When a solid dissolves in water, heat may be evolved or absorbed. The heat of di
ID: 588371 • Letter: W
Question
When a solid dissolves in water, heat may be evolved or absorbed. The heat of dissolution (dissolving) can be determined using a coffee cup calorimeter. In the laboratory a general chemistry student finds that when 1.62 g of CoCl2(s) are dissolved in 110.50 g of water, the temperature of the solution increases from 24.20 to 26.39 °C. Thermometer Cardboard or Styrofoam lid The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.84 J/oc Nested Styrofoam cups Reaction occurs in solutiorn 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 equal to the specific heat of water Hdissolution- kJ/mol Next 2Explanation / Answer
Ans. #1: Heat absorbed (or lost) by water is given by-
q = m s dT - equation 1
Where,
q = heat change
m = mass
s = specific heat
dT = Final temperature – Initial temperature
# Heat gained by calorimeter is given by-
q1 = C x dT - equation 2
where, C = Calorimeter constant
# Mass of solution = 1.62 g (mass of CoCl2) + 110.50 g (mass of water) = 112.12 g
It’s assumed that the specific heat of solution remains the same as that of pure water.
# Total heat gained by solution and calorimeter must be equal to the total amount of heat released during dissolution of CoCl2 –
Or,
Heat released during dissolution of CoCl2, -Q = q + q1
Or, -Q = 112.12 g x (4.184 J g-10C-1) x (26.39 – 24.20)0C + (1.84 J 0C-1) x (26.39 – 24.20)0C
Or, -Q = 1027.3510752 J + 4.0296 J
Hence, Q = -1031.3806752 J
The –ve sign of Q indicates that heat is being released during dissolution of CoCl2.
# Therefore, total heat released during dissolution of 1.62 g CoCl2 = -1031.3806752 J
# Moles of CoCl2 = Mass / Molar mass
= 1.62 g / (129.8386 g mol-1)
= 0.012477 mol
Now,
dHdissolution = Total heat released during dissolution / Moles of CoCl2
= -1031.3806752 J / 0.012477 mol
= -82662.36 J/ mol
= -82.66 kJ/mol
#2. # Mass of solution = 4.45 g (mass of NH4Cl) + 102.50 g (mass of water) = 106.95 g
It’s assumed that the specific heat of solution remains the same as that of pure water.
# Total heat gained by solution and calorimeter must be equal to the total amount of heat released during dissolution of NH4Cl –
Or,
Heat released during dissolution of NH4Cl, -Q = q + q1
Or, -Q = 106.95 g x (4.184 J g-10C-1) x (22.32 – 25.40)0C + (1.77 J 0C-1) x (22.32 – 25.40)0C
Or, -Q = -1378.234704 J + (-5.4516 J)
Hence, Q = +1383.686304 J
The +ve sign of Q indicates that heat is being absorbed during dissolution of NH4Cl.
# Therefore, total heat absorbed during dissolution of 4.45 g NH4Cl = 1383.686304 J
# Moles of NH4Cl = 4.45 g / (53.4912 g mol-1) = 0.08319 mol
Now,
dHdissolution = Total heat absorbed during dissolution / Moles of NH4Cl
= 1383.686304 J / 0.08319 mol
= 16632.84 J/ mol
= 16.63 kJ/mol
#3. # Mass of solution = 3.68 g (mass of KClO4) + 118.50 g (mass of water) = 112.18 g
It’s assumed that the specific heat of solution remains the same as that of pure water.
# Total heat gained by solution and calorimeter must be equal to the total amount of heat released during dissolution of KClO4 –
Or,
Heat released during dissolution of KClO4, -Q = q + q1
Or, -Q = 112.18 g x (4.184 J g-10C-1) x (22.06 – 25.00)0C + (1.66 J 0C-1) x (22.32 – 25.40)0C
Or, -Q = - 1379.9216928 J + (-4.8804 J)
Hence, Q = +1384.8020928 J
The +ve sign of Q indicates that heat is being absorbed during dissolution of KClO4.
# Therefore, total heat absorbed during dissolution of 3.68 g KClO4 = 1384.8020928 J
# Moles of KClO4 = 3.68 g / (138.5486 g mol-1) = 0.02656 mol
Now,
dHdissolution = Total heat absorbed during dissolution / Moles of KClO4
= 1384.8020928 J / 0.02656 mol
= 52138.63 J/ mol
= 52.14 kJ/mol
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