Zinc metal reacts with hydrochloric acid according to the following balanced equation.

Zn (s) + 2HCl (aq) →ZnCl2 (aq) + H2 (g)

When 0.121 g of Zn (s) is combined with enough HCl to make 52.5 mL of solution in a coffee-cup calorimeter, all of the zinc reacts, raising the temperature of the solution from 21.8 ∘C to 23.9 ∘C. Find the Enthalpy change (in kJ/mol) for this reaction as written. (Use 1.0g/mL for the density of the solution and 4.18 J/g * Celcius as the specific heat capacity.)

The enthalpy change is - 249.1 kJ/mole

Explanation:

Step 1: The balanced equation

Zn (s) + 2HCl (aq) →ZnCl2 (aq) + H2 (g)

Step 2: Data given

mass of Zn = 0.121 grams

volume of the solution = 52.5 mL = 0.0525 L

Initial temperature = 21.8°C

Final temperature = 23.9 °C

Density of the solution = 1g/mL

Specific heat capacity = 4.18 J / g°C

Step 3: Calculating mass of solution

mass = density * volume

mass = 1g / mL * 52.5 mL = 52.5 grams

Step 4: Calculate heat transfer

Q = m*c*ΔT

with m = mass of the solution = 52.5 grams

with c = the specific heat capacity of the solution = 4.18 J/g °C

with ΔT = the change in temperature = final - initial temperature = 23.9 - 21.8 °C = 2.1 °C

Q = 52.5 grams * 4.18 J/g°C * 2.1 °C = 460.845 J

Step 5: Calculate number of moles of Zn

moles = mass of Zinc / Molar mass of Zinc

moles of Zinc = 0.121 grams / 65.38 g/mol = 0.00185 moles

Step 6: Calculate enthalpy change

ΔHrxn = heat / moles

ΔHrxn = 460.845 J / 0.00185 moles = 249105.4 J/mole = 249.1 kJ/mole

Since reaction is exothermic, ΔHrxn is negative.

ΔHrxn = - 249.1 kJ/mole