Question

1a )When 50.0 mL of 0.500 M HCl at 25.0°C is added to 50.0 mL of 0.500 M NaOH at 25.0°C in a coffee cup calorimeter, the temperature of the mixture rises to 28.2°C. What is the enthalpy of reaction per mole of acid? Assume the mixture has a specific heat capacity of 4.18 J/(g ? K) and that the densities of the reactant solutions are both 1.00 g/mL.

1b) Consider the following reaction: 2 PbS (s) + 3 O2 (g) ? 2 PbO (s) + 2 SO2 (g) ?H = -827.4 kJ How many grams of PbS have reacted if 975 kJ of heat is produced? 1c) Consider the following reaction carried out under constant pressure 3MgO(s) + 2NH3(g) ? 3H2O(l) + Mg3N2(aq) ? Hrxn = 5.78×102 kJ Calculate the heat associated with the complete reaction of 46.0 g of MgO with 17.1 g of NH3.

Answer 1

a)

Calculate the mass of reactants

since density is 1, then the number of ml will be equal to the number of grams

mass = density * volume

mass = 1 g/ ml * 50 ml = 50 grams, since both reactants have the same conditions then there 50 grams of each reactant so total mass is 50 + 50 = 100 grams

Now calculate the temperature diference:

28.2 - 25 = 3.2 C

Now apply the equatipn

Heat = mass * heat capacity * Temperature difference

Heat = 100 grams * 4.18 * 3.2 = 1337.6 Joules or 1.337 KJ

now calculate the moles of acid

moles = Molarity * volume = 0.5 M * 0.05 L = 0.025 moles

so 0.025 moles of acid releases 1.337 KJ so:

Heat rxn = 1.337 / 0.025 = 53.48 KJ / mol

b)

According to the reaction

2 PbS (s) + 3 O2 (g) ? 2 PbO (s) + 2 SO2 (g)

In this reaction

?H = -827.4 kJ

this means that 2 moles of PbS releases 827.4 KJ so

2 moles of PbS / 827.4 KJ * 975 KJ = 2.35 moles of PbS

molar mass of PbS is 239.26 , calculate the mass of reactant using this information

mass = moles * molar mass = 2.35 * 239.26 = 563.88 grams of PbS have reacted,