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 22.24 g of Cs2SO4(s) are dissolved in 106.70 g of water, the temperature of the solution drops from 25.62 to 22.57 °C. The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.65 J/°C. Based on the student's observation, calculate the enthalpy of dissolution of Cs2SO4(s) in kJ/mol. Assume the specific heat of the solution is equal to the specific heat of water.

26.85kJ/mol is the heat of dissolution of Cs₂O₄

Explanation:

The process of dissolution of Cs₂SO₄ in water occurs as follows:

Cs₂O₄(s) → 2Cs⁺(aq) + SO₄²⁻(aq) + ΔH

Where ΔH is the change in heat per mole of Cs₂O₄

The first we can see is ΔH > 0 because heat is absorbed (Temperature is decreasing) when the reaction occurs.

Now, the change in heat of reaction is:

q = Heat calorimeter + Heat solution

q = ΔT*1.65J/°C + S*m×ΔT

Where q is the heat of reaction.

ΔT is change in temperature: 25.62°C - 22.57°C = 3.05°C

S is specific heat of solution = Specific heat water = 4.184J/g°C

m is mass of solution = 22.24g + 106.70g = 128.94g

Replacing, heat of reaction is:

q = 3.05°C*1.65J/°C + 4.184J/g°C*128.94g×3.05°C

q = 1650J are absorbed when 22.24g of Cs₂O₄ reacts

Moles of 22.24g of Cs₂O₄ are - Molar mass: 361.87g/mol-:

22.24g * (1mol / 361.87g) = 0.06146 moles

That means, when 0.06146 moles of Cs₂O₄ react, the heat absorbed is 1650J. That means the heat absorbed per mole of Cs₂O₄ (Enthalpy of dissolution) is:

1650J / 0.06146 moles = 26847J / mol =

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