Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: brycebb on August 06, 2012, 08:44:54 PM
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The meals ready to eat in the military can be heated on a flameless heater. You can purchase a similar product called heater meals. Just pour water into the heater unit wait a few minutes and you have a hot meal. The source of energy is
Mg(s) + 2H2O(l) -----> Mg(OH)2(s) + H2(g)
Calculate the enthalpy change under standard conditions, in joules, for this reaction. What quantity of magnesium is needed to supply the energy required to warm 25ml (1.00g/ml) from 25C to 85C?
Now the first part I calculated -352.88kj for the enthalpy change, which was correct. Now when I get to the second part I have some trouble. Normally I would set it up as
q1+q2=0 Where the energy released and absorbed equal zero because both come to thermal equilibrium. So I proceed with naming q1 the water.
q1= (4.184J/g.C)(25g)(60C)= 6276J. So in order to increase 25g of water 60C, you need 6276J. Now this is where I am stuck, the problem doesn't give the specific heat capacity for the magnesium, so I looked it up and did this
q2= (1.20J/g.C)(x)(60)... now this is where I think i did something wrong, not only because when i do this i do not get the right answer, but because the temperature values for the magnesium were not given and heat is produced by the chemical reaction and not the differences in temperature. Can someone help me out with this problem? I feel I have to use the enthalpy change value to eventually get the amount of Magnesium required but do not know exactly what to do.
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The meals ready to eat in the military can be heated on a flameless heater. You can purchase a similar product called heater meals. Just pour water into the heater unit wait a few minutes and you have a hot meal. The source of energy is
Mg(s) + 2H2O(l) -----> Mg(OH)2(s) + H2(g)
Calculate the enthalpy change under standard conditions, in joules, for this reaction. What quantity of magnesium is needed to supply the energy required to warm 25ml (1.00g/ml) from 25C to 85C?
Now the first part I calculated -352.88kj for the enthalpy change, which was correct. Now when I get to the second part I have some trouble. Normally I would set it up as
q1+q2=0 Where the energy released and absorbed equal zero because both come to thermal equilibrium. So I proceed with naming q1 the water.
q1= (4.184J/g.C)(25g)(60C)= 6276J. So in order to increase 25g of water 60C, you need 6276J. Now this is where I am stuck, the problem doesn't give the specific heat capacity for the magnesium, so I looked it up and did this
q2= (1.20J/g.C)(x)(60)... now this is where I think i did something wrong, not only because when i do this i do not get the right answer, but because the temperature values for the magnesium were not given and heat is produced by the chemical reaction and not the differences in temperature. Can someone help me out with this problem? I feel I have to use the enthalpy change value to eventually get the amount of Magnesium required but do not know exactly what to do.
nevermind, I figured out the problem