MOD 6 Flashcards
Which scientist proposed that “acids contain oxygen”? And what were the limitations of his model?
Lavoisier, supported by CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)
Limitations: HCl (does not contain oxygen but is acidic), NaOH and MgO (basic)
Which scientist proposed that “acids contain hydrogen”? And what were the limitations of his model?
Sir Humphrey Davy
Limitations: CH4 (methane is a base and not an acid) as well as Hydrogen.
What was Arrhenius’s model of acids and bases?
Acids dissociate to produce H3O+ ions in solution.
Bases dissociate to produce OH-ion in solution
What are the limitations of Arrhenius?
Could’nt explain:
- NH3 -> NH4 (ammonia dissociation)
- The discovery that Ammonia in its gaseous form could neutralise HCl.
- The Arrhenius theory couldn’t account for salts being acidic/basic rather than neutral. e.g. ZnCl₂ is acidic although NaCl is neutral.
Definition limited to aqueous acid and base interactions. However insoluble carbonates and metal oxides also neutralise acids, suggesting that acid/base behaviour was prevalent beyond just the aqueous medium.
What was the Bronsted Lowry model of acids and bases?
Acids donate a proton, Bases accept a proton
(conjugate pairs, BL acid/conj base and BL base/conj acid)
What was the Bronsted-Lowry definition able to explain?
- Acid/base reactions are not restricted to the aqueous media
- Could explain ammonia dissociation: NH₃ (g) + HCl (g) → NH₄⁺ (g) + Cl⁻ (g) → NH₄Cl (s)
- Furthermore, Bronsted-Lowry theory also explains the basic nature of substances that do not contain the hydroxide (OH-) ion.
- Explores hydrolysis of salts which explains why salts like ZnCl₂ (aq) are acidic and others are basic although NaCl(aq) is neutral.
- Explains the amphiprotic nature of some substances by considering that acidity and basicity a very relative characteristics (an acid is only an acid because the other substance is behaving as a base and vice versa)
What are the limitations of Bronsted-Lowry?
- BF3+NH3, ammonia can’t accept and Br cannot donate H+
- CoCl2, SO2, N2O4
What was the Lewis model of acids and bases?
Lewis acid – electron pair acceptor
Lewis base – electron-pair donor
What was the Lewis model able to explain?
Allowed acids and bases to be defined and identified even when there were no protons involved and no aqueous media present (explains BF3 + NH3) - The boron atom in BF3 is a Lewis acid because it accepts a lone pair from the nitrogen in NH3 which is a Lewis base, forming BF3NH3
Lewis’s theory is much broader than the Bronsted-Lowry theory and encompasses a much larger range of compounds. As a result, the Lewis theory identifies some substances as acids or bases which the Bronsted Lowry theory doesn’t.
While Brønsted and Lowry focused on the movement of protons, Lewis’ theory emphasised the movement of pairs of electrons.
How is red cabbage used as an indicator?
Natural acid−base indicators are found in plants such as red cabbage. Anthocyanin is the most readily available acid/base indicator; it is the plant pigment that makes red cabbage purple. Red cabbage extract turns a different colour in; concentrated acid, dilute acid, neutral solution, dilute base and concentrated base. It changes colour from red in acid solution to purplish to green in mildly basic solution to yellow in very basic solution.
Assess the effectiveness of red cabbage as an indicator
Effective in telling the concentration of acid or base, but ineffective in identifying strength
If the solution is dark, the colour change is not viable.
How to prepare and use a red cabbage indicator?
Preparation:
1. Chop a small red cabbage leaf into pieces
2. Grind up the pieces in a mortar and pestle
3. Boil for a few minutes in 100mL distilled water
4. Decant the cabbage extract into a clean 100mL beaker
Testing:
To determine the colour of the indicator over the full pH range from 1 to 14.
Place 2 mL of each NaOH and HCl into clean separate test tubes. Add a few drops of the indicator to each test tube until a definite colour is observed. Record the indicator and its colour. Repeat this with the other substances in separate test tubes, classifying them as acidic, basic or neutral (white vinegar, household ammonia, lemon juice, lemonade, bicarbonate soda, washing powder, antacid tablet, salt water)
What is a standard solution?
A standard solution must be used to determine the concentration of an ‘unknown’ substance and must have a highly accurate known concentration itself. A primary standard is the substance used to make a standard solution.
What is the method used to prepare a standard solution? Justify each step
- Dry analytical grade anhydrous Na2CO3 in an oven at 150 degrees and store in a desiccator to ensure it is free of moisture
Justification: The initial weighing would lead to an
overestimation of the concentration, due to impurities - Weigh the amount of dry primary standard that is required, in a clean 50mL beaker on an electronic balance to 3dp
Justification: The standard solution requires high
accuracy for the known concentration of at least 3sf. - Dissolve this mass of sodium carbonate by adding a small amount of distilled water using a wash bottle and stirring with a short fine glass rod. Always keep the glass rod in the beaker and ensure the solid is completely dissolved
Justification: Distilled water is free of impurities, If the
glass rod is removed some of the dissolved sodium
carbonate may also be removed. It is important for it to
be completely dissolved so that the calculated
concentration isn’t overestimated. - Pre-clean and rinse the graduated volumetric flask three times with distilled water
Justification: Distilled water is pure and doesn’t contain
dissolved impurities, the flask can be left wet as water will
be added later - Transfer the dissolved primary standard solution into a clean 250mL volumetric flask with the aid of a small glass funnel placed in the neck of the volumetric flask. Use the wash bottle to rinse the glass rod, funnel and beaker, adding these washings to the flask.
Justification: washings are transferred so no moles of the
primary standard are lost. This ensures accuracy in the
calculated concentration of the standard solution. - Add distilled water to the volumetric flask with the help of the small glass funnel until the water level in the volumetric flask is about 1 cm below the engraved mark on the flask.
- Remove the funnel and add distilled water drop by drop using a pasteur pipette until the bottom of the meniscus is aligned with the engraved mark. Avoid parallax error by making readings at eye level
- Stopper the flask and invert and swirl five times
- Label the solution, displaying the name and concentrations.
How to conduct a titration
- Clean and Rinse __ mL burette three times with distilled water then with the solution to be used in it prior to use. Discard the washings
Justification: ensures that the presence of leftover water droplets does not dilute the solution and make the experiment invalid. - Clamp the burette to the retort stand with a burette clamp
- Fill the burette with the standard solution using a small clean glass funnel to above the zero mark
- Turn the stopcock and let the solution drain from the burette into a waste beaker until the base of the meniscus is on the zero line and the solution fills up the tip of the burette
Justification: to test if stopcock is working - Draw up distilled water into a 25 mL bulb pipette with the aid of a bulb pipette filler and then drain to rinse the pipette filler. Repeat 3 times
- Transfer approximately 40mL of the analyte from the volumetric flask into a clean dry beaker and use the 25 mL bulb pipette to repeat the above rinsing process with about 10 mL of standard solution. Discard of all washings and the standard solution in the beaker
Justification: This is to prevent any contamination of the original solution from foreign materials in/on the pipette - Fill the 25 mL bulb pipette with 25 ml of standard solution by ensuring that the bottom of the meniscus lies on the engraved mark. Wipe the outside of the pipette to ensure no extra solution is transferred to the conical flask later. Preparation of pipette done
- Clean and rinse a conical flask 3 times with distilled water. The flask can be left wet as the number of moles of standard solution and analyte will be unaffected.
- Drain the aliquot of the standard solution to the conical flask with the aid of gravity, ensuring that the tip of the pipette rests against the inside glass wall of the flask.
Justification: this is to ensure that adhesive forces will allow the liquid to be fully transferred into the flask. Do not shake out or blow out the remaining drop in the pipette as manufacturers have calibrated for its presence - Add three drops of a suitable indicator to the conical flask containing the analyte, set both flasks above white paper/tiles
Justification: White - to allow the comparison of colour - Open the stopcock of the burette and slowly add titrant solution into a conical flask whilst swirling the flask until the required colour change of the indicator occurs
- Record the volume of acid added from the burette that achieved this endpoint. This is called the titre
- Repeat all steps until at least 2 readings are within 0.1 mL of each other
- Average the volume of the titre obtained to obtain a value for the titre volume.
