q 25 2020 h&c chem = suffering (1 Viewer)

[izzyheslop]

Hi guys! I just wanted to make sure I fully understand the answer to question 25 from last years paper. Is it true that we can say the point of intersection between the two graphs is the equivalence point because it shows where the reaction goes from producing a specific amount of energy per mole of substance that is neutralised to the energy released during the dissociation of the basic substance?

 

[CM_Tutor]

Hi guys! I just wanted to make sure I fully understand the answer to question 25 from last years paper. Is it true that we can say the point of intersection between the two graphs is the equivalence point because it shows where the reaction goes from producing a specific amount of energy per mole of substance that is neutralised to the energy released during the dissociation of the basic substance?

The point of intersection of the two lines is the point at which citric acid and sodium hydroxide are present in their stoichiometric ratio. The increasing trend for lower volumes of citric acid is because the citric acid is the limiting reagent, hence more citric acid means more reaction and so a greater release of heat and a larger temperature change. Beyond this point, citric acid is present in excess and the sodium hydroxide is the limiting reagent, so more citric acid gives no more reaction, and so the temperature increase becomes constant. The temperature change is the same as the total volume being heated is the same because more citric acid means less water as each case has a total solution volume of 14.0 mL.