Determining Enthalpy Change of a Displacement Reaction

Topics: Zinc, Thermodynamics, Chemical reaction Pages: 7 (1501 words) Published: February 20, 2011
Determining an enthalpy change of a displacement reaction

To determine the enthalpy change for the displacement reaction between zinc powder and copper (II) sulphate solution. Zn (s) + Cu2+ (aq) → Cu2+ (s) + Zn2+ (aq)

For the displacement reaction, an excess amount of zinc powder is added to the measured amount of aqueous copper (II) sulphate. The temperature change over a period of time has to be measured and thus, the enthalpy change for the reaction can be measured.

The equation of the above reaction is:
Zn (s) + Cu2+ (aq) → Cu2+ (s) + Zn2+ (aq)

* Safety spectacles
* Polystyrene cup with lid
* Pipette, 25 cm3
* Pipette filler
* Weighing bottle
* Spatula
* Balance
* Thermometer, 0-100∘C (0.1∘graduations)
* Timer
* Beaker,
* 100 cm3 (X2)
* 600 cm3 (X1)
* 500 cm3 (X1)

* Zinc powder
* Copper (II) sulphate solution, 1.00M


Toxic Harmful

1) The pipette was washed with water first and then with copper (II) sulphate solution. 2) 25 cm3 of copper (II) sulphate solution was transferred in the polystyrene cup. 3) About 6 g of zinc powder was weighed in the weighing bottle. Since the zinc powder was in excess, there was no need to be accurate. 4) Thermometer was put through the hole in the lid, stirred and the temperature was recorded to the nearest 0.1∘C every half minute for 2½ minutes. 5) At precisely 3 minutes, the weighed zinc powder is transferred in the polystyrene cup. 6) Stirring was continued and temperature was recorded for an addition of 6 minutes. 7) The results were tabulated and a graph of change in temperature of the solution at different time intervals was plotted.

Time (min)| 0.0| 0.5| 1.0| 1.5| 2.0| 2.5| 3.0| 3.5| 4.0| 4.5| Temperature (∘C)| 20.4| 20.4| 20.4| 20.4| 20.5| 20.5| –| 63.4| 65.5| 65.7| Time (min)| 5.0| 5.5| 6.0| 6.5| 7.0| 7.5| 8.0| 8.5| 9.0| 9.5| Temperature (∘C)| 65.2| 65.1| 64.4| 63.5| 62.6| 61.9| 60.9| 60.6| 59.5| 58.6|

Change in temperature of the reaction mixture

It was assumed that the density of the solution was same as that of water i.e. 1 g cm-3 and the specific heat capacity of the solution (c1) was also as same as that of the water i.e. 4 200 J kg-1 K-1. Moreover, it was also assumed that there was no heat absorbed by the polystyrene cup, thus its specific heat capacity was assumed to be 0 J kg-1 K-1.

Mass of copper (II) sulphate solution (m1)
= 25 cm3 X 1 g cm-3
= 25 g
= 0.025 kg

Temperature change of the reaction mixture (ΔT)
= [(273+70) – (273+20)] K
= 50 K

Heat evolved
= m1c1ΔT
= 0.025 kg X 4 200 J kg-1 K-1 X 50 K
= 5 250 J
= 5.250 kJ

No. of moles of copper (II) sulphate solution
= 1.00 mol dm-3 X (25/1000) dm3
= 0.025 mol
Since copper (ii) sulphate solution is a limiting reagent and zinc powder is in excess; therefore, no. of moles of zinc
= 0.025 mol {mole ratio of zinc powder and copper (II) sulphate solution is 1:1}

Heat given out per mole of zinc and copper (II) sulphate solution is
= 5.250 kJ / 0.025 mol
= 210 kJ mol-1

Therefore, the enthalpy change for one mole of zinc and copper (II) sulphate solution is – 210 kJ mol-1.{Since it is an exothermic reaction, the value of enthalpy change
has a negative sign.}

The thermochemical equation for the reaction between zinc powder and copper (II) sulphate solution is: Zn (s) + Cu2+ (aq) → Cu2+ (s) + Zn2+ (aq)ΔH= –210 kJ mol-1

The reaction between zinc powder and copper (II) sulphate solution was a displacement reaction.

Since zinc is more reactive than copper. Zinc atoms loose electrons more easily than copper atoms. In this reaction when zinc was reacted with copper (II) sulphate solution, each zinc atom transfers two electrons to each copper (II) ion. Thus, copper (II) ions...
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