V. Enthalpy of Redox Mass of magnesium added to calorimeter (g)O toa Moles of Mg

Question

V. Enthalpy of Redox Mass of magnesium added to calorimeter (g)O toa Moles of Mg (mol)--o or a nulm 2 Initial Temperature (019·"c Final Temperature () Molar mass of Mg 2430 glnol a0aawaar-- Temperature change C)5.7 Balanced Molecular Equation: Net lonic Equation: Calculations: Hredox (kJ/mol) ost-lab Questions: Why is it that the concentrated sulfuric acid was not ALL added into the calorimeter? In the enthalpy of neutralization, why is the number of moles of base used to calculate the H,neutralization?

Explanation / Answer

V. enthalpy of reaction,

mass of Mg = 0.102 g

moles of Mg = 0.102 g/24.305 g/mol = 0.0042 mol

dT of rection = 5.7 oC

Balanced molecular equation,

Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g)

net-ionic equation,

Mg(s) + 2H+(aq) --> Mg2+(aq) + H2(g)

Calculations,

dH (kJ) = -mCpdT

m = mass of solution in calorimeter (HCl + Mg)

since mass of HCl is missig, for calculations we would assume it to be 50 g (50 ml x 1 g/ml)

So,

dH = -50 x 4.18 x 5.7/1000 = -1.1913 kJ

dH(kJ/mol) = -1.1913 kJ/0.004 mol = -297.825 kJ/mol

Q's

Concentrated sulfuric acid has 2 reactive hydrogens instead of one as is the case with HCl, therefore not all sulfuric acid is added in this case.

In the enthalpy for neutralization reaction calculation, number of moles of base is used instead of acid. This is because base is present in lower moles in the reaction, that is once all the base is consumed, no more acid would react and reaction would stop. Base is thus a limiting reactant.

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