Chemistry Lab Report- Determining the Enthalpy of Enthalpy Change, H, for a Redox Reaction (
by
florianb (student)
Chemistry Lab Report- Determining the Enthalpy of Enthalpy Change, ΔH, for a Redox Reaction (DCP, CE Criterion)
Aim: The aim of this investigation is to experimentally determine the change in the enthalpy for the following redox (displacement) reaction:
Zn (s) + Zn2+(aq) ---->+ Zn2++ Cu (s)
Data Collection and Processing
Recording Raw Data:
Data Table:
Minimum temperature of trial 1: 21 °C
Maximum temperature for trial 1:59°C
Temperature difference of trial 1: 38 °C
Minimum temperature of trial 2: 21 °C
Maximum temperature for trial 2:59 °C
Temperature difference of trial 2: 38 °C
Processing Raw Data:
Average of Temperature rise (ΔT): 38°C+38°C 2 trials= 38°C
This value will be used to calculate the enthalpy change throughout this experiment.
Number of Moles (Cu + Zn2+) = Mass/Mr
Mass=Mass of CuSO4 Solution (g) (±0.05g) + Mass of Zinc Powder (g) (±0.05g)
Mass= 25.0g (±0.05g) +4.0g (±0.05g)
Mass= 29.0g (±0.05g)
Convert to Kg: 29.0g/1000= 0.0290Kg
Mr=
Cu-1; 63.55 x 1=63.55
S-1; 32.06 x 1=32.06
O-4 16.00 x 4=64.00
Total relative mass of = 63.55 + 32.06 + 64.00
Total relative mass of ethanol= 159.61= 159.6 (to 3 significant figures)
Number of Moles (Cu + Zn2+) = 29.0g159.6= 0.182 moles
Enthalpy change (heat energy change) (ΔH) measured in kJ per mole
Mass (m) measured in (Kg)
Specific heat capacity ...
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Number of Moles (Cu + Zn2+) = Mass/Mr
Mass=Mass of CuSO4 Solution (g) (±0.05g) + Mass of Zinc Powder (g) (±0.05g)
Mass= 25.0g (±0.05g) +4.0g (±0.05g)
Mass= 29.0g (±0.05g)
Convert to Kg: 29.0g/1000= 0.0290Kg
Mr=
Cu-1; 63.55 x 1=63.55
S-1; 32.06 x 1=32.06
O-4 16.00 x 4=64.00
Total relative mass of = 63.55 + 32.06 + 64.00
Total relative mass of ethanol= 159.61= 159.6 (to 3 significant figures)
Number of Moles (Cu + Zn2+) = 29.0g159.6= 0.182 moles
Enthalpy change (heat energy change) (ΔH) measured in kJ per mole
Mass (m) measured in (Kg)
Specific heat capacity (c) measured in (kJ per kg per kelvin)
Temperature change (ΔT) = the change in temperature from start to finish in degrees Celsius (°C)
In other words the equation can be written as:
ΔH= Mass (m) x Specific heat capacity (c) x Temperature change (ΔT)
The standard specific heat capacity of water is 4.18 kJ per kg per kelvin. This means that it requires 4.18 kilojoules (kJ) of energy to raise the temperature of one kilogram of water by one kelvin.
Change)
M (Mass) = 0.0290Kg
c (Specific Heat Capacity)= 4.18kJ
Temperature change (ΔT)= 38°C
∆H of (Cu + Zn2+)= 0.0290Kg x 4.18kJ x 38°C
∆H of (Cu + Zn2+)=4.606 kJ
The ΔH= -4.606 kJ because it is exothermic
Number of Moles (Cu + Zn2+) = Mass/Mr
Mass=Mass of CuSO4 Solution (g) (±0.05g) + Mass of Zinc Powder (g) (±0.05g)
Mass= 25.0g (±0.05g) +4.0g (±0.05g)
Mass= 29.0g (±0.05g)
Convert to Kg: 29.0g/1000= 0.0290Kg
Mr=
Cu-1; 63.55 x 1=63.55
S-1; 32.06 x 1=32.06
O-4 16.00 x 4=64.00
Total relative mass of = 63.55 + 32.06 + 64.00
Total relative mass of ethanol= 159.61= 159.6 (to 3 significant figures)
Therefore; Number of Moles (Cu + Zn2+) = 29.0g/159.6
Number of Moles (Cu + Zn2+) = 0.1817 moles
ΔH of combustion= ΔH /Number of moles
ΔH of combustion= 4.6060.1817 moles= 25.34kJ
Since this reaction is exothermic there is a negative sign in front of 25.34kJ,therefore ΔH= - 25.34kJ
Propagating Error:
Dealing with Errors:
Analogue instrument: half of the smallest unit readable
Digital instrument: the smallest unit readable
Water in calorimeter was measured by a beaker first.
Beaker: Analogue instrument- half of smallest measurement readable
Half of 1 g = +_ 0.5g
Multi meter measuring temperature: Digital instrument-smallest unit measurable
= 1.0°C
Weighting scale: Digital instrument-smallest unit measurable
=0.1g
%Uncertainty error of mass of Zn=(+_ 0.1g_4.0g) x 100%
= 2.5%
%Uncertainty error of mass of CuSO4 = (+_ 0.1g_25.0g) x 100%
=0.4%
Total %Uncertainty error of mass= %Uncertainty error of mass of Zn + %Uncertainty error of mass of CuSO4
Total %Uncertainty error of mass= 2.5% + 0.4%= 2.54%
%Uncertainty error of temperature change of Cu + Zn2+ (1°C/ 38.0°C) x 100%
=2.6%
Total %Uncertainty error of Cu + Zn2+=5.14%
5.14%= random error
This is the total random error caused by recording our results with the use of instruments, including the digital instruments and as well as the analogue instrument.
Observed value= -25.34 kJ per mole
Accepted value -217 kJ per mole
Total % Error=(observed value – accepted value) – accepted value
Total %Error = (25.43 –217) / 217 x 100= 88.2%
88.2%= Total % of experimental error
Total random error= 5.14% of 25.34 kJ per mole
=1.26 kJ per mole
Total experimental error > than random error
This means that there was a problem with the way the experiment was set up.
Conclusion and Evaluation:
Conclusion: In conclusion, the redox reaction, which we experimented with zinc and copper, was an exothermic reaction. The reason for this to happen, is that an incredible amount of heat was given off, which increased the temperature of the surroundings, from 21°C to 59°C. This signifies that there was a lot of energy released from the reaction. Also, as the temperature of the surroundings increased there was some lost in the energy from the system. We know that there was an excess in zinc, because there was some left overs with black powdery substance from the reaction.
Evaluating procedure(s):
:
There are weaknesses and limitations in this experiment. Systematic error, such as poor insulation of calorimeter has caused the results to be inaccurate in relation to the accepted value.
From the graph, we see that there was a temperature decrease, which logically means that the insulation from the aluminum foil was poorly made, and therefore, more room for error.
The hole made in the aluminum foil was not accurately made for least heat to get lost. However, since the hole was too big, there was some energy that had left between the calorimeter and the outside surroundings.
Random error: The precision of the devices we used to measure our data. Naturally, there are always uncertainties in the measuring devices we used; therefore, there is room for error for every measuring device used. However, this weakness is not major compared to the errors in relation to the calorimeter. S
Human error: Some insignificant errors include; the transfer of copper sulphate solution from the 25ml beaker to the calorimeter where maybe not all the solution went to the calorimeter and some droplets could have been left behind in the 25ml beaker. Also, the transfer of the zinc powder from the weighing boat to the calorimeter; where some zinc powder might be left over on the weighing boat.
Improving the experiment:
The weaknesses and limitation suggest that there are improvements, which can be made to improve this experiment.
For the closed system used, there was poor insulation for heat to not get lost. Therefore, a more sophisticated calorimeter with maximum insulation could prevent such heat loss. This could prevent energy to be transferred with the outside surroundings.
Instead of using a beaker, we could use a pipette to measure the amount of copper sulphate to be used. Also, for the zinc transfer from the weighing boat to the calorimeter, we could use a more solidified zinc powder. The reason for this is that it will be easier for to transfer the zinc to the calorimeter with minimal zinc being left behind.
