Theloshua
  • Theloshua
If 30.0 mL of 0.150 M aqueous sodium hydroxide is mixed with 30.0 mL of 0.150 M aqueous hydrochloric acid in a calorimeter at an initial temperature of 25.0 degrees Celsius, what is the enthalpy change of this reaction if the final temperature reached in the calorimeter is 27.5 degrees Celsius? NaOH + HCl yields NaCl + H2O
Chemistry
  • Stacey Warren - Expert brainly.com
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SOLVED
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jamiebookeater
  • jamiebookeater
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aaronq
  • aaronq
For calorimetry reactions, use the equation: \(q=m_{water}*C_p*\Delta T\) where m is the mass of water (or solution in this case) Cp is the specific heat capacity for the system (i would assume it to be the same as water here) \(\Delta T\) is change in temperature the system underwent
aaronq
  • aaronq
oh yea and q is the heat evolved (which is equal to the change in enthalpy under constant pressure, which I assume it's valid here)
Theloshua
  • Theloshua
okayy..... so..... wait

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aaronq
  • aaronq
lol so \(C_p=4.18 ~J/~ ^oC*g\) So we have \(\sf q=m*(4.18 ~J/~ ^oC*g)*(T_{final}-T_{initial})\) we need the mass and the temperatures..
Theloshua
  • Theloshua
ummm.... but water isnt involved in the reaction...
aaronq
  • aaronq
I know, but we're measuring the temperature of the water (solution), and that is how (indirectly) we're getting the change in enthalpy
Theloshua
  • Theloshua
but the solution isnt water XD so you dont know its heat capacity....
Theloshua
  • Theloshua
so isnt it irrelevant?
aaronq
  • aaronq
It's common in these problems to make the simplification that the specific heat capacity is equal to that of water, unless you're told otherwise... same with the density of the solution, it's equal to that of water
Theloshua
  • Theloshua
okayy.... I hope your right... :p
aaronq
  • aaronq
Unless you're told (in the question) otherwise, i'm right lol
Theloshua
  • Theloshua
okay
Theloshua
  • Theloshua
so ummmm hold on let me plug in the equation
Theloshua
  • Theloshua
what is the mass of the equation? :/
aaronq
  • aaronq
the mass of the solution, the density is assumed to be the same as water's so \(\sf mass=density*volume=1~g/mL*60~mL=60~g\)
Theloshua
  • Theloshua
im confused right now....
aaronq
  • aaronq
What are you confused about?
Theloshua
  • Theloshua
you took the density and volume of water to find the mass right?
aaronq
  • aaronq
yep, the definition for density is: \(\sf density=\dfrac{mass}{volume}\) so i just rearranged
Theloshua
  • Theloshua
but doesnt the density and volume of water depend on the amount? or no?
aaronq
  • aaronq
the density depends on a lot of things, most importantly temperature. Again, these problems make the simplification that the density of the solution is the same as that of water .. which is further simplified and rounded to 1 g/mL. The volume is given in the question
Theloshua
  • Theloshua
oh, you added the two volumes?
aaronq
  • aaronq
yeah
aaronq
  • aaronq
Thats the volume of the solution
Theloshua
  • Theloshua
okayyyy so \[Q = 60 \times 4.18 \times (27.5 - 25)\]
Theloshua
  • Theloshua
right?
aaronq
  • aaronq
yep. theres one final thing, because this is the energy absorbed by the water it is an endothermic process, but the reaction is exothermic as energy was released. \(\sf q_{absorbed}=-q_{released}\) you need to change the sign
Theloshua
  • Theloshua
oh okay, but what about the values given (0.250 M)?
Theloshua
  • Theloshua
where do those come in?
aaronq
  • aaronq
they don't, unless they want the molar enthalpy of the reaction. You'll come across questions that include more info than you need and you have to know what to use and what is irrelevant
Theloshua
  • Theloshua
okay, so hold on
Theloshua
  • Theloshua
The answer would be -627?
aaronq
  • aaronq
yep, just tack on some units to that
Theloshua
  • Theloshua
okay, thnx so much!!!!
aaronq
  • aaronq
no problem !

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