Question

a 0.258 g piece of potassium solid is placed inot water inside a coffee cup calorimeter resulting in a vigorous reaction. assume a total volume of 100 ml for the resulting solution. the temperature of the solution changes from 22 to 25.1 due to the reaction. how much heat in kj is generated per gram of potassium for this reaction? assume the density of the solution after the reaction is the same as the density of water

Answer 1

Answer:

5 kJ/g  

Explanation:

There are two energy flows in this reaction.

q₁ = heat from reaction

q₂ = heat to warm the solution

 q₁   +    q₂     = 0

m₁ΔH + m₂CΔT = 0

Data:

m₁ = 0.258 g

V₂ = 100 mL

  C = 4.184  J°C⁻¹g⁻¹

T_i = 22 °C

T_f = 25.1 °C

Calculations

(a) Mass of solution

$\text{Mass} = \text{100 mL} \times \dfrac{\text{1.00 g}}{\text{1 mL}} = \text{100 g}$

(b) ΔT

ΔT = T_f - T_i = 25.1 °C - 22 °C = 3.1°C

(c) ΔH

$\begin{array}{ccccl}m_{1}\Delta H & +& m_{2}C \Delta T& = &0\\\text{0.258 g}\times \Delta H& + & \text{100 g} \times 4.184 \text{ J ^{\circ} C ^{-1} g ^{-1} } \times 3.1 \, ^{\circ}\text{C} & = & 0\\0.258 \Delta H \text{ g} & + & \text{1300 J} & = & 0\\&&0.258 \Delta H \text{ g} & = & \text{-1300 J} & & \\& &\Delta H & = & \dfrac{\text{-1300 J}}{\text{0.258 g}}\\\\& & & = & \text{-5000 J/g}\\& & & = & \textbf{-5 kJ/g}\\\end{array}$

$\text{The reaction produces \large \boxed{\textbf{5 kJ}} per gram of potassium.}$

Note: The answer can have only one significant figure because you measured the initial temperature of the water only to the nearest degree.