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a level chemistry question help - enthalpy change of solution / q = mct

hi guys, can someone explain how the mark scheme arrives at this number? i got 0.8... what formula did they use / rearrange and where did they get the values 4000, 10 and 58.5 from!!! thanksScreenshot_38.pngScreenshot_39.pngScreenshot_40.png
(edited 2 years ago)
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Original post by medicalmentality
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answer for a i is -370 btw if that helps
Original post by medicalmentality
hi guys, can someone explain how the mark scheme arrives at this number? i got 0.8... what formula did they use / rearrange and where did they get the values 4000, 10 and 58.5 from!!! thanksScreenshot_38.pngScreenshot_39.pngScreenshot_40.png

This is an ennthalpy of solution question.

When sodium chloride dissolves you have the equation:

NaCl(s) + (aq) ===> Na+(aq) + Cl-(aq) ...................... this is the enthalpy of solution

This can be broken down into two energetic steps:

NaCl(s) ==> Na+(g) + Cl-(g) ........................ this is the endothermic lattice enthalpy (i.e. positive)

and

Na+(g) + (aq) ===> Na+(aq) ....................... this is the hydration enthalpy
Cl-(g) + (aq) ===> Cl-(aq) ........................... this is the hydration enthalpy

So if you add the energy change of the three equations you will get the energy change of dissolution:

ΔH(solution) = ΔH(lattice enthalpy) + ΔH(hydration enthalpy of sodium ions) + ΔH(hydration enthalpy of chloride ions)

now simply substitute data
Original post by medicalmentality
hi guys, can someone explain how the mark scheme arrives at this number? i got 0.8... what formula did they use / rearrange and where did they get the values 4000, 10 and 58.5 from!!! thanksScreenshot_38.pngScreenshot_39.pngScreenshot_40.png

For part b)

You know the energy change per mol of NaCl dissolved.

You only have 10g so the mol = 10/58.5

This gives you the energy from 4.0 * 10/58.5 kJ

Then you apply the calorimetry equation:

E = mcΔT

4.0 * 10/58.5 = 0.21 * 4.2 * ΔT

or if you want to work in Joules and grams

4000 * 10/58.5 = 210 * 4.2 * ΔT

Then rearrange to make ΔT the subject

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