boaaambcalorimetry troisi 9nov

8/13/2019 BoAAAmbCalorimetry Troisi 9Nov

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First, the fuse wire will generate a bit of heat. So that the equation relating the Q of the reaction

with the increase of temperature should be modified by the amount of heat generates by the fuse

wire fe .

f calQ e T C

fe can be determined from the amount of fuse that has burned (see detail below).

A second complication is that the calorimeter can itself exchange energy with the environment. The

stirrer will cause a slight constant increase in the temperature of water before the reaction is

initiated as the mechanical work is transformed into heat (incidentally this is Joules observation that

led to the formulation of the first principle of

thermodynamics). Moreover, a few minutes after the

combustion has taken place, the temperature of water

may be still increasing because of the stirrer or it maydecrease because the slightly warmer calorimeter is

releasing heat to the surrounding environment. A typical

profile of temperature versus time is in figure 2 (see

section 5). For this reason the quantity T , that goes in

the formula above, cannot just be the increase of

temperature before and after the reaction (this value

keeps changing with time), but it is evaluated via a

simple procedure using the data points in a graph of

temperature versus time (described in section 5).

3. Relation between heat generated Q and the thermodynamic functions U and H

The reaction in the container has taken place at constant volume, not pressure. According to the first

principle of thermodynamics, considering that the container cannot do any work at fixed volume,

the measured heat is equivalent to the change of internal energy U and not the change in

enthalpy H . However, the relation between enthalpy and free energy is

H U pV

The variation of the product pV (pressure times volume) at a given absolute temperature Tcan be

computed using the law of gasses if we know the stoichiometry of the reaction. In fact pV nRT ( n

is the number of moles of gas and R the gas constant) and pV n RT, where n is the

variation in the number of moles between gaseous reactants and gaseous products. It is assumed

that the volume of the solid reactant is so small compared with that of the gas that it can be ignored.

In summary, the heat measured by the calorimeter is U of the reaction happening inside the

bomb, but the enthalpy of the reaction can be also computed via H U n RT .

Figure 1


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Pre-lab questions

1. Find the formula of benzoic acid(if not known) and express the balanced chemical reactionof combustion for this compound. Find out what the meaning of combustionis, in case it is

not known.

2.

What is the difference between enthalpy change ( H) and internal energy change (

U )when 1.00 g of this compound is combusted at 300 K.3. The heat capacity of 1 g of water at 14 C is 1.00 cal C-1. How many calories are needed to

warm up a litre of water by 3 C.

4. Express the relation between the units cal, Cal and kcal.5. Discuss briefly (less than 50 words) the possible risks associated with this experiment

5. Schedule of the experiment and samples to be considered

Divide yourself into two sub-groups. Each sub-group will perform two runs of the experiment. Each

of the four runs will analyse a different sample:

Run 1: benzoic acid in preformed pellets (available in a blue

bottle).

Run 2: sucrose (table sugar) pellet to be formed using the press

Run 3: a candy (defined as Candy A) provided or brought in the

lab by you

Run 4: another candy as above (defined as Candy B)

The initial set up in the schedule to the right consists into taking

what is needed from the drawer and performing step 1 of the

procedure. All other steps need to be performed each time. The

clear up step consists into leaving everything as it was found and

where it was found. The extra time needed by sub-group 1 to do

this operation is compensated by the fact that sub-group 2 has to

prepare a pellet sample (the instructions for that are given separately in a laminate). The detail of

the data analysis required is given in section 7. Additional questions to be answered in your report

and not requiring the data are in section 8. Note that the results of both groups are needed for the

lab report so you need to plan an efficient way to exchange data before the lab is finished. The

timing is only to give an idea.

5. Procedure (see also pictures at the end of these instructions)

The procedure is identical for calibration with benzoic acid pellets and measurements of new

samples (sucrose or candies) and so it is reported only once.

PreliminaryBomb Preparation

1. Install the thermistor by sliding it through the opening of the calorimeter cover. Adjust the length

of the thermistor to match the length of the stirring shaft. Tighten by hand, the nut to secure the

thermistor.


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6. Calculating the heat of combustion

The heat of combustion, the increase in temperature, and the heat capacity of the calorimeter are

related by

cal f Q T C e

where fe is the correction in calories for heat of combustion of fuse wire and is given by the length

of the burnt wire (in cm) multiplied by the predetermined constant 2.3 cal/cm. How to evaluate T

is explained below.

The equation above is used to determine Q if calC of the calorimeter in known and the property

of the sample are unknown. The same (rearranged) equation is used to determine calC of the

calorimeter when an experiment is run with a reaction that releases a known amount of heat Q in

the calibration of the instrument.

Standardization is carried out with pellets of benzoic acid, which are known to release 6318 cal/g.

T is approximately the increase of temperature after the ignition. However, as the temperature

changes continuously during the experiment we need a standardized procedure to make the

measurement more reproducible and to account for the extra heat given by the stirrer and released

by the bucket to the environment. The procedure is as follows:

We consider 5 special points in the plot of temperature versus time. As an example consider the

data in figure 2. The first four are simple:

(i) The point at time 0t =0min when the experiment begins; (ii) the point at time at =5 min when the

bomb is fired (iii) the point at time dt when the experiment was interrupted because for 5 minutes

the rate of temperature change was constant (iv) the point ct , five minutes before dt .

(v) The fifth point is defined at a time bt where 60% of the temperature rise between at and bt has

taken place.

A short table can be built with these data points (data are only to provide an example)

Data Point t / min T / C

0 0 24.340

a 5 24.420

b 6.3 26.292c 13 27.540

d 18 27.515

The standardized temperature variation can be computed from the formula

0

0

( ) ( )( ) ( )

( ) ( )

a d c

c a b a c b

a d c

T T T T T T T t t t t

t t t t

The heat of combustion should be reported per gram (for complex substances) or per mole (for pure

substances). The former is the specific heat of combustion the latter is the molar heat of

combustion.


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Figure 2. A typical run of the calorimeter.

7. To be included in your report

State the subgroup (1 or 2) you have been working in

(i) Describe in less than 100 words the scope of each of the four runs.(ii) Report all the data needed for the computation of the heat capacity or the heat of

combustion with appropriate error (for each run). A table like the one below can be

useful and compact (not compulsory). The error of each measurement should be

estimated.

(iii) Report the detail of the calculations made to compute heat capacity or heat ofcombustion, including error propagation (all runs).

(iv) Report the plot of temperature versus time only for the two runs your subgroup hasperformed and show in the plot the five points used to compute T .

(v) For each run report the specific heat of combustion and, when it can be computed, themolar heat of combustion.

(vi) Do notreport the procedure but you may include important or unusual observations(e.g. incomplete combustion, important data point missing, etc)

Run 1 Run 2 Run 3 Run 4

Sample Type

Sample Weight

Wire length

0t | 0T

at | aT

bt | bT

ct | cT

a

b

c d


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dt | dT

T

Q

calC

Comments

8. Questions to be answered in your report

You can search for any data you need to answers these questions on books, web etc Discussion

with other students is allowed.

1. A large portion of the calorimeter is constituted by ~2 Kg of water. Can you estimate theapproximate heat capacity of the calorimeter? Discuss if the value that you have measured is

acceptable.

2.

Describe possible sources of systematic errors in the bomb calorimetry experiment.3. Show that the formula used to compute T becomes simply the difference between thefirst and last measured point if the temperature is constant before ignition and after the

rising period.

4. Describe how the enthalpy (not the internal energy) of combustion of sucrose can beevaluated from the data you have collected (or you will collect).

5. A substance handled in air is easily contaminated. What is the systematic error in computingthe molar heat of combustion of sucrose if a sample is contaminated for 0.1% in weight by

atmospheric water.


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Figure 3. Photographs Illustrating of some steps of the procedure.

boaaambcalorimetry troisi 9nov

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