Reactions in Chemisty

The 4 ways to control a chemical reaction are, thermochemistry, electrochemistry, photochemistry, and, starting around 2017, polariton chemistry.

Combination reactions can be exothermic or endothermic, but are more often exothermic, and negative entropy. Decomposition reactions are therefore mostly endothermic, and positive entropy. Precipitation reactions are a type of combination reactions, and the opposite of precipitation is dissolution reactions (dissolving).

Furthermore, for a change in entropy to be positive, are reactions with more moles of gas in the product side (decomposition reactions).

Heating a reaction shifts it to the endothermic side, while cooling it shifts to the exothermic side. Increasing the pressure on an equilibrium mixture (decreasing the volume) will shift the reaction to the side containing the fewest number of gas molecules, and increasing the volume (decreasing the pressure) will shift it to the side containing the most moles of gas.

Fusion reactions are exothermic as long as the starting material are bigger than Fe-56, with products less than Fe-56. They are endothermic if products are bigger than Fe-56. For fission reactions, are vice versa.

Chain reactions can be endothermic, but such a reaction would need to be in contact with a heat source in order to carry out its chain process.

The fastest reactions in chemistry, by far, are acid-base reactions in water, as the molecules don’t have to touch each other. The 2nd fastest reactions are generally the electron-transfer reactions. However, depending on the context, photochemical reactions are faster, such as photoisomerization.

Combination reactions favor the higher oxidation state, replacement reactions favor the lower oxidation state.

For an explosion to occur, the decomposition or combustion of the explosive must be exothermic (so that a detonation wave spreads), and the reaction products must be gaseous. The most dangerous endothermic reactions are when gases are produced (which would be from a hot gas to a cold gas). And the most dangerous reactions where no gases are produced, are exothermic reactions going from a cold solid to a hot liquid (or something almost a gas, due to the pressure change).

For all cases, 1 should never mix strong oxidants with oxidizable material whenever there is the possibility of a reaction to produce gas.

The act of oxygen combining with iron to form rust is hugely exothermic, about -826 kJ/mol, which means that about 100 grams of iron, in the course of rusting, would release enough energy to raise 1 liter of water's temperature about 200 degrees C. However, rust happens so slowly (over the course of years to decades) that that energy comes out in such a small trickle, we don't notice it.

While it is true that increasing the temperature will decrease the activation energy (Ea) of the reaction, and will decrease the reaction time, that is not the case for where the x-axis does not include time as a dimension, but have "reaction progress" as the x-axis.

Reaction progress is a representation of the reorientation of atoms in space, where 1 side is the product and the other is the starting material. The farther to the right, the more closely the intermediate or transition state resembles the product, in some arbitrary amount of movement.

RCD (reaction coordinate diagrams) always has the x-axis as the reaction coordinate which is the basically the proximity of the reactants to each other towards the formation of the transition state followed by the resolution of the transition state and the formation of products. There would not be a time component or a concentration component there.

Reaction progress also isn't progress of the entire jar of molecules. It's progress of the individual components of 1 reaction along the path from reactants to products.

Imagine it like you made a movie of an individual set of chemicals reacting from A + B -> C. When the bonds break the energy is higher because bonds lower energy. Then they reform and lower the total energy. Reaction coordinate is how far into the movie you are. The jar of molecules might react faster with higher temperature, but individual reactions are the same speed. Higher temperature just makes the reactions more likely to happen.

It is impossible to predict S and H a priori without any computer quantum calculations. This lack of predictions, will explain whether a combination reaction will be exothermic or endothermic, and whether something that dissolves in water will be exothermic or endothermic.

If solubility decreases with higher temperature, then change in S must be negative. Then the salt will dissolve if change in H is negative enough to give an overall negative result.

Ammonium bromoplatinate, sulfate octahydrate, hydroxystannate, sodium selenite, sodium dihydrogen pyrophosphate hexahydrate, sulfate octahydrate, ytterbium sulfate, and virtually any substance that's a gas at ordinary temperatures, including nitrogen, oxygen, hydrogen, helium, carbon dioxide, and ammonia.

Note: while &DeltasH is an approximate of bonds broken - bonds formed, that is still only an approximation, from the products - reactants formula. There is no ΔS version of bonds broken - bonds formed.

All gas reactions are either exothermic and spontaneous, and not endothermic and spontaneous, or, if you flip the reaction around, are endothermic and non-spontaneous, and not exothermic and non-spontaneous.

Gas reactions are further either combination or decomposition reactions, all that are exothermic in their spontaneous-side. (So if the gas reactions are spontaneous, then they are supposed to be exothermic.). You could, theoretically, have a endothermic spontaneous reaction, in their decomposition form, which would require the gases to absorb heat from their surroundings. But these would not be dependent on the identity of the gas, only the controlled environments.

When you have AB + CD -> AC + BD, which is a double replacement reaction, how do you know whether the reactants are in equilibrium or the products are in equilibrium, at STP?

Reactions tend to happen after an activation energy, Ea. The Ea is the same whether at equilibrium. The equilibrium constant, K, symbolizes equilibrium when it is 1. Make sure ΔG and R are in the same units, so if using kJ for ΔG, move the decimal 3 places to the right.

So take 2Na + Cl₂ -> 2NaCl. In real life, we use a flame on the sodium metal when it's in a tube of chlorine gas. What is the minimum temperature needed for the reaction to react?

The answer is any temperature, for some reaction to occur, just that even at subzero temperatures, the speed is microscopic. But for the reaction at equilibrium (K = 1), T comes to be above their melting and boiling point. (There is a formula, for 1 = e^(ΔG/RT), but, the values for ΔG and R are defined for 298 K, so you can't use that formula).

When K = 1, ΔG⁰ = 0, and T = ΔH/ΔS. (Because ΔG⁰ = -RT ln(K), and ln(1) = 0.).

Do not use the product of formation for ΔH and ΔS, use the products - reactants.

-Forming of rust. 4Fe + 3O₂ -> 2Fe₂O₃.
-Formation of quicklime. CaO + H₂O -> Ca(OH)₂.
-Decomposition of hydrogen peroxide. 2H₂O₂ -> 2H₂O + O₂.

In a battery, in the anode (where the oxidation occurs) you want a low reduction potential, and in the cathode (where the reduction occurs) you want a high reduction potential. Low reduction potential correlates with high oxidation potential and vice versa. So at the anode, you want high Coloumbic efficiency. The electrolyte (for solvation effects) must have high ionic conductivity. However, a paradox in electrochemistry is electrolytes that have good reductive stability, have poor oxidative stability, and vice versa, but they all need to have good ionic conductivity as a 3rd requirement. Ethers, for example, have excellent reductive stability, so are good at the anode, while Li has low reduction potential.

In a lithium ion battery, the Li metal has a low reduction potential at the anode, whereas LiCoO₂ is commonly used at the cathode because it can accept electrons.

Unfortunately, many solvents and electrolytes degrade at 5 V. Lithium-ion batteries typically operate in the range of 3 to 4.2 V per cell. At voltages above 4.5 V, many electrolyte solutions produce gas that may lead to swelling or leakage in the battery. You need high ion pairing at the electrolytes. A newer discovery and promising solutions to this are ionic liquids, which do not degrade at 5 V (high oxidative stability). Molten salts are even better than ionic liquids.

Most solid-state batteries have liquid electrolytes in the cathode, and solid electrolytes in the anode.

An electrochemical oscillator is a system where the voltage, current, or chemical concentrations oscillate over time instead of reaching equilibrium. Electrochemical oscillators appear when nonlinear feedback between electrode reactions and mass transport causes self-sustained oscillations.

The Stuart-Landau oscillator is the normal form of a Hopf bifurcation, or the simplest nonlinear oscillator near the onset of oscillation. It describes both amplitude and phase dynamics.

The Kuramoto-Sakaguchi oscillator is a phase-only oscillator model (amplitude is assumed constant), and captures synchronization phenomena (oscillators locking in a phase).

Nonlinear coupling between oscillations mean the interactions involves products, powers, or nonlinear functions of the oscillators' states. So instead of just k(x₁ - x₂), you might see (X₁ - x₂)³, a sine functions, etc. This means the coupling strength depends on the amplitude and phase of oscillations in a more complicated way.

(Coupling means the oscillators are not independent, so the state of 1 influences the other, like 2 pendulums connected by a spring, where the motion of 1 affects the other.). Nonlinear coupling is important because it can create synchronization (lock phases even if natural frequencies differ) and amplitude deth (oscillation quenching where oscillations cancel out).

Homogeneous oscillators means all oscillators are identical (same natural frequency), whereas heterogeneous difer in frequencies, amplitudes, or parameters. Heterogenous oscillators stabilize synchronization.

SN2: losing a halogen and doing a replacement. C temporarily has 5 bonds, no carbocation or carbanion. The nucleophile bonds to the electrophilic carbon as the substrate leaves. SN2 reactions require nucleophiles to be strong.

SN1: rearrangement (losing halogens and forming carbocations). Something falls off 1st, then something else bonds on. A substitution / replacement, involving weak nucleophiles that are not strong enough to do SN2.

E1: carbocation formed. A base abstracts a proton from the C adjacent to the carbocation. E1 reactions almost always occur together with SN1.

E2: requires a strong base present (like SN2, so no carbocations formed) but SN2 mechanism is blocked due to being hindered. So a strong base abstracts a proton on a C adjacent to the C with the leaving group, forming a double bond on the adjacent carbon. This makes E2 a concerted reaction as bonds break and form at the same time. No rearrangements.

SN2 and SN1 both stand for nucleophilic substitution, and we do have addition reactions.

Nucleophilic addition reaction: the nucleophilic carbon of the reagent attacks the electrophilic carbon of the carbonyl group.

Electrophilic addition reaction: involve the addition of electrophiles to a double or triple bond. Such as the addition of hydrogen halides to alkenes. Or adding water (hydration) to alkenes.

Electrophilic substitution reactions involve the substitution of a hydrogen atom in an organic compound with an electrophile. Common examples include electrophilic aromatic substitution reactions.

DNA replication (SN2). During DNA replication, DNA polymerases catalyze the incorporation of nucleotides into a growing DNA strand. This process involves SN2 reactions where a nucleophile (incoming nucleotide) attacks the electrophilic phosphate group on the DNA backbone.

Protein folding uses both SN2 and SN1, while photosynthesis and fermentation uses both E1 and E2.

Glutathione (a tripeptide containing a thiol group), serves as a mild reducing agent to detoxify peroxides and maintain the cysteine residues of hemoglobin in the reduced state. It can also detoxify alkylating agents, where the thiol of glutathione reacts with methyl iodide by an SN2 reaction, making the methyl iodide harmless and preventing its reaction with other molecules in the body.

Common aromatic systems have 2, 6, or 10 pi electrons, whereas anti-aromatic systems have 4, 8, or 12 pi electrons. Benzene (and pyridine, pyrrole, and imidazole) have 6.

Deactivating groups (electron-withdrawing) are for nucleophilic substitution and activating groups (electron-donating) are for electrophilic aromatic substitution.

Electron-withdrawing substituents (such as nitro groups and carboxyl groups) activate the ring toward nucleophilic aromatic substitution. These deactivating groups (such as carbonyls (aldehydes, ketones) and halogens) withdraws electron density by both the inductive effect (through the sigma bonds) and the resonance effect which involves pi systems.

Likewise, things that increase electron density of the ring (like hydroxyl groups, amine groups, and methyl groups), boosts electrophilic substitution.

An electron-withdrawing substituent deactivates primarily the ortho and para positions (such as nitro groups, for nucleophilic substitution), and an electron-donating substituent activates primarily the ortho and para positions (such as phenols, for electrophilic aromatic substitution). However, by deactivating the ortho and para position, we mean it primarily activates the meta position.

So an application of this, is when methylbenzene (toluene) reacts with strong acids (activating), they react a lot quicker than benzene does, and they react on the ortho and para positions. But when nitrobenzene reacts with strong acids (deactivating), it is much less reactive than benzene is, and the products that do form, form primarily in the meta position. (With strong bases like NaOH or Grignards, they are not strong enough to pull hydrogens from benzene or methylbenzene, but can from hydroxybenzene (phenol)).

Do note that in nitro groups, no matter how we position the electrons in a Lewis dot diagram, the nitrogen always has a formal positive charge, so it inductively withdaws electron density from the aromatic ring.

Electrophilic aromatic substitutions are far more important than nucleophilic aromatic substitutions.

2 types of polymer reactions: step-growth (which includes condensation reactions), and chain-growth (which includes free-radical and ionic reactions). Step-growth has a slow rate of increasing the molecular weight, whereas chain-growth has a fast rate of increasing the molecular weight. In step-growth reactions, water or carbon dioxide are typically a byproduct, and not in chain-growth. In chain growth, free-radical reactions are generally faster than ionic reactions, due to less stabilization.

Step-growth reactions tend to be endothermic, whereas chain-growth reactions tend to be exothermic after the initiator (the initiator is endothermic).

Chemicals are dangerous depending on the type of reactions and route of exposures. Examples of types of reactions as listed:

Includes acids and bases (dangerous to touch). Strong bases dissolve tissues. Sulfuric acid is both a strong acid, a strong oxidizer, and a strong dehydrating agent (absorbs water), whereas HNO3 is both a strong acid and a strong oxidizer, and HCl is just a strong acid and KMnO4 is just a strong oxidizer. The most dangerous commercial chemical is HF, a weak acid, but not because of the acid, but because of the fluorine.

Compounds that are safe to touch, but dangerous to breathe or digest. Examples are carbon monoxide and cyanide gas (CO and HCN), as well as solid KCN and LiCN.

For example, in cyanide gases, cyanide bonds to Fe(III) in ferricytochrome oxidase enzyme, preventing reduction to Fe(II) in the oxidative phosphorylation process by which the body utilizes O₂ (which prevents utilization of O₂ in cells, so metabolic processes cease). And CO binds to hemoglobin to convert oxyhemoglobin to carboxyhemoglobin, at 210 times more than oxygen (so the pain is a lack of oxygen, and a lack of oxygen ceases production of ATP).

Other examples of chemicals besides cyanide, are H₂S, azides, formates, NO-radicals, and PH₃.
Other examples of chemicals besides carbon monoxide, are chemicals that form methemoglobin, such as nitroglycerin, sulfonamides, chlorobenzene, and arsine.

Examples are organophosphates, where they inhibit (or prevent) acetylcholinesterase (an enzyme essential for nerve function), which accumulates excess acetylcholine at cholinergic synapses, with overstimulation of muscarinic and nicotinic cholinergic receptors. Excess acetylcholine causes paralysis of muscles needed for breathing and heartbeat. These are through absorption of the skin.

Another example is atropine, an alkaloid which can counter the effects of pesticides and nerve gas by blocking the receptors they over-activate.

In chemical engineering, we want reactions to be low Ohmic resistance, low vapor pressure, and faster kinetics, which means more reactions. More electrons means slower kinetics. High voltage in acids corrodes everything.

In electrochemistry, in a voltaic cell, electrons flow from the anode through the external circuit to the cathode, which is electrons flowing spontaneously toward the electrode with the more positive electrical potential.

2H₂O -> 4H⁺ + 4e^- + O₂, ΔH = 286 kJ, E = 1.23 V.
3H₂O -> 6H⁺ + 6e^- + O₃, ΔH = 1000 kJ, E = 1.51 V.

The 1st reaction has lower voltage, so it's better (the easier it is to oxidize). The 2nd reaction is also not the common way for generating ozone.

C₂H₆ -> C₂H₄ + H₂, ΔH = 131 kJ.
2CO₂ + 2H₂O -> C₂H₄ + 3O₂, ΔH = 1304 kJ.

The 1st reaction is 1400 kW-hour/ton, 2nd is 14,000 kW-hour/ton. The 1st reaction is called steam cracking of ethane, while the 2nd reaction can be done a few ways, but mainly done electro-reduction of CO₂.

Obviously, the 2nd reaction is 10x more costly than the 1st, however, the 2nd reaction will remove CO₂ by 2.30%, while the 1st increases CO₂ by .74%. However, the 2nd reaction would increase electricity usage globally by 22%, whereas the 1st reaction decreases by .3%. (This assumes the energy efficient of 47% for electro-reduction, a 66% Faradaic efficiency, and .58 V overpotential, and the electricity does not generate CO₂).

As of Dec. 2022, there are 438 active nuclear plants in the world. As of 2021, 2,653 terawatt hour electricity worldwide was from nuclear plants (about 10%). Of those 438 nuclear plants, 92 come from the U.S., 56 from France, 55 from China, 37 from Russia, and 33 from Japan.

In nuclear power plants, the heat produced by nuclear fission is used to create steam, which then drives the turbines that generate electricity.

The most common type of reaction in power plants, is nuclear fission, where the nucleus of a heavy atom, usually uranium-235 or plutonium-239, is split into 2 smaller nuclei when struck by a neutron, which releases a tremendous amount of energy in the form of heat.

Haber-Bosch uses 8.6 * 10¹⁸ J of world's energy in 2021 (about 80% efficient). About .18 gigatonness (180 million tonnes) of ammonia produced, or about 3.5 * 10¹⁸ J. The flip side is about .42 gigatons of carbon dioxide are produced. Ammonia is about the same price as a $2 gallon of gasoline. Nearly 50% of the nitrogen found in human tissus originated from the Haber-Bosch process, which could explain why this is the donator of the population explosion, enabling the world population to increase from 1.6 billion in 1900 to 7.7 billion by Nov. 2018.

Modern plants typically use 10 kWh/kg of ammonia produced. There's about 400 Haber-Bosch plants in the world.

Combustion: ammonia can be combusted in the presence of oxygen to release heat energy.

This reaction can be utilized in combustion engines or power plants to generate heat, which can then be converted into electricity or used for other purposes.

Another method is to electrolyze ammonia. Ammonia fuel cells: ammonia is oxidized at the anode and reduced at the cathode, with the overall reaction producing electricity and nitrogen gas as byproducts. The specific reactions depend on the type of fuel cell used. In a direct ammonia fuel cell (DAFC), the following reactions occur:

Anode (oxidation): 6NH₃ + 6H₂O -> 6NH₄⁺ + 6e^-.
Cathode (reduction): 6NH₄⁺ + 6e^- + 6O₂ -> 6 NO₃^- + 12H₂O.
Overall reaction: 6NH₃ + 6O₂ -> 6NO₃^- + 12H₂O + 6e^-.

The electrons generated from the oxidation reaction flow through an external circuit, producing electricity. Meanwhile, the oxidized ammonia (NO₃^-) combines with water to form nitrate ions.

Catalysts can affect rates and selectivities, but not thermodynamic state variables (such as heats of reactions). Approximately 90% of industrially produced chemicals require catalysts. Ammonia, widely used as a precursor in fertilizer manufacturing, is an example. The thermocatalytic Haber-Bosch process utilized to produce ammonia, however, operates at high temperatures and pressures, accounts for 2% of global carbon emissions, and is near its thermodynamic limits. Low temperature plasma (LTP) is an electrified technology that can produce ammonia using renewable energy and at ambient pressures and temperatures. LTP is a highly reactive state of a matter characterized by extremely hot electrons and low temperature neutral, radical, and ionic species. The interaction of heterogeneous nanoscale catalysts (nanocatalysts) with the plasma species is of crucial importance to improve energy efficiency, conversion, and selectivity of ammonia.

Heterogeneous catalysis refers to the use of catalysts in a different phase than the reactants. They are usually a solid phase, with reactants in the liquid or gas phase.

-Catalytic converters in automotive exhaust systems.
-Haber-Bosch process for ammonia synthesis using iron catalysts.
-Catalytic cracking in petroleum refining.

Heterogeneous catalysis has long served as the bedrock of fuels and chemical manufacturing.

In homogeneous catalysis, the catalyst is in the same phase as the reactants. Typically, both the catalyst and reactants are in the liquid phase, but they can also be in the gas phase. Since the catalyst and reactants are in the same phase, they mix thoroughly, allowing for uniform interaction and high catalytic efficiency.

-Acid-base catalysis in solutions.
-Enzyme catalysis in biochemical processes.
-Organometallic catalysts in polymerization and hydrogenation reactions.

Homogeneous catalysts often exhibit high selectivity and specificity for particular reactions, as they can be finely tuned at the molecular level.

There is also oxidation and reduction reacts that use catalysts. The metal catalysts that are used for oxidation reactions (Mn, Cr, and V) are different than the metals used in reduction reactions (Pt, Pd, and Ni, for hydrogenation reactions). All of these are heterogenous catalysis.

Electrolyzers use electricity to split water into hydrogen and oxygen. A fuel cell is composed of 3 main components: an anode, a cathode, and an electrolyte membrane. The magic of the proton exchange membrane (PEM) fuel cell is its proton exchange membrane, which looks like a piece of construction paper, and works by passing hydrogen through the anode side and oxygen through the cathode side. At the anode side, the hydrogen molecules are split into electrons and protons. The protons pass through the electrolyte membrane, while the electrons are forced through a circuit, generating an electric current and excess heat. At the cathode side, the protons, electrons, and oxygen combine to produce water molecules.

So, a hydrogen fuel cell is an electrochemical power generator that combines hydrogen and oxygen to produce electricity, with water and heat as by-products. Hydrogen fuel cells work much like batteries by generating electricity from an electrochemical reaction. However, hydrogen fuel cells are refueled with more hydrogen instead of being recharged like a traditional battery. Although clean hydrogen holds great promise as a potential emissions-free fuel source, it's costly to produce. It costs about $1.50 per kilogram to produce hydrogen from natural gas and $5 per kilogram to produce clean hydrogen. The U.S. Department of Energy wants to reduce the cost of clean hydrogen to $1 per kilogram over the next decade to make it a more competitive fuel source.

In Jan. 2024, the largest electrolytic liquid hydrogen production plant, and largest PEM electrolyzer deployment operating in the U.S. opened up in Woodbine, Georgia, by Plug Power. The plant is designed to produce 15 tons per day of liquid electrolytic hydrogen, enough to power approximately 15,000 forklifts per day. Through 8 5-megawatt (MW) PEM electrolyzers, water is separated into hydrogen and oxygen. The hydrogen gas is then condensed into liquid form at -423 F to be delivered to customers’ hydrogen fueling stations. The 1st operational green hydrogen plant in the U.S. is H2B2’s SoHyCal facility in California (near Fresno), going live in Nov. 2023. Plug Power is 2nd, in Jan. 2024, but at 5x more (15 tons/day from 3 tons/day).

In sol-gel chemistry, the compounds are irreversible and have no control. But metal-oxide nanocrystal gels can self-assemble a bit, without organic ligands.

Sol-gel chemistry involves hydrolysis and condensation of metal alkoxides or salts, formation of a colloidal suspension (sol), and transformation into a network (gel). Then there is drying or and heat treatment to yield glass, ceramics, or porous materials. A gel is a porous solid phase permeated by liquid. Upon drying or and heat tretment, it becomes a porous xerogel or aerogel.

In sol-gel systems, you want gelation, and not precipitation. Colloids can either precipitate or self-assemble into gels, depending on the conditions. In sol-gel chemistry, you intentionally drive gelation to create porous materials.

ExxonMobil, headquartered in Houston, TX, has over 18,000 scientists and engineers, with over 1500 of them having PhDs, with most of their PhDs researching what they got their PhD in. Their headquarter building alone has some 10,000 employees, as of 2024.

Around 2021, ExxonMobil started a low carbon solutions business research team, most of it headquartered in their Clinton, NJ facility. They research hydrogen, lithium, carbon-capture utilization and storage, gas and liquid separation, etc. ExxonMobil are also trying to research an alternative to steam cracking, which is ethane dehydrogenation to get ethylene. Steam cracking requires an 800 C furnance, with 70-80% yields. ExxonMobil are considering using membrane reactors as an alternative: they do conversion and separation at the same time. That is, the membrane reactors remove hydrogen as it's produced via a membrane, shifting the equilibrium to allow a higher conversion of ethane to ethylene at the same temperature and pressure.