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Manhattan College

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Problem 108

Meals-ready-to-eat (MREs) are military meals that can be heated on a flameless heater. The heat is produced by the following reaction:
$$\mathrm{Mg}(s)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Mg}(\mathrm{OH})_{2}(s)+2 \mathrm{H}_{2}(g)$$
(a) Calculate the standard enthalpy change for this reaction. (b) Calculate the number of grams of Mg needed for this reaction to release enougy energy to increase the temperature of 75 mL of water from 21 to $79^{\circ} \mathrm{C}$ .

(a)$$-353.0 \mathrm{kJ}$$
(b)1.2 $\mathrm{gMg}$

## Discussion

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## Video Transcript

very guys were doing from one of eight in Chapter five chemistry Central Science it. So they want us to calculate the withstands the enter key for that reaction. So in this reaction, this is is reaction that any Jews in many meals ready to eat thes air military meals that could be heated on a flameless heater. This done because of the key producing reaction, were magnesium brexit quarter produced magnesium hydroxide and action gas. So remember again, do you have his law? Which is healthy reaction, physical devastation, product minus Delta Age of reacting like that and married delta H of hydrogen of each to and also magnesium that zero because that element in its standard state So magnesium hydroxide minus two nine hundred twenty four point seven, minus negative. Two hundred eight. Two times two hundred eighty five eight three Larry so that the age is equal to minus three five dory point zero four. Kill it, Jules Ramon. That's the That's the age for this reaction in minus three fifty three points here for killer Jules Permal, and now they want to know, calculate the number of grounds the envy needed distraction to release enough energy to whose to temperature of of the water that twenty five millions of water from twenty one degrees to twenty nine degrees. So let's add another sheet here. So so we have seven five million's water and the density of what is one gram per mil eater. That means that mass of water is equal to seventy five twenty five grams. Now we're going to his cue is equal to NC Delta T A. Q is going to be the heat release. Seventy five grand is gonna beat my ass for water. She is going to be four point one at a time. Four point one eight Jules Per Jules program killed him and Delta T. That's going to be there. Then be seventy nine. I'm minus twenty one. We don't have to worry about conversion because the difference in Degrees Celsius scene. It's a different than Calvin s. So Q is going to be equal to eighteen. Eighteen thousand and one hundred eighty three. The jewels, which is also going to equal eighteen point one eight three. Kill a Jules. Now we gotta find the number of malls, and then we can find Mole's at Maggie's, and then we could find the Master museum. So eighteen point one, it three over three five three point oh four. That's the same. Produce your point zero for two malls of magnesium. Now we're going to multiply that by the molecular weight two grand from all. So that's good. B B one point two six grams of magnesium is what's needed to produce that much heat.