqueous environments (oceans, rivers, bloodstream etc) pH, temperature and ion concentration must be maintained within narrow boundaries, bc major changes to these factors can cause an organism to be unable to function, cause structural or biological damage, or can result in death.
37
How do buffers work
By LCP. Weak acids/bases and their conjugates will set up equilibrium systems that will shift to the left or right by Le Chatelier’s principle to adjust to minimise the effect of addition of an acid/base. If an acid or base is added, the system will shift so that the concentration of hydronium ions will revert to the original levels, thus keeping the pH level relatively constant.
38
Common buffer solutions
Carbonic acid (H2CO3) or hydrogen carbonate ion (HCO3-).
39
What happens when a strong acid is added to a weak acid-conjugate base buffer system. HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A⁻(aq)
- Initially the pH will decrease due to the added acid - The system will move to the left to reduce the amount of hydronium ions - The pH will increase back to near its original level
40
What happens when a strong base is added to a weak acid-conjugate base buffer system. HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A⁻(aq)
- Initially the pH will increase due to the added hydroxide ions - The hydroxide ions will react with the hydronium ions (already in eqlm as per equation) thus reducing the concentration of H3O+ ions - The equilibrium will shift to the right to increase the level of hydronium ions - The pH will decrease back to near its original level
41
What is the buffer capacity
The amount of acid or base that can be added to a buffer system without causing a significant change in pH.
41
What solutions have the greatest buffer capacity and why.
A solution with equal moles (equimolar amounts) of acid/base is able to easily adjust to both acid and base addition (small addition). The buffer can neutralise roughly equal amounts of added acid or base, thus giving max. capacity to resist pH changes in either direction
41
What can influence how great the buffer capacity is
The buffer solutions concn with higher concn, buffer solution is greater – there are more moles of acid/base in the buffer solution.
41
What happens with a weak acid-strong base titration (referring to a buffering effect)
When a weak acid is titrated with a strong base, the formation of the conjugate base of the acid causes a buffering effect to occur in the solution. We have our acid reacting with a base HA + OH- → A− + H2​O If any more base (OH-) is added, the HA can react with it If any acid (H3O+) is added, the A- can react with it A− + H3​O+ → HA + H2​O
42
What is the half equivalence point
The point when the volume of base added is HALF the volume needed for the equivalence point.
43
At the half equivalence point, what is equal
The concentrations of the acid and the conjugate base and equal.
44
What is the Henderson-Hasselbalch eq and what can it be used for
For the dissociation of any weak acid, HA(aq) --> H+(aq + A-(aq) pH = pKₐ + log([A-]/[HA]) Can be used to find the pH of a buffer solution, or solve problems involved with these titrations
45
What can we derive from the Henderson-Hasselbalch eq at the half equivalence point
At half e.p. --> The concentrations of the acid and the conjugate base and equal ie. [A-] = [HA] pH = pKₐ + log([A-]/[HA] --> pH = pKₐ + log(1) --> pH = pKₐ + 0 --> pH = pKₐ *Only at half equivalence point
46
A strong acid produces produces x, and a weak acid produces y. what is x and y
x = a weak or neutral conjugate base y = strong conjugate base
47
A strong base produces produces x, and a weak base produces y. what is x and y
x = a weak/neutral conjugate acid y = strong conjugate acid
48
What is produced when a strong acid is mixed with a strong base
Neutral salt
49
[H₃O⁺] = [OH-] means the solution is what
Neutral
50
[H₃O⁺] > [OH-] means the solution is what
Acidic
51
[H₃O⁺] < [OH-] means the solution is what
Basic
52
Which reactions go to completion and which ones do not reach completion 1) Strong acid + Strong base 2) Strong acid + Weak base 3) Weak acid + Strong base 4) Weak acid + Weak base
1 --> Completion 2 --> Completion 3 --> Completion 4 --> Doesnt reach completion
Chem y12
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