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wizzkids
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This is how you unpack Question 19 -
Step 1. Identify which acids are monoprotic, diprotic or triprotic etc.
Step 2. Write the balanced chemical equation for each neutralisation.
Step 3. Figure out if there is a limiting reagent, using the standard methods of chemical stoichiometry.
Step 4. Calculate how many moles of protons are going to be neutralised in each chemical reaction, and select the reaction with the highest number of moles.
If you want to go further, you could multiply the number of moles by 56 kJ/mol (because that's how much energy is released by neutralising 1 mole of protons).
For @=)(=
Regarding the enthalpy of ionisation of ethanoic acid in (d), yes there is an enthalpy change here of about +5kJ/mol, but it is negligible compared to the enthalpy of neutralisation.
Step 1. Identify which acids are monoprotic, diprotic or triprotic etc.
Step 2. Write the balanced chemical equation for each neutralisation.
Step 3. Figure out if there is a limiting reagent, using the standard methods of chemical stoichiometry.
Step 4. Calculate how many moles of protons are going to be neutralised in each chemical reaction, and select the reaction with the highest number of moles.
If you want to go further, you could multiply the number of moles by 56 kJ/mol (because that's how much energy is released by neutralising 1 mole of protons).
For @=)(=
Regarding the enthalpy of ionisation of ethanoic acid in (d), yes there is an enthalpy change here of about +5kJ/mol, but it is negligible compared to the enthalpy of neutralisation.
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wizzkids
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Question 30.
Think about the pH function mathematically. It says pH = - log [H]
Double the concentration [H] and the pH only changes by - log[2] which is -0.3 pH units.
What this means is the pH curve is quite insensitive at the low pH end, so you can't rely on the drawn shape of the pH curve at very low values. The most reliable part of the pH curve is the part close to the equivalence point.
There is a second factor which comes into play at high concentrations, which is some strong acids are not fully dissociated at high concentrations, but they are fully dissociated at low concentrations.
Anyway, the lesson is - don't rely on the initial part of the acid neutralisation curve for quantitative answers.
Think about the pH function mathematically. It says pH = - log [H]
Double the concentration [H] and the pH only changes by - log[2] which is -0.3 pH units.
What this means is the pH curve is quite insensitive at the low pH end, so you can't rely on the drawn shape of the pH curve at very low values. The most reliable part of the pH curve is the part close to the equivalence point.
There is a second factor which comes into play at high concentrations, which is some strong acids are not fully dissociated at high concentrations, but they are fully dissociated at low concentrations.
Anyway, the lesson is - don't rely on the initial part of the acid neutralisation curve for quantitative answers.
wizzkids
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- Joined
- Jul 13, 2016
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- 1998
Question 11.
Remember: At the equivalence point, there are still reactants and products in the flask. Don't think that because you have reached the equivalence point that the reaction has gone to completion.
No, it hasn't.
One of the most important techniques about titraton is to slow down when nearing the equivalence point, and not to over-shoot, because you don't want the reaction to go to completion. Think about it.
What you have done in titration is to shift the chemical equilibrium to a known value of hydrogen ion concentration, as shown by the change in the chemical indicator, and from knowledge of the amount of standard solution that was added (the titre), you can deduce the amount of moles of the unknown substance.
Remember: At the equivalence point, there are still reactants and products in the flask. Don't think that because you have reached the equivalence point that the reaction has gone to completion.
No, it hasn't.
One of the most important techniques about titraton is to slow down when nearing the equivalence point, and not to over-shoot, because you don't want the reaction to go to completion. Think about it.
What you have done in titration is to shift the chemical equilibrium to a known value of hydrogen ion concentration, as shown by the change in the chemical indicator, and from knowledge of the amount of standard solution that was added (the titre), you can deduce the amount of moles of the unknown substance.
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