Answer to Question #140619 in General Chemistry for Celeste

Question

To determine the enthalpy change for a metal displacement reaction by adding a known mass of zinc powder to excess aqueous copper(II) sulfate solution and recording the rise in temperature.
Initial mass of weighing boat / g = 2.54
Mass of weighing boat and zinc / g = 4.57
Final mass of weighing boat / g = 2.55
Mass of zinc added / g = 2.02
Volume of aqueous CuSO4 used = 40 cm3
Temperature change / ºC = 29

Answer 1

To determine the enthalpy change for a metal displacement reaction by adding a known mass of zinc powder to excess aqueous copper(II) sulfate solution and recording the rise in temperature.

Initial mass of weighing boat / g = 2.54

Mass of weighing boat and zinc / g = 4.57

Final mass of weighing boat / g = 2.55

Mass of zinc added / g = 2.02

Volume of aqueous CuSO₄ used = 40 cm³

Temperature change / ºC = 29⁰C

Solution:

mass of zinc reacted = 2.02g.

Volume of aqueous CuSO₄ used = 40cm³ .

Temperature change (⁰C) = 29⁰C.

The reaction of Zn metal and CuSO₄ (aq) is given as

Zn(s) + CuSO₄ (aq) ---> ZnSO₄ (aq) + Cu(s)

Our first step will be to find the Heat produced due to displacement of copper by zinc.

Step 1: Find the heat produced due to the displacement of copper by zinc.

The formula which relates heat and specific heat capacity is

Q = m * s * ΔT; ------------------------------- Equation 1

where,

m = mass of water = volume of copper sulphate solution (cm³) = 40cm³.

s = specific heat capacity of the solution (water) = 4.184 J/mol-K.

ΔT = change in temperature = 29⁰C.

The density of water is 1.00g/cm³

So mass of water = 40cm³ * 1.00g/cm³ = 40g

Substitute, m= 40g, s = 4.184 J/g C, and ΔT = 29 ⁰C in the Equation 1, we have

Q = 40g * 4.184J/g⁰C * 29⁰C

Q = 4853.44J.

Step 2: To find the moles of zinc reacted in the reaction.

In the given experiment known amount of Zinc is used and CuSO₄ solution used is in excess.

So Zinc is the limiting reactant and we will use its moles for finding the molar enthalpy of reaction.

Atomic mass of zinc = 65.38g/mol

moles of zinc = 2.02g of Zn * 1mol Zn/ 65.38g Zn = 0.03090 mol Zn.

Step 3: To find the molar enthalpy of the reaction.

Molar enthalpy of displacement of copper by zinc = -Q/ (mol of Zn)

= - 4853.44J / 0.03090 mol Zn

= - 157088 J/mole

Convert this to ‘kJ/mol’ using 1kJ = 1000J.

Molar enthalpy (kJ/mol) = - 157088 J/1mole * 1kJ/1000J = -157.088 kJ/mol

Hence molar enthalpy for the displacement of copper by zinc is -157 kJ/mol