My Friend Sent This To Her Professor Today

The two kinds of water

Credit: University of Basel

Pre-sorted ortho-water and para-water molecules with differently oriented nuclear spins (blue or red arrows) react with diazenylium ions (centre left) at different speeds.

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Researchers from the University of Basel’s Department of Chemistry, Switzerland, has investigated how the two forms of water differ in terms of their chemical reactivity – the ability to undergo a chemical reaction. Both forms have almost identical physical properties, which makes their separation particularly challenging.

It is less well-known that water exists in two different forms (isomers) at the molecular level. The difference is in the relative orientation of the nuclear spins of the two hydrogen atoms. Depending on whether the spins are aligned in the same or opposite direction, one refers to ortho- or para-water.

The was made possible by a method based on electric fields. Using this, researchers were able to initiate controlled reactions between the pre-sorted water isomers and ultracold diazenylium ions (protonated nitrogen) held in a trap. During this process, a diazenylium ion transfers its proton to a water molecule. This reaction is also observed in the chemistry of interstellar space.

It was discovered that para-water reacts about 25% faster than ortho-water. This can be explained in terms of the nuclear spin also influencing the rotation of the water molecules. As a result, different attractive forces act between the reaction partners. Para-water is able to attract its reaction partner more strongly than the ortho-form, which leads to an increased chemical reactivity. 

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Alkanes: Crash Course Organic Chemistry #6:

Alkanes are kind of the wallflowers of organic chemistry, but they still have important functions in the world around us. In this episode of Crash Course Organic Chemistry we’re building our knowledge of organic molecules by learning all about these so called couch potatoes from how they are separated from crude oil to how to use Newman projections to predict torsional strain and steric hinderance. We’ll also learn the names of some common conformers and get an introduction to cycloalkanes. 

Breaking Down Alkanes - isn’t it cracking?

Unfortunately, if you’re sitting your A Level chemistry exam, you need to know a little more than the basic properties of alkanes outlined in my last post. Luckily though, this post takes you through fractional distillation and the two types of cracking - isn’t that convenient?

Crude oil contains carbon compounds formed by the effects of pressure and high temperature on plant and animal remnants. It is viscious, black and found in rocks beneath the earth’s surface. It is a mixture of mainly alkane hydrocarbons which are separated by a process called fractional distillation. Crude oil is essential because it is burned as a fuel and each fraction has different properties e.g. diesel, petrol, jet fuel.

Fractional distillation is the continual evaporation and condensation of a mixture which causes fractions to split due to a difference in boiling point. It is important to note that fractional distillation does not separate crude oil into pure compounds but rather less complex mixtures. Fractions are groups of compounds that have similar boiling points and are removed at the same level of a fractionating column.

The first step in this process is to heat crude oil in a furnace until some changes state from a liquid to a vapour. This mixture goes up a fractionating tower or column which is hotter at the bottom than the top and reaches a layer which is cool enough to condense and be collected. Shorter chain molecules are collected at the top where it is cooler since they have lower boiling points.

As you go down the fractionating column, bear in mind that: the column temperature increases, the boiling point increases, the number of carbon atoms increases and the strength of the Van der Waals’ between molecules increases.

Different fractions have different usefulnesses and often, it is the fractions with lower boiling points and shorter chains which are much more purposeful. Therefore there needs to be a process to getting shorter chains because they are the least abundant in crude oil samples. To meet demand, long chain molecules that are less useful are broken down into shorter chain molecules. This is done by cracking.

Cracking is a process where long chain hydrocarbon molecules are broken down into shorter chain molecules which are in high demand. This can be done one of two ways - thermal or catalytic.

Thermal cracking involves heating long chain alkanes to high temperatures - usually between 1000 - 1200K. It also uses high pressures up to 70atm and takes just one second. It only needs a second because the conditions could decompose the molecule completely to produce carbon and hydrogen instead. The conditions produce shorter chain alkanes and mostly alkenes.

A typical equation for this:

Decane -> octane + ethene

C10H22 -> C8H18 + C2H4

Catalytic cracking also breaks down long alkanes by heat under pressure using the presence of a zeolite catalyst. Temperature used is approx. 800-1000K and the pressure is often between 1-2 atm. Zeolite is an acidic mineral with a honeycomb structure, made from aluminium oxide and silicion dioxide. The honeycomb structure gives the catalyst a larger surface area which increases ROR. Factories which catalytically crack are often operated continuously for around 3 years at a time and produce branched alkanes, cycloalkanes and aromatic compounds.

You need to be able to compare the conditions of catalytic and thermal cracking for the A Level exam. Know that thermal cracking has a high temperature and pressure, a short duration, no catalyst and produces a high percentage of alkenes and some short chain alkanes. Catalytic uses a catalyst, a high temperature, a low pressure and produces aromatic hydrocarbons and motor fuels.

SUMMARY

Crude oil contains carbon compounds formed by the effects of pressure and high temperature on plant and animal remnants. I It is a mixture of mainly alkane hydrocarbons which are separated by a process called fractional distillation.

Fractional distillation is the continual evaporation and condensation of a mixture which causes fractions to split due to a difference in boiling point. 

It is important to note that fractional distillation does not separate crude oil into pure compounds but rather less complex mixtures.

Fractions are groups of compounds that have similar boiling points and are removed at the same level of a fractionating column.

The first step in this process is to heat crude oil in a furnace until some changes state from a liquid to a vapour. This mixture goes up a fractionating tower or column which is hotter at the bottom than the top and reaches a layer which is cool enough to condense and be collected. Shorter chain molecules are collected at the top where it is cooler since they have lower boiling points.

As you go down the fractionating column, bear in mind that: the column temperature increases, the boiling point increases, the number of carbon atoms increases and the strength of the Van der Waals’ between molecules increases.

Fractions with lower boiling points and shorter chains are much more purposeful but are the least abundant in crude oil samples. To meet demand, long chain molecules that are less useful are broken down into shorter chain molecules. 

Cracking is a process where long chain hydrocarbon molecules are broken down into shorter chain molecules which are in high demand. 

Thermal cracking involves heating long chain alkanes to high temperatures - usually between 1000 - 1200K. It also uses high pressures up to 70atm and takes just one second. It only needs a second because the conditions could decompose the molecule completely to produce carbon and hydrogen instead. The conditions produce shorter chain alkanes and mostly alkenes.

Catalytic cracking also breaks down long alkanes by heat under pressure using the presence of a zeolite catalyst. Temperature used is approx. 800-1000K and the pressure is often between 1-2 atm. Zeolite is an acidic mineral with a honeycomb structure, made from aluminium oxide and silicion dioxide. The honeycomb structure gives the catalyst a larger surface area which increases ROR. 

You need to be able to compare the conditions of catalytic and thermal cracking for the A Level exam. Know that thermal cracking has a high temperature and pressure, a short duration, no catalyst and produces a high percentage of alkenes and some short chain alkanes. Catalytic uses a catalyst, a high temperature, a low pressure and produces aromatic hydrocarbons and motor fuels.

Happy studying!

#ChemAdvent day 22 features flaming Christmas puddings! 🔥 https://ift.tt/3h7ELcS

Plenty of opportunities to wear sunglasses this week! 😎 Here’s the science behind how the protect your eyes from the sun’s UV radiation in C&EN: https://ift.tt/2XW7h8L https://ift.tt/3gT8PI6

Shapes of Molecules

his post is more information than trying to explain something - the truth is, you just need to learn shapes of molecules like you do with anything. I’ve got a physical chemistry mock tomorrow that I’m dreading since I’ve done zero revision. The fact that I run a study blog yet don’t revise myself is odd, but what else can I do? Oh, wait … revise. So here it is, my last minute revision for myself and you too - I present, shapes of molecules!

VSEPR stands for valence shell electron pair repulsion theory. If you’ve ever seen a moly-mod or a diagram of a molecule in 3D space, you may wonder how they decided it was that shape. Well, VSEPR answers all.

The theory essentially states that electron pairs are arranged to minimise repulsions between themselves - which makes sense, since electrons carry the same charge and therefore try to repel each other. Of course, there are different types of electron pairs, lone and bonding. The strongest repulsions happen between lone pair - lone pair followed by lone pair - bonding pair and finally, bonding pair - bonding pair have the least repulsion. 

Since the repulsion governs the shape of the molecule, to work out a molecule’s shape you must look at dot and cross diagrams or electron configurations to see how a molecule is bonded. There are many methods to do this, but the bottom line is that you must work out how many bonding pairs of electrons and how many lone pairs are involved.

The easiest shape to learn is linear. This has two bonding pairs and no lone pairs at an angle of 180 degrees, since that is the furthest the two can get away from each other. Examples of linear molecules include carbon dioxide and beryllium chloride.

Next up is trigonal planar. This has three bonding pairs and no lone pairs, each at the angle of 120 degrees. Trigonal means three and planar means on one plane, this should help you in identifying the molecules since after a fourth pair of electrons, the shape becomes 3D. Examples of trigonal planar molecules include boron trifluoride and sulfur trioxide.

What if you were to have two bonding pairs and two lone pairs? Well, then you’d have a bent molecule. Water is a good example of a bent molecule. Since it has two lone pairs that repel the other two bonding pairs more than they repel each other, the bond angle is 104.5. I’d be careful though, as in many textbooks it shows a bent molecule to have one lone pair and a different bond angle.

Another variation of the bent molecule I’ve seen is the one with two bonding pairs and one lone pair. It is deemed as bent with a bond angle of 109 or sometimes less than 120 degrees.

Tetrahedral molecules have four bonding pairs and no lone pairs. The bond angle is 109.5 degrees. Examples of this include the ammonium ion, methane and the phosphate ion. A good thing to note here is how these molecules are drawn. To demonstrate the 3D shape, where the molecule moves onto a plane, it is represented with a dashed line and triangular line along with a regular straight line. 

Trigonal pyramidal, sometimes just called pyramidal, is where there are three bonding pairs and a lone pair. Bond angles are roughly 107 degrees due to the repulsion from the lone pairs. An example of a trigonal pyramidal molecule is ammonia, which has a lone pair on the nitrogen.

Having five bonding pairs gives a trigonal bipyramidal structure. I guess the three bonding pairs on the trigonal plane accounts for that part of the name, where the rest comes from the position of the remaining two. These molecules have no lone pairs and have a bond angle of 90 degrees between the vertical elements and 120 degrees around the plane. Diagrams below are much clearer than my description! Examples of this include phosphorus pentachloride.

Six bonding pairs is an octahedral structure. I know this is confusing because octahedral should mean 8 but it’s one of those things we get over, like the fact sulfur isn’t spelt with a ph anymore. It’s actually to do with connecting the planes to form an octahedral shape.There are no lone pairs and each bond angle is a nice 90 degrees. Common examples include sulfur hexafluoride.

Square planar shapes occur when there are six bonding pairs and two lone pairs. All bond angles are 90 degrees! They take up this shape to minimise repulsions between electrons - examples include xenon tetrafluoride.

The final one to know is T-shape. This has three bonding pairs and two lone pairs. These molecules have bond angles of (less than) 90 degrees, usually a halogen trifluoride like chlorine trifluoride.

There are plenty more variations and things you could know about molecular geometry, but the truth is, there won’t be an extensive section on it. It’s a small part of a big topic!

I’m not going to do a summary today since I’d just be repeating the same information (I tried to keep it concise for you guys) so instead I’ll just leave you with, 

Happy studying!

If you are scrolling through Tumblr trying to distract yourself from something you don’t want to think about, or you’re looking for a sign. It is going to be okay. Just breathe. You are alive and you matter. 

Good news! You CAN rewire your brain. Through the same mechanism as forming habits, you can change your automatic thought patterns. When you catch yourself with unhealthy thoughts, STOP, and make a conscious effort to replace that thought pattern with a more beneficial/healthy thought pattern. Keep doing that whenever those thoughts occur. Slowly but surely your mind’s default voice will match what you’re teaching it.

Follow @productive-tips for more tips and content like this posted daily! Handpicked and curated with love :)

Happy #StPatricksDay2021! Ever wondered why the bubbles in Guinness appear to fall rather than rise, and what causes its dark colour? This graphic in C&EN has the answers! https://ift.tt/2WfNI8j https://ift.tt/3rY7DIW