Acids And Bases
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Acid: Increase in concentration of H+ ions, equilibrium moves to the left. Alkali: OH reacts with H+ ions, equilibrium moves to the right (to replace the H+ ions) Concentration of H+ remains (almost) constant
Suggest sources of error in using the burette to carry out a titration.
• not removing funnel - droplet from funnel could enter beretta • air bubble in jet
During the titration, the inside of the conical flask is rinsed with distilled water. Suggest why rinsing improves the accuracy of the titre.
Ensures all reagents are able to react / mix / come into contact
Explain why adding this extra water does not change the volume of EDTA solution that is required in the titration.
The added water does not affect the mols / amount of reagents / reactants
Suggest a suitable piece of apparatus that could be used to measure out the sodium hydroxide solution in a titration. Explain why this apparatus is more suitable than a pipette for this purpose.
Burette Because it can deliver variable volumes
Use information from the curve in the figure above to explain why the end point of this reaction would be difficult to judge accurately using an indicator.
The change in pH is gradual / not rapid at the end point An indicator would change colour over a range of volumes of sodium hydroxide
Two solutions, one with a pH of 4.00 and the other with a pH of 9.00, were left open to the air. The pH of the pH 9.00 solution changed more than that of the other solution. Suggest what substance might be present in the air to cause the pH to change. Explain how and why the pH of the pH 9.00 solution changes
Co2 Ph decreases Acidic gas/reacts with OH-
Suggest why chloroethanoic acid is a stronger acid than ethanoic acid.
Cl is more electronegative so withdraws electrons Weakens O-H bonds
pH = 0.49
State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement.
Over time / after storage meter does not give accurate readings
Describe how you would obtain the pH curve for the titration.
The dissociation of methanoic acid in aqueous solution is endothermic. Deduce whether the pH of a solution of methanoic acid will increase, decrease or stay the same if the solution is heated. Explain your answer.
Decrease Equilib shifts to RHS To reduce temperature/oppose increase in temp
Which is an acid/base
BB
Identify which is base/acid
• AA • BA
pH [HA]=[A-] pH = pKa
Why is ethylamine a weak alkaline
Slightly dissociate Reaction/equilib lies to left or low [OH-]
State which is the stronger base, ammonia or ethylamine. Explain your answer.
Ethylamine Alkyl gorup is electron releasing Increases electron density of N(H2) - increased availability of lp
Give the formula of an organic compound that forms an alkaline buffer solution when added to a solution of ethylamine.
pH = 1.77
indicator/ conc HCL Colour for an alkali/white fumes