Login | Register
Enrol now for our new online tutoring program. Learn from the best tutors. Get amazing results. Learn more.

Welcome, Guest. Please login or register.

April 22, 2021, 08:47:22 am

Author Topic: Conductivity Graph  (Read 1140 times)  Share 

0 Members and 1 Guest are viewing this topic.

Jefferson

  • Forum Regular
  • **
  • Posts: 93
  • Respect: 0
Conductivity Graph
« on: November 05, 2019, 07:59:19 pm »
0
Source: additional NESA sample questions
https://www.educationstandards.nsw.edu.au/wps/wcm/connect/59826472-38f1-4ed7-8c8a-a3bb42842fd1/chemistry-2019-additional-sample-hsc-questions.pdf?MOD=AJPERES&CVID=

For Nesa's graph below, why is the conductivity initially increasing even before the equivalence point?
My understanding is that the concentration of ions would first decrease until equivalence point (H+ reacts with OH- to form H2O), then after there will be excess base molecules and thus OH- ions, hence conductivity should only increase AFTER equivalence point. This aligns with attachment 4.

Attachments:
1. Question part (a)
2. Question part (b)
3. Graph answer part (b)
4. ConquerHSC weak acid + strong base

louisaaa01

  • MOTM: NOV 19
  • Forum Regular
  • **
  • Posts: 82
  • Respect: +33
Re: Conductivity Graph
« Reply #1 on: November 05, 2019, 08:16:31 pm »
+1
Source: additional NESA sample questions
https://www.educationstandards.nsw.edu.au/wps/wcm/connect/59826472-38f1-4ed7-8c8a-a3bb42842fd1/chemistry-2019-additional-sample-hsc-questions.pdf?MOD=AJPERES&CVID=

For Nesa's graph below, why is the conductivity initially increasing even before the equivalence point?
My understanding is that the concentration of ions would first decrease until equivalence point (H+ reacts with OH- to form H2O), then after there will be excess base molecules and thus OH- ions, hence conductivity should only increase AFTER equivalence point. This aligns with attachment 4.

Attachments:
1. Question part (a)
2. Question part (b)
3. Graph answer part (b)
4. ConquerHSC weak acid + strong base

Hi Jefferson,

To my understanding, conductivity increases from the outset because citric acid, being a weak triprotic acid, will continue to dissociate as base is added (the dissociation equilibrium shifts to the right as base is added, by Le Chatelier's Principle, producing additional H+ and C6H5O73-) - this increases the concentration of ions in the solution, thus conductivity rises. However, since neutralisation is still taking place, and water is formed, the gradient will be lower compared to that beyond equivalence.

It is indeed a confusing concept as your graph, in addition to being from Conquer HSC, also aligns perfectly with what the Chemistry in Focus textbook suggests. I'm not sure how such a discrepancy would be dealt with at the Marking Centre.
« Last Edit: November 05, 2019, 08:22:00 pm by louisaaa01 »
2019 ATAR: 99.95

2020 - 2026

USYD BSci / Doctor of Medicine

Jefferson

  • Forum Regular
  • **
  • Posts: 93
  • Respect: 0
Re: Conductivity Graph
« Reply #2 on: November 09, 2019, 01:24:05 pm »
0
Hi Jefferson,

To my understanding, conductivity increases from the outset because citric acid, being a weak triprotic acid, will continue to dissociate as base is added (the dissociation equilibrium shifts to the right as base is added, by Le Chatelier's Principle, producing additional H+ and C6H5O73-) - this increases the concentration of ions in the solution, thus conductivity rises. However, since neutralisation is still taking place, and water is formed, the gradient will be lower compared to that beyond equivalence.

It is indeed a confusing concept as your graph, in addition to being from Conquer HSC, also aligns perfectly with what the Chemistry in Focus textbook suggests. I'm not sure how such a discrepancy would be dealt with at the Marking Centre.

Thank you,
That makes sense! :)