Hess PPQs

Question 1

In Q=mC delta t, what is the m

Answer 1

mass of WATER

Question 2

When doing Q/moles to get enthalpy, what do you do the moles of

Answer 2

reactant in excess

Question 3

how many significant figures does 56.4 have

Answer 3

3

Question 4

how many significant figures does 0.005 have

Answer 4

1

Question 5

If the hess cycle is to do with dissolving and anhydrous salts etc, when you add excess water to both the anhydrous and hydrous salt, what will the thing at the bottom of the cycle be?

Answer 5

XY (aq)

so it doesn’t have to be XY + H20 to show you’ve added water, it is just now an aqueous solution represented by the (aq)

Question 6

anhydrous to hydrous salt hess cycle Na2CO3 example - what went on the top and bottom?

Answer 6

top -

Na2CO3 (s) + 10H20 (l) ——————> Na2CO3*10H20
[^this is the anhydrous salt having water added to it]

bottom-

Na2CO3 (aq)
[this is the prdoct when both the reactant and product have had loads of water added]

Question 7

X loses some water of crystallisation when left in air, explain how the enthalpy using an old sample of X would compare to the value in the data book

Answer 7

• it would be a lower value
• because NaCO3 reacts exothermically with water (so if there is less water then less to react with so less exothermic air let off …)

Question 8

In a calorimetry experiment, explain how the mass of the fuel to burn would be prepared

Answer 8

measure the mass before and after (to get mass of alcohol burned)

Question 9

Wy do you need to measure the mass of the alcohol burn in a calorimetry experiment

Answer 9

so that when you do Q/moles you have the mass of the fuel (and the rfm) to work out the moles

Question 10

the results of this experiment are inaccurate due to heat loss, suggest another source of error

Answer 10

incomplete combustion

Question 11

Why might a bond enthalpy value be different to the data booklet

Answer 11

bond energy varies with environment