matt b asked in Science & MathematicsChemistry · 8 years ago

# ΔH if given mass and Cp?

Hi i am having trouble on my chemistry homework. The question is: Cp for the calorimeter (including the HCl solution and the Mg) was calculated to be 0.232 kJ/°C . For the temperature curve shown above, 0.197 g Mg was reacted with HCl. What is the ΔH of the reaction? The initial and final temps of the curve they are talking about are 22, and 28 degrees C respectively. The initial time of reaction was 60s.

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• 8 years ago

Was 28C the highest temperature reached, or was it the final temperature? If it was the final temperature, then you should extrapolate that temperature back to the time at which the two solutions were mixed. That temperature would then be the actual final temperature.

So, you can calculate the heat released in the reaction as:

q = Cp (T2-T1)

q = 0.232 kJ/C X (28-22) = 1.39 kJ

Now, the reaction of the magnesium released that much heat.

Now, calculate the moles of Mg used: 0.197 g / 24.3 g/mol = 8.11 X 10^-3 mol Mg

So, per mole of Mg reacted:

DeltaH = -1.39 kJ/ 8.11X10^-3 mol = -171 kJ/mol