ocr as level paper question

Solid ammonium thiocyanate, NH 4 SCN, reacts with solid barium hydroxide, Ba(OH) 2 , as shown in the equation below

2NH 4 SCN(s) + Ba(OH) 2 (s) Ba(SCN) 2 (s) + 2H 2 O(l) + 2NH 3 (g

A research chemist carries out an experiment to determine the enthalpy change of this reaction.

(Diagram)

boiling tube mixture of ammonium thiocyanate and barium hydroxide insulated beaker thermometer 50.0g of water In the experiment, 15.22 g of NH 4 SCN is reacted with a slight excess of Ba(OH) 2 . The reaction absorbs energy, cooling the 50.0 g of water from 21.9 °C to 10.9 °C.


(i) Calculate the energy absorbed, in kJ, during this reaction. The specific heat capacity of water = 4.2 J g –1 K –1 . energy =

................................ ......................kJ [2]

(ii) Calculate the amount, in moles, of NH 4 SCN used by the research chemist. amount = ................................ ................... mol

(iii) Calculate the enthalpy change of reaction. Include the sign in your answer. Give your answer to two significant figures.


On question 3b iii) in the mark scheme why do they multiply 2.3 divided 0.2 by 2?

Paper is Jan 2011