Question

Potassium nitrate, KNO3KNO3, has a molar mass of 101.1101.1 g/mol. In a constant-pressure calorimeter, 44.644.6 g of KNO3KNO3 is dissolved in 201201 g of water at 23.00 °C23.00 °C.

KNO3(s)H2O−−→K+(aq)+NO−3(aq)KNO3(s)→H2OK+(aq)+NO3−(aq)

The temperature of the resulting solution decreases to 17.70 °C17.70 °C. Assume that the resulting solution has the same specific heat as water, 4.184 J/(g⋅°C)4.184 J/(g·°C), and that there is negligible heat loss to the surroundings.

How much heat was released by the solution?

What is the enthalpy of the reaction?

Answer 1

Given, Mass of potassium nitrate = 44.6 g

Mass of water = 201 g

Mass of solution(m) = Mass of potassium nitrate + Mass of water = 44.6 g + 201 g = 245.6 g.

• As,the solution has the same specific heat as water. The amount of heat released by solution or calorimeter is given by

qcalorimater = mCpΔT where m= mass of solution, Cp= specific heat and ΔT= change in temperature

Cp = 4.184 J/(g⋅°C)

ΔT= 23 - 17.7 = 5.3 °C   

So,  qcalorimater= (245.6 g) (4.184 J/g-°C) (5.3 °C ) = 5446.23 Joules= 5.45 KJ

• We can see that temperature of the solution decreased because heat was released by the solution (q < 0) and this heat was absorbed by potassium nitrate solution when dissolved in water. Because the temperature of the solution decreased, shows that this reaction is endothermic. So enthalpy of reaction would also be positive that we need to give some heat to make happen this reaction
• ΔHrxn = qcalorimeter = 5.45 KJ

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