Determining an Enthalpy Change of Reaction

5/14/2018 Determining an Enthalpy Change of Reaction - slidepdf.com

http://slidepdf.com/reader/full/determining-an-enthalpy-change-of-reaction 1/7

Determining an Enthalpy Change of Reaction

Aim: to determine the enthalpy change for the displacement reaction:

By adding an excess of zinc powder it a measured amount of aqueous copper (II)

sulphate and measuring the temperature change over a period of time, you then

calculate the enthalpy change for the reaction.

Formula

This will give us heat which is opposite to enthalpy.

Diagram

25.5

00.00.00

TemperatureDisplay

Stopwatch

Zinc powder

Temperature

probe

Lid



Data Collection

Time (s) Temperature (°C) ± 0.1 °C

Trial 1 Trial 2 Trial 3

0 22.8 22.4 22.5

30 22.8 22.4 22.6

60 22.8 22.4 22.5

90 22.8 22.5 22.5

120 22.8 22.5 22.5

150 39.8 40.8 51.2

180 55.3 67.9 69.8

210 65.5 70.2 69.5

240 69.4 69.0 67.9

270 68.2 67.3 66.5

300 66.6 65.9 65.1

330 65.3 64.5 63.7

360 63.9 63.2 62.5

390 62.7 61.9 61.3

420 61.5 60.7 60.0

450 60.4 59.5 58.9

480 59.2 58.5 57.8

510 58.2 57.4 56.8

540 57.4 56.3 56.0

570 56.4 55.3 55.1

600 55.6 54.4 54.4

630 54.9 53.5 53.4

660 54.2 52.7 52.7



Qualitative Observations

690 53.6 51.9 52.0

720 53.0 51.1 51.3

This data was then

represented in 3 graphs,

1 for each trial. The

graphs showed the

change in temperature

over the time span. See

attached hand drawn

graphs.



Before reaction takes placeThe copper sulphate, mostly made up

of water, is a clear bright blue.

During reaction

As the zinc is added to the copper

sulphate, the solution begins to heat up

until it reaches a maximum at which

point it begins to cool down

After reaction takes place

After the reaction is complete we are

left with the products, reddish copper

and white zinc sulphate.

Data Processing

Graphs

On the graphs the red line represents the slope at the time of cooling or the rate

of cooling, and the green line indicates where the reaction began. The point at

which the two lines meet is the theoretical maximum temperature for that trial.

(see graphs). This is the max temperature value we will use for our calculations.

Trial 1 2 3

Max

theoretical

Temperature

found from

graph (°C)

74.0 74.0 72.8

Calcualtions

average max temp = 73.6°C

As we have two of the

same values for our

temperature (74.0) the

last value may be an

outlier but there is no

way to be sure of this

with only three trials so

we will need to include it

in our calculations.

Here we are finding the

average of our maximum

temperatures, to have

one final, more accurate

value for our enthalpy

change formula.In the f ormula we

need the change in temperature

therefore we must calculate theaverage minimum temperature

of the three trials. This is just

the lowest temperature

recorded for each trial.



average min temp = 22.6°C

= 51°C

Finding the Uncertainty

To find you subtract the

average minimum temperature

from the average maximum

temperature.

Then we use the heat equation

to find Q, assuming that the

copper sulphate is made up

completely of water and has a c

value of 4.18 KJKg-1

We used 25ml of copper

sulphate which equals 25g =

We want to find the heat

change per mole so we divide

by the number of moles

This is now the heat produce

per mole of copper sulphate

reacted.

We take the uncertainty of the

instrument and divide it by the

smallest measurement taken togive us the biggest uncertainty

then we add up the 2 (pipette

and temperature) and times by

100 to give us the percentage

uncertainty.



Final answer

Q= ± 0.44KJmol-1

But because we are looking for enthalpy change we just have to switch the sign

as enthalpy change is opposite to heat change.

Final value for Enthalpy change

Conclusion

When we compare our value of approximately -213KJmol-1 it is very close to the

accepted value of -217KJmol-1. However it is not within the uncertainty. When I

did the percentage yield formula I found that I was less than 2% off (only -1.76%). There was bound to be a difference as experimental conditions are

never perfect. Also I did wonder if because we used the specific heat capacity of

water and not copper sulphate if it made a difference.

Evaluation



Error Explanation Improvement

Heat loss

This was our major point of

error and probably the

main reason why our value

was below the literature

value. The calorimeter was

polystyrene but the top

would have let out a lot of

heat to the surroundings

that would not have shown

in our data as they would

not have warmed thetemperature probe

To improve this I would

proposed using 2 or maybe

3 calorimeters and placingthem inside each other and

then taping them together

airtight. This would add

insulation but also create

airspace, which would

insulate well. Then I would

cut out a polystyrene top

with a much smaller hole so

that the temperature probe

can fit more snuggly. This

should minimize heat loss

significantly and hopefully

give a more accurate value

for ∆H.

C value of water or

copper sulphate

I felt that by using the

specific heat capacity of

water for our calculations

we decreased the accuracy

of our result, as we did not

know the specific heat

capacity of copper sulphate.There is no way to know if

this made our value smaller

or larger.

The SHC of copper sulphate

is 4.184KJKG-1K-1. Obviously

this does not seem like it

would make a big difference

but I recommend that it wasused in future calculations

Stirring the zinc so

it reacts fully

When the zinc is added it

often gets stuck on the sides

and does not all react. It is

in excess but it may still

have affected our results

If this experiment were re

done I would recommend

larger quantities of both

copper sulphate and zinc to

make sure that they can be

mixed properly although

you will need to make sure

you are careful not to spill

them over the side.

Determining an Enthalpy Change of Reaction

Documents

Transcript of Determining an Enthalpy Change of Reaction