I have been very lucky. I have always found these halos visually exhilarating so I produced a poster about them for the 2019 Royal Society of Chemistry Twitter Poster competition in teh Chemcial Education Category .Then this came through the emails. We are delighted to let you know that you have been awarded Further Education prize in #RSCEdu for your poster entitled; "Chemical halos - the beauty of chemistry" The subject chairs thought your poster really stood out and contained some fantastic science – congratulations! |
If you you would to see how you and your students can make these halos and how they can help in teaching more about how cheksiry works, then read on.
Catching the students’ imagination through practical work is one reason teachers do it. Going that extra mile/metre and using the evidence before your eyes to delve into the inner workings of chemical processes in another matter. It requires both teacher and students to work with very tiny particles which you cannot see. However, you can see the effects they are having during chemical reactions. Letting the particles work for you though microchemistry techniques can be far more rewarding than just a "magical "event. I can still remember being excited (and I was in my 60s at the time ―sad, I know) in seeing the first alkali/acid halo.
Here is parade of halos for you.
Here is parade of halos for you.
The effect works because gases evaporate from solution (ammonia) or are evolved via a chemical reaction (sulfur, nitrogen or carbon dioxide). The gas particles (molecules) move through all the oxygen and nitrogen molecules in air within the Petri dish and finally enter the puddle.
Diffusion of gas molecules
We need have to get this into perspective. The molecules at 1atm and 300K are moving on average at speeds of 500metres per second (over 1100 miles per hour). There will be 1,000,000,000 to 10,000,000,000 collisions per second.
Stop a moment and let those figures sink in. That velocity is as fast as a bullet from a rifle, double the speed of an airliner). It would take you between 30 and 300 years to count that number of collisions at the rate of one a second.
The first diagram below represents ammonia diffusing; the first is a simplistic view of brown ammonia molecules diffusing from the container of ammonia solution to the puddle; this is typical of what you might find in a text book. The second diagram puts in 80 nitrogen molecules and 20 oxygen molecules. However this not a true reflection of the situation. I would have to draw 100 versions of the third diagram to ensure that an ammonia molecule is present as shown in the last diagram.
Stop a moment and let those figures sink in.
As well as the invisibility of the particles to our eyes, the students and teachers are dealing with enormous numbers of molecules of oxygen and nitrogen that the ammonia has to diffuse through.
Diffusion of gas molecules
We need have to get this into perspective. The molecules at 1atm and 300K are moving on average at speeds of 500metres per second (over 1100 miles per hour). There will be 1,000,000,000 to 10,000,000,000 collisions per second.
Stop a moment and let those figures sink in. That velocity is as fast as a bullet from a rifle, double the speed of an airliner). It would take you between 30 and 300 years to count that number of collisions at the rate of one a second.
The first diagram below represents ammonia diffusing; the first is a simplistic view of brown ammonia molecules diffusing from the container of ammonia solution to the puddle; this is typical of what you might find in a text book. The second diagram puts in 80 nitrogen molecules and 20 oxygen molecules. However this not a true reflection of the situation. I would have to draw 100 versions of the third diagram to ensure that an ammonia molecule is present as shown in the last diagram.
Stop a moment and let those figures sink in.
As well as the invisibility of the particles to our eyes, the students and teachers are dealing with enormous numbers of molecules of oxygen and nitrogen that the ammonia has to diffuse through.
Ammonia reaches the puddle of water
The ammonia molecules have been buffeted by air molecules and moved to the out parts of the puddle where they encounter water molecules. The NH₃ molecules become incorporated because there is intermolecular attraction between a water and ammonia molecules (see diagram on the right). This is electrostatic attraction. It arises because the molecular array has reduced its potential energy via this attraction. We give all sorts of names to these electrostatic attractions but they exist between particles from the structure to the atom to the extremely week induced dipole effects between inert gas molecules.
The ammonia molecules have been buffeted by air molecules and moved to the out parts of the puddle where they encounter water molecules. The NH₃ molecules become incorporated because there is intermolecular attraction between a water and ammonia molecules (see diagram on the right). This is electrostatic attraction. It arises because the molecular array has reduced its potential energy via this attraction. We give all sorts of names to these electrostatic attractions but they exist between particles from the structure to the atom to the extremely week induced dipole effects between inert gas molecules.
So how do the numbers add up here? You can detect the odour of ammonia at 1 ppm, that is, one part in a million. The picture on the left below shows one green ammonia molecule with 1000 molecules of water. I would have had to draw 1000 versions of the diagram below right to find that diagram on the left with the "green" ammonia molecule.
Stop a moment and let those figures sink in.
Ammonia in water is quite lonely. Many of our diagrams in textbooks give the wrong impression.
Stop a moment and let those figures sink in.
Ammonia in water is quite lonely. Many of our diagrams in textbooks give the wrong impression.
Ammonia is alkaline in water
Ammonia in water is weakly alkaline because not all molecules react with water to form ammonium and hydroxide ions. However, I am interested in the hydroxide ions because they are the ions that will ultimately react with the indicator molecule and change the colour on the outer reached on the puddle. On average, there is one hydroxide ion for every 240 ammonia molecules entering the water.
So, I would have to draw the water diagram 240 000 times to find 240 molecules of ammonia in which ONE will have dissociated ammonium and hydroxide ions
Stop a moment and let those figures sink in.
The presence of ammonium and hydroxide ions are even more remote. I admit I have not addressed the dynamic nature of this relationship between water and ammonia.
Ammonia in water is weakly alkaline because not all molecules react with water to form ammonium and hydroxide ions. However, I am interested in the hydroxide ions because they are the ions that will ultimately react with the indicator molecule and change the colour on the outer reached on the puddle. On average, there is one hydroxide ion for every 240 ammonia molecules entering the water.
So, I would have to draw the water diagram 240 000 times to find 240 molecules of ammonia in which ONE will have dissociated ammonium and hydroxide ions
Stop a moment and let those figures sink in.
The presence of ammonium and hydroxide ions are even more remote. I admit I have not addressed the dynamic nature of this relationship between water and ammonia.
Hydrochloric acid is a strong acid in solution
The puddle consist of 1 drop of water, one drop of Universal Indicator and 1 drop of 0.1M hydrochloric acid. The acid is effectively 0.03M. The diagrams below each contain 1000 molecules of water and that is about the right number (1800) to surround one yellow chloride ion and black hydrogen (or hydronium ion) which are both found on the left hand side.
Stop a moment and let those figures sink in.
The puddle consist of 1 drop of water, one drop of Universal Indicator and 1 drop of 0.1M hydrochloric acid. The acid is effectively 0.03M. The diagrams below each contain 1000 molecules of water and that is about the right number (1800) to surround one yellow chloride ion and black hydrogen (or hydronium ion) which are both found on the left hand side.
Stop a moment and let those figures sink in.
The Movement of Ions in Water
The majority of positive and negative ions, with their closely associated water molecules, which surround (solvate) the ion, move through water molecules, rather like the way you exit a football match. Here are measured values (μ/10⁻⁸m²s⁻¹V⁻¹)
But for the hydroxide and hydrogen ions the rate is 20.6 and 36.2 μ/10⁻⁸m²s⁻¹V⁻¹ respectively. This is much quicker than expected. These tow ions do not diffuse but move via transport precesses.
The majority of positive and negative ions, with their closely associated water molecules, which surround (solvate) the ion, move through water molecules, rather like the way you exit a football match. Here are measured values (μ/10⁻⁸m²s⁻¹V⁻¹)
But for the hydroxide and hydrogen ions the rate is 20.6 and 36.2 μ/10⁻⁸m²s⁻¹V⁻¹ respectively. This is much quicker than expected. These tow ions do not diffuse but move via transport precesses.
The Movement of Hydroxide Ions in Water
As the OH- shunts to a water molecule that water molecule ejects another hydroxide ion and so on. The negative charge is moving though the solvent water molecules, not the hydroxide ion itself. You can see this on https://gph.is/g/ZrPj8jE. Again this is far more involved than what you see here.I am limited by time and just the sheer complxity of making computer models.
As the OH- shunts to a water molecule that water molecule ejects another hydroxide ion and so on. The negative charge is moving though the solvent water molecules, not the hydroxide ion itself. You can see this on https://gph.is/g/ZrPj8jE. Again this is far more involved than what you see here.I am limited by time and just the sheer complxity of making computer models.
The Movement of Hydogen Ions in Water
The hydrogen ion is a proton and there is evidence of clusters of water molecules with them. The simplest cluster (H₃O⁺) is shown between the square brackets. If a water molecule has enough energy to hit a cluster in the right position, it forms a cluster itself and the positive charge moves though the solvent water molecules. Effectively it is "hole" that is moving through the water.
The Reaction
The hydrogen ion is a proton and there is evidence of clusters of water molecules with them. The simplest cluster (H₃O⁺) is shown between the square brackets. If a water molecule has enough energy to hit a cluster in the right position, it forms a cluster itself and the positive charge moves though the solvent water molecules. Effectively it is "hole" that is moving through the water.
The Reaction
The billions of particles in puddle are in constant movement and colliding. At some point, the hydroxide negative ion with its extra electron will encounter "a hole" and the charge neutralised. There is a sudden drop in potential energy manifested in the heat of reaction which increases the temperature of the puddle and some of it is distributed amongst all the micro-energy states of water (movement, rotation and vibration). This is an increase in entropy.
The approximate relative number of particles in the puddle can be seen on the left. You see just how "lonely" many of our chemicals are in our watery solutions.
Much of the research education is dealing with how we understand the nature and interactions of particles in chemical reactions. This is because equations in examinations rely on this understanding. And if you get the equations right, more marks are obtained in examinations.
So not only does the school student (and teacher) have to imagine in the view of chemistry, particles so small they cannot be individually weighed or seen directly, the student (and teacher) also have to cope with phenomenally large numbers. It is said there are 1,000,000,000,000,000,000,000 stars in the observable universe (1 x 10²¹) but in 18 ml of water there are 6 x 10²³ molecules. This uber-macroscopic number of interacting particles is as much an issue with the understanding of chemistry as the sub-microscopic nature of the particles. This needs a HUGE imagination!
Visualising particles in aqueuos solutions via computer is also is very difficult.
The nearest I have come to is Roy Tasker's work on http://www.visichem.thelearningfederation.edu.au/topic09.html.
Much of the research education is dealing with how we understand the nature and interactions of particles in chemical reactions. This is because equations in examinations rely on this understanding. And if you get the equations right, more marks are obtained in examinations.
So not only does the school student (and teacher) have to imagine in the view of chemistry, particles so small they cannot be individually weighed or seen directly, the student (and teacher) also have to cope with phenomenally large numbers. It is said there are 1,000,000,000,000,000,000,000 stars in the observable universe (1 x 10²¹) but in 18 ml of water there are 6 x 10²³ molecules. This uber-macroscopic number of interacting particles is as much an issue with the understanding of chemistry as the sub-microscopic nature of the particles. This needs a HUGE imagination!
Visualising particles in aqueuos solutions via computer is also is very difficult.
The nearest I have come to is Roy Tasker's work on http://www.visichem.thelearningfederation.edu.au/topic09.html.