2. Meals, Ready- to Eat (MREs) are military meals that can be heated on a flame-

Question

2. Meals, Ready- to Eat (MREs) are military meals that can be heated on a flame-les heater. The heat is produced by the following reaction: Mg(s) + 2H,0n) Mg(OH)(6)+Haa) a Caleulate the standard enthalpy change (Alxn) for the reaction. b. Calculate the mass of Mg (in g) needed for this reaction to release enough energy to increase the temperature of 25 mL of water from 15 C to 85° C. The density of water is 1.00 g/mL. 3. Given the information below, determine the ?? for the following reactions. b. 3A +B+6D +4F2G a. 6C-9A +3B c. 6A +2B +G+5C+3D+ 2F

Explanation / Answer

2.

(a)

deltaH0r = deltaH0f(Mg(OH)2) + deltaH0f(H2) - deltaH0f(Mg) - 2 * dletaH0f(H2O)

= - 924.8 + 0 - 0 - 2 * ( - 285.8 )

= - 353.2 kJ

(b)

Heat produced,

q = m * s * ( t2 - t1 )

q = 25.0 * 4.184 * ( 85 - 15 )

q = 7322 J

q = 7.322 kJ

From the balanced equation,

To release 353.2 kJ of heat it requires 1 mol (or) 24 g. of Mg

Then,

to release 7.322 kJ of heat, 7.322 * 24 / 353.2 = 0.498 g. of Mg

3.

(a)

Reverse the given equation and multiply by 3 to get equation (a).

Hence,

deltaH = - 3 * ( - 8 ) = + 24 kJ

(b)

Multiply equation (2) by 2 and add it with (1) to get the equation (b).

Hence,

deltaH = - * + 2 * ( 5 ) = + 2 kJ

(c)

Multiply (1) by 2 and add it with the reverse of equation (2) to get equation (c)

Hence,

dletaH = 2 * ( - 8 ) - 5 = - 21 kJ

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