Which Changes Would Cause The Reaction To Become Darker Brown?

In this sit-in, students observe changes in the color and volume of an equilibrium mixture of dark-brown nitrogen dioxide and colourless dinitrogen tetroxide as it is compressed, heated or cooled. Use the experiment to introduce Le Chatelier's principle, or encourage students to apply the principle to make predictions

For this activeness, the demonstrator fills a gas syringe with an equilibrium mixture of chocolate-brown nitrogen dioxide and colourless dinitrogen tetroxide. The effect of force per unit area on the equilibrium is shown to be in accordance with Le Chatelier's principle past compressing the mixture and observing the modify in colour intensity.

Similarly, the effect of temperature is demonstrated past heating or cooling the mixture and observing the modify in colour, or the change in volume of the mixture compared with that of a similar volume of air.

The demonstration tin can be used to introduce Le Chatelier'southward principle or to ask students to utilise it in predicting the changes expected when force per unit area and temperature are changed.

Preparation must be carried out in a fume cupboard. The filled syringes may be brought out onto a lecture bench in a well ventilated room for better visibility provided they are sealed and do not leak when put under pressure. The nitrogen oxides involved are very toxic and students with animate bug should avoid inhaling them equally they could trigger an asthma attack.

A white background will profoundly enhance the visibility of the colour changes.

The time for conveying out the demonstration, including preparation of the dinitrogen tetroxide, should be about 30–40 minutes. More time will be needed for the quantitative extension selection.

Equipment

Apparatus

Eye protection (for the demonstrator)
Access to a fume closet
Clear glass gas syringes, 100 cm³, x2 or x3 (annotation 3)
Safe septum caps to fit syringe tips, x2 or x3 (note 4)
Plastic tubing, short lengths (optional; annotation 5)
Screw clips (Hoffmann) for rubber tubing, x2 or x3 (optional; note 5)
Boiling tube with a sidearm
Hard glass (borosilicate) boiling tube
One-holed rubber stoppers to fit the boiling tubes, x2
Glass tubing, short lengths, x2 (to fit stoppers and aptitude as shown in the diagram below)
Bound clip (Mohr) for rubber tubing (optional)
Iii-way stopcock or glass (or plastic) T-piece
Beaker, 400 cm³
Beakers, two dm³, x2
Thermometer (-10–110 °C)
Bunsen burner
Tripod
Gauze
Bosses, clamps and stands equally required

Chemicals

Lead(Ii) nitrate(5) (TOXIC, Unsafe FOR THE Environment), 10–twenty k (note half-dozen)
Crushed water ice, well-nigh 400 cm³
Common table salt (sodium chloride) or crushed rock salt, about 100 g
Light lubricating oil (such as WD-xl), a few drops

Health, condom and technical notes

Read our standard health and safety guidance.
Vesture eye protection and work in a fume cupboard.
Two or 3 gas syringes is ideal, simply the experiment tin be done with one.
The syringes should all be airtight nether pressure. If non, lubricate the plunger with a few drops of light mineral oil, such equally sewing auto oil or WD-40.
Instead of using septum caps to seal the syringes, a short length of plastic tubing carrying a screw clip tin be used to connect the syringe to the sidearm tube. The tubing must fit the syringe tightly plenty not to be dislodged when the gas is put nether force per unit area. Soften it by warming it in hot water before attaching it to the syringe. However, it must not fit the sidearm tube so tightly that it cannot easily be disconnected from it when the syringe has been filled.
Lead(Two) nitrate(V), Atomic number 82(NO₃)₂ (south), and lead(2) oxide, PbO(s), (both TOXIC, DANGEROUS FOR THE Surround) – meet CLEAPSS Hazcards HC057a and HC056.
The lead(II) nitrate should exist thoroughly dried by heating overnight in an oven set up to around 100 °C. Shop in a desiccator later heating unless used immediately.

The equation for the decomposition of lead nitrate is Pb(NO₃)₂ (s) → PbO(southward) + 2NO_ii (g) +ⁱ/₂ O_ii (1000).

1 g of lead(Ii) nitrate(5) should therefore produce nearly 150 cm³ of NO_two gas at room temperature and pressure. Adjust the mass of lead nitrate accordingly (but apply at least five chiliad) depending on the number of syringes of gas required, bearing in mind the number of times they are flushed (see below) earlier finally filling.

The atomic number 82(Ii) oxide remainder will soften the glass of the test tube and fuse with it. This combined residue should be nerveless and labelled as chancy waste, for eventual collection by a licensed contractor – come across CLEAPSS Hazcard HC056.
Nitrogen dioxide, NO₂ (thou), and dinitrogen tetroxide, North₂ O₄, (both VERY TOXIC) – come across CLEAPSS Hazcard HC068B.
Sodium chloride, NaCl(s) – meet CLEAPSS Hazcard HC047b.

Process

Before the demonstration

In a fume cupboard, ready the apparatus for drove of dinitrogen tetroxide shown in the diagram below. Stand the smaller chalice on a gauze on a tripod and clamp both of the test tubes near the top. Keep the length of the safe connecting tube as brusque as possible as nitrogen dioxide attacks safe.

It is vital that a syringe does not leak nether pressure level. Exam if a syringe is airtight by sealing information technology using a rubber septum cap, with near lx cm³ of air in the syringe. Hold the syringe vertically with septum cap resting on a business firm support, such as the bench surface. Push in the plunger to shrink the gas equally far every bit comfortably possible, agree for a few seconds, then release the pressure. The plunger should return to its original position if the syringe is closed. If non, cheque the septum cap, or supercede the syringe.
Mix the table salt with the crushed ice to produce a freezing mixture and place information technology in the chalice.

The demonstration

Working in a fume closet, heat the lead nitrate gently to decompose information technology to nitrogen dioxide (and oxygen). Heating too vigorously may decompose the nitrogen dioxide into nitrogen monoxide. As the gas meets the cold wall of the sidearm test tube, it condenses as liquid dinitrogen tetroxide, which may announced greenish in colour due to dissolved water if the lead nitrate was not absolutely dry. The oxygen is lost to the temper.
When nigh 2 cm³ of liquid has been collected, stop heating, tighten the screw clip and disconnect the tube containing the lead nitrate residue, leaving the screw prune sealing the tube continued to the receiver. The liquefied dinitrogen tetroxide can be kept for some fourth dimension in the freezing mixture, and so that this part of the experiment could exist done before the demonstration if desired.
Connect the sidearm of the tube containing the dinitrogen tetroxide to a syringe nozzle via the iii-way stopcock or T-slice and short lengths of safety tubing (meet diagram below). If a T-slice is used, the third outlet tin be opened or closed using a gloved finger (for preference, use nitrile gloves) or a brusque length of rubber tubing fitted with a spring clip.
Set the stopcock or T-piece to open the connection betwixt the syringe and the sidearm tube and partly fill up the syringe with the brown gas mixture by removing the water ice bath and gently warming the sidearm tube in your paw or a chalice of warm water. Every bit the nearly colourless dinitrogen tetroxide evaporates, it decomposes to course some dark-brown nitrogen dioxide.
Flush the first filling of gas out of the syringe into the fume cupboard using the three-way tap or the T-piece, and repeat the filling and flushing cycle two or three more times to ensure that in that location is no air in the system. Finally make full the syringe to the l–lx cm³ mark, release information technology from the connecting rubber tubing and rapidly seal it with a septum cap. Leave the tap or T-piece open to allow any excess gas to affluent out into the smoke closet (run across diagram beneath).

The effect of force per unit area on the equilibrium

In good view of the class and confronting a white background, hold the syringe with the septum cap resting on a hard surface. Compress the gas mixture quickly past pressing in the plunger of the syringe as far every bit it will comfortably go and hold it there for a few seconds. The colour of the gas mixture will initially become darker equally the concentration of the gases increases with the decrease in book. Within a few seconds, withal, it will become paler as the equilibrium responds to the increased pressure and brown nitrogen dioxide is converted into colourless dinitrogen tetroxide.
Pull the plunger out to its starting position. Annotation the colour changes as the pressure is decreased. They should be the reverse of the above.

The effect of temperature on the equilibrium

Make full one large beaker about two-thirds full with ice-cold water and another with hot water at virtually 60–70 °C.
Fill up one, ii or three syringes with 50–threescore cm³ of the dark-brown gas mixture equally above and seal them with septum caps. If only i syringe is used, let the class observe the colour of its contents against a white background, then place information technology in the beaker of hot water. The gas volition expand and become darker as the equilibrium adjusts to the higher temperature by converting dinitrogen tetroxide to nitrogen dioxide.
Transfer the syringe (or employ another syringe full of gas) into the common cold water. The gas volition contract but become lighter in colour as the equilibrium readjusts to the decrease in temperature. If 3 syringes of gas are used, ane tin be kept at room temperature for comparison.

Didactics notes

The gaseous equilibrium studied here is: Due north₂O₄(chiliad) ⇌ 2NO_two(g) ΔH^o = +58 kJ mol^–1

Le Chatelier's principle predicts that the equilibrium will move to the correct with an increase in temperature as the frontward reaction is endothermic. The brownish colour intensifies. The equilibrium constant, K_p, has a value of 48 atm at 400 K (127 °C), and the equilibrium volition lie almost completely over to the correct at 140 °C.

Increasing the pressure moves the equilibrium to the left as this decreases the number of moles of gas present in the mixture. The brown colour becomes paler.

Remind students that the value of K_p will increment with increase in temperature, whereas information technology will not be afflicted by changes in pressure despite a shift in the position of equilibrium.

A quantitative extension: comparison with ideal gas behaviour (optional)

By recording the volume of the gas mixture at intervals equally it is warmed up, a plot of volume vs temperature can be obtained – see graph below. This can exist compared with volume readings obtained from a syringe containing a similar book of air.

Clamp ii syringes, 1 containing the mixture of nitrogen oxides and one containing a similar volume of air (about l cm³), vertically in a 2 dm³ beaker of h2o on a tripod and gauze, then that they are immersed up to the 100 cm³ marking.
Notation the volume readings of both syringes, which should be as close as possible, and the temperature of the water. Rut the water gently with a Bunsen burner and tape the temperature and the volume of gases every 10 °C or so. Earlier taking each reading, remove the Bunsen burner and stir the water for a couple of minutes to ensure that the temperature of the gases is the same as that of the h2o. Twist the plungers of the syringes earlier taking each reading, to ensure that they are not sticking.
Continue taking readings until the temperature is between 70 to eighty °C. Plot graphs of volume against temperature for both gases on the same axes. The nitrogen dioxide/nitrogen tetroxide mixture volition expand more than air (see diagram below) equally the equilibrium responds to the increment in temperature by producing more than nitrogen dioxide. If at that place is fourth dimension take further readings as the water cools, to check for leaks.

This experiment can be done without a 2d syringe of air if necessary. The predicted volume of an platonic gas can exist worked out using Charles' Law for each temperature reading and compared with the observed one (encounter graph in a higher place). For example, if the volume of the gas mixture is l cm³ at 25 °C (298 K), the predicted volume of an ideal gas at fifty °C (323 G) would be l ten 323/298 = 54.2 cmⁱⁱⁱ. Students could lookout man the demonstration and plot the points equally the readings are taken. Alternatively they could do the Charles' Police calculations as the equilibrium mixture is warmed up.