Question

The standard enthalpy change for the following reaction is -170 kJ at 298 K. 2 Cu(s) + 1/2 O2(g) Cu2O(s) ΔH° = -170 kJ What is the standard enthalpy change for the reaction at 298 K? 4 Cu(s) + O2(g) 2 Cu2O(s)

A student determines the heat of dissolution of solid cobalt(II) chloride using a coffee-cup calorimeter of negligible heat capacity. When 1.33 g of CoCl2(s) is dissolved in 111.00 g of water, the temperature of the solution increases from 25.00 to 26.84 °C. Based on the student's observation, calculate the enthalpy of dissolution of CoCl2(s) in kJ/mol. Assume the specific heat of the solution is 4.184 J/g°C.

Answer 1

1-

The standard enthalpy change (ΔH^o) of a reaction is the net heat absorbed or released in a reaction, Now for the given reaction-

2 Cu(s) + 1/2 O2(g) ----------------> Cu2O(s) ΔH° = -170 kJ

Now if we double the moles of the gases involved, then the heat change will also doubled. i.e

2 * [2 Cu(s) + 1/2 O2(g) ----------------> Cu2O(s)]   ΔH° = 2 * [-170 kJ ]

i.e

4 Cu(s) + O2(g) ----------------> 2 Cu2O(s) ΔH° = -340 kJ

2-

Now when we preform a dissolution reaction in water, generally heat is released from the reaction. Now this heat is absorbed by water. So the temp of the water increases. In this case

Heat released from the reaction (-Q_rxn) = Heat absorbed by water (Q_water)

Here the value of heat trasnfered will be the same. Only are opposite in signs

Now the heat absorbed by water is calculated by the formula-

(Q_water) = m * C * del T

where

m = mass of water taken = 111.00 g

C = specific heat of water = 4.184 J/g°C

del T = change in temp of water

= final temp - initial temp

= 26.84 °C - 25.00 °C

= 1.84‬°C

Now putting these values-

(Q_water) = m * C * del T

= 111.00 g * 4.184 J/g°C * 1.84‬°C

= 854.54 J

So heat released from the dissolution reaction = -854.54 J

Again mass of CoCl2(s) taken = 1.33 g

Then mols of CoCl2(s) taken = mass / molar mass of  CoCl2

= 1.33 g / 129.839 g/mol

= 0.0102 mols

So enthalpy of dissolution of CoCl2(s) = heat released from the dissolution reaction / mols of CoCl2(s) taken

= -854.54 J/ 0.0102 mols

= 83,778 J/mol

= 83.778 J/mol