# Whats the enthalpy change for this displacement reaction

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#1
Results from the displacement reaction:
Initial mass of weighing boat / g = 2.54
Mass of weighing boat and zinc / g = 4.57
Final mass of weighing boat / g = 2.55
Mass of zinc added / g = 2.02
Volume of aqueous CuSO4 used = 40 cm3
Temperature change / ºC = +29
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1 month ago
#2
I think you could you use the formula: q=mc∆t, (convert "q" into KJ by dividing by 1000) and then use the formula ∆H = q/n
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#3
Is the answer 65.4 kJ mol-1?
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1 month ago
#4
(Original post by celesteee04)
Is the answer 65.4 kJ mol-1?

Also the answer should be negative because the reaction caused a temperature rise
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#5
Mass of zinc powder dissolved = zinc added - final initial weighing boat mass
Zinc added = 4.57 - 2.54
= 2.03
Zinc after the reaction = 2.55 - 2.54
0.1
Mass of zinc that dissolved = 2.03 - 0.1
= 2.02
Initial temperature = 25
Final temperature = 54
Temperature rise = 54 - 25
= 29
From the results, 2.02g of zinc dissolved in 40cm3 of water and produced a temperature change of 29.
Enthalpy change = -(mass of water X specific heat capacity X temperature change)
= -(40 X 4.18 X 29)
= -4848.8
Enthalpy change for 2.02 g of zinc is -4.8 Kj
The enthalpy change for 1.0 mole of zinc = ( - 65.4 / 4.8 ) X 4.8 = 65.4 kJ mol-1
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1 month ago
#6
(Original post by celesteee04)
Mass of zinc powder dissolved = zinc added - final initial weighing boat mass
Zinc added = 4.57 - 2.54
= 2.03
Zinc after the reaction = 2.55 - 2.54
0.1
Mass of zinc that dissolved = 2.03 - 0.1
= 2.02
Initial temperature = 25
Final temperature = 54
Temperature rise = 54 - 25
= 29
From the results, 2.02g of zinc dissolved in 40cm3 of water and produced a temperature change of 29.
Enthalpy change = -(mass of water X specific heat capacity X temperature change)
= -(40 X 4.18 X 29)
= -4848.8
Enthalpy change for 2.02 g of zinc is -4.8 Kj
The enthalpy change for 1.0 mole of zinc = ( - 65.4 / 4.8 ) X 4.8 = 65.4 kJ mol-1
to work out the mol of zinc you need to do mass/molar mass --> 2.02 / 65.4=0.030887...
then divide q/n --> 4.8488 / 0.030887= 157kj INCLUDING THE NEGATIVE: final answer is -157kj mol-1

(depending on whether you rounded q, the answer may be slightly different- i kept it as 4.8488)
Last edited by alisha.h; 1 month ago
0
1 month ago
#7
(Original post by celesteee04)
Mass of zinc powder dissolved = zinc added - final initial weighing boat mass
Zinc added = 4.57 - 2.54
= 2.03
Zinc after the reaction = 2.55 - 2.54
0.1
Mass of zinc that dissolved = 2.03 - 0.1
= 2.02
Initial temperature = 25
Final temperature = 54
Temperature rise = 54 - 25
= 29
From the results, 2.02g of zinc dissolved in 40cm3 of water and produced a temperature change of 29.
Enthalpy change = -(mass of water X specific heat capacity X temperature change)
= -(40 X 4.18 X 29)
= -4848.8
Enthalpy change for 2.02 g of zinc is -4.8 Kj
The enthalpy change for 1.0 mole of zinc = ( - 65.4 / 4.8 ) X 4.8 = 65.4 kJ mol-1
All correct, but the last step, the mass of zinc that reacted was 2.02 g, so it's
-65.4/2.02 x 4.8 = ...

And you didn't need to find the mass of zinc again, it's already in the table of results. You know that 2.02 g reacted
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