A 0.471 g sample of magnesium (mg) is dropped into a constant pressure coffee-cu

Question

A 0.471 g sample of magnesium (mg) is dropped into a constant pressure coffee-cup calorimeter containing 20.0 g of dilute HCl (hydrochloric acid). The temperature increase from 21.3 degree C to 32.2 degree C as the magnesium reacts (Mg is completely consumed and HCl is in excess). The HCl solution has the same specific heat as water, 4.184 J/k degree C. Calculate delta H for this reaction ()in joules) and indicate whether this chemical process is endothermic or exothermic (circle one) Ethanol C_2 H_3OH (M = 46.05 g/mol). is mixed with gasoline and sold as gasohol. Heat is evolved when burning fuels, like gasohol. Use the following to calculate the energy (in kJ) produced when 2986.68 g of ethanol is burned

Explanation / Answer

Q1

m = 0.471 g of Mg

mol = mass/MW = 0.471/24.3 = 0.01938 mol of Mg

m = 200 g of HCl

mol = mass/MW = 200/36 = 5.5555 mol of HCl

dT = 32.2-21.3 = 10.9 °C

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

first, identify total heat:

Q = m*C*dT = 200*4.184*10.9 = 9121.12 J

then

mol reacted, assume Mg is limiting

HRxn = -Q/(mol) = -9121.12/0.01938 = -470646.026J = -470.65 kJ/mol

this must be exothermic, since it is increasing temperature

b)

mol of ethanol = mass/MW = 2986.68/46.05 = 64.857 mol of ethanol

so..

1 mol --> -1235 kJ

64.85 mol --> -1235*64.85 = -80089.75 kJ =

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