Heat of Combustion: Magnesium PRE-LAB LECTURE and PRE-LAB The enthalpy (AH) is d

Question

Heat of Combustion: Magnesium PRE-LAB LECTURE and PRE-LAB The enthalpy (AH) is defined as the proceeds from heat lost or gained as a reaction reactants to products at constant as part of a thermochemical equation, such pressure It is often given asthat shown for the combustion of hydrogen (1) 572 kJ sign for AH indicates that this reaction is exothermic. Thus, when two moles of hydrogen gas and one mole of oxygen gas combine to form two moles of liquid water, S2 of heat energy is released. Conditions of constant atmospheric pressure (reaction vessel is an open container) and near room temperature (2s c) are assumed in the above value, although heats of reaction do not vary appreciably with temperature. A calorimeter is a device for measuring heat flow (q), one carries out the reaction in the calorimeter and measures the temperature change (At). The equation for calculating heat change is given by q m s At where m is the mass of the substance in grams and s is the specific heat of the substance. The unit of q is u) joule or k, and the unit for s is J/g c. Assuming ideal adiabatic conditions (no heat loss), for exothermic reactions all of the heat produced in the reaction goes into warming the calorimeter and the solution placed in the calorimeter. Because no heat enters or leaves the system throughout the process, the heat change of the system must be zero, and we have where qcal and qren are the heat changes for the calorimeter and the reaction, respectively. Thus, This means that the heat released by the reaction will be equal to the heat absorbed by the solution in the calorimeter However, in the lab, calorimeters have heat leaks. Plus the thermometers cannot respond quickly enough to give an accurate indication of the temperature at the time of mixing (performing reaction) and shortly thereafter. As a result of heat losses and thermal lag, a time-temperature curve is plotted to determine the actual temperature of the mixture at the time of mixing.

Explanation / Answer

The equations are:

(I) : Mg(s) + 2HCl (aq) ---> MgCl2 (aq) + H2(g) ; dH1 = -436.9 kJ/mol

(II) : MgO(s) + 2HCl (aq) ---> MgCl2 (aq) + H2O(l) ; dH2 = -121.3 kJ/mol

(III) : 2H2 (g) + O2(g) ---> 2H2O (l) ; dH3 = -572 kJ/mol

Next, multiply (I) and (II) by 2.

The, reverse the (II) reaction and the sign of dH2. The result is:

(I) : 2Mg(s) + 4HCl (aq) ---> 2MgCl2 (aq) + 2H2(g) ; dH1 = -436.9 kJ/mol

(II) : 2MgCl2 (aq) + 2H2O(l) ---> 2MgO(s) + 4HCl (aq) ; dH2 = +121.3 kJ/mol

(III) : 2H2 (g) + O2(g) ---> 2H2O (l) ; dH3 = -572 kJ/mol

Now, when the above three reaction are added, we get:

(III) : 2Mg (s) + O2(g) ---> 2MgO (s) ; dH4 = -436.9+121.3-572 = -887.6 kJ/mol

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