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Thermochemistry with Heat of Reactions and Standard Enthalpies

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1. (Stoichiometry/Heats of Reactions)
a. How many kilojoules of heat energy are released when a 63.4-g sample of magnesium metal is completely converted to magnesium oxide according to the equation below?
b. How many grams of oxygen gas are used up during an enthalpy change of -292 kJ?
2 Mg(s) + O2(g) -> 2 MgO(s) delta H = -1204 kJ

2. (Measuring Heats of Reactions)
A 0.500-g sample of CaC2 is added to a coffee cup calorimeter containing 75.0 mL of water at 25.0°C. What is the final temperature of the mixture when the reaction shown below goes to completion? The solution has a density of 1.00 g/mL and a specific heat of 4.184 J/g?°C.

CaC2(s) + 2 H2O(l) -> Ca(OH)2(s) + C2H2(g) delta H = -127.2 kJ

3. (Stnd Enthalpies of Formation) Complete the following calculations:

a. Calculate the standard enthalpy change for the reaction shown below using standard enthalpies of formation.

H2O2(l) + COCl2(g) -> H2O(l) + CO2(g) + Cl2(g) delta H° = ?

b. Calculate the standard enthalpy of formation for B2H6(g) using standard enthalpies of formation and the reaction below.

B2H6(g) + 3 O2(g) -> B2O3(s) + 3 H2O(l) delta H° = -2147.5 kJ

Solution Summary

This solution contains step-by-step stoichiometry calculations using the heat equation and enthalpy change. All steps are shown for further understanding.

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1. (Stoichiometry/Heats of Reactions)
a. How many kilojoules of heat energy are released when a 63.4-g sample of magnesium metal is completely converted to magnesium oxide according to the equation below?
Sol: Atomic weight of Mg = 24.3 g/mol
Moles of Mg in 63.4 g sample = 63.4/24.3 = 2.61 moles.
2 moles of Mg would release 1204 kJ of energy.
Therefore, 2.61 moles of Mg would release 2.61*1204/2 = 1571.22 kJ of energy,

b. How many grams of oxygen gas are used up during an enthalpy change of -292 kJ?
2 Mg(s) + O2(g)  2 MgO(s) H = -1204 kJ
Sol: ...

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