A student dissolves...

Question

A student dissolves 12.3 g of lithium chloride (LiCl) in 250. g of water in a well-insulated open cup. She then observes the temperature of the water rise from 22.0 °C to 30.3°C over the course of 7.9 minutes.

Use this data to answer the questions below about this reaction:

LiCl(s)→  Li+(aq)+  Cl−(aq)

You can make any reasonable assumptions about the physical properties of the solution. Be sure answers you calculate using measured data are rounded to the correct number of significant digits.

1. Is this reaction exothermic, endothermic, or neither?
2. If you said the reaction was exothermic or endothermic, calculate the amount of heat that was released or absorbed by the reaction in this case.
3. Calculate the reaction enthalpy ΔHrxn per mole of LiCl.

Thank you!

Answer 1

1. The temperature rises so the reaction is EXOTHERMIC, so the sign of the ∆H will be negative.
2. q = mC∆T where

q = heat = ?

m = mass = 250. g + 12.3 g = 263.3 g = 263 g

C = specific heat of water = 4.184 J/gº (assuming the solution has same specific heat as water)

∆T = change in temperature = 30.3º - 22.0º = 8.3º

q = (263 g)(4.184 J/gº)(8.3º) = 9133.25 J = 9130 J = 9.13 kJ of heat was released (negative value of q)

1. ∆Hrxn is calculated based on moles of reactant(s) present, so in this case we need to find out how many moles of LiCl are undergoing dissolution

12.3 g LiCl x 1 mol LiCl/42.39 g = 0.290 moles

∆Hrxn = 9.13 kJ/0.290 moles = 31.5 kJ/mole