A student dissolves 12.1 g of potassium chloride (KCl) in 250. g of water in a well-insulated open cup. He then observes the temperature of the water fall from 21.0 °C to 17.1 °C over the course of 6.9 minutes. Use this data, and any information you need from the ALEKS Data resource, to answer the questions below about this reaction: KCl(s) → K (aq) + Cl (aq)

a. Is this reaction exothermic, endothermic, or neither?
b. Calculate the reaction enthalpy ΔH. rxn per mole of KCI.

a. Endothermic

b. 26.37kJ/mol

Explanation:

a. As we can see, the temperature of the water is decreasing when the reaction is occurring, that means the reaction is absorbing heat and is endothermic

b. To find the enthalpy we must find the change in heat when 12.1g of KCl are dissolved. Using the equation:

Q = -m*ΔT*C

Where Q is change in heat

m the mass of solution (250g + 12.1g = 262.1g)

ΔT is change in heat (17.1°C - 21.0°C = -3.9°C)

And C is specific heat of the solution (4.184J/g°C assuming is the same than the specific heat of water).

Replacing:

Q = -262.1g*-3.9°C*4.184J/g°C

Q = 4277J = 4.28kJ

As reaction enthalpy is the change in heat per mole of reaction, we must find the moles of 12.1g of KCl:

Moles KCl -Molar mass: 74.55g/mol-:

12.1g KCl * (1 mol / 74.55g) = 0.1623 moles KCl

The reaction enthalpy us:

4.28kJ / 0.1623mol = }