What is Reactions: Definition and 649 Discussions

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.
The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions.
Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms.
Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium. Reactions that proceed in the forward direction to approach equilibrium are often described as spontaneous, requiring no input of free energy to go forward. Non-spontaneous reactions require input of free energy to go forward (examples include charging a battery by applying an external electrical power source, or photosynthesis driven by absorption of electromagnetic radiation in the form of sunlight).
A reaction may be classified as redox in which oxidation and reduction occur or nonredox in which there is no oxidation and reduction occurring. Most simple redox reactions may be classified as combination, decomposition, or single displacement reactions.
Different chemical reactions are used during chemical synthesis in order to obtain a desired product. In biochemistry, a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) form metabolic pathways. These reactions are often catalyzed by protein enzymes. Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperatures and concentrations present within a cell.
The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions, radioactive decays, and reactions between elementary particles, as described by quantum field theory.

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  1. D

    Is there a way to collect all one's reactions to others' posts?

    Is there a way to trace all the reactions I have had to others' posts in various threads of this forum? Like an index? If only to retrieve all the useful information I've learned from them, but have forgotten already now where it was located. Also, I thought OP meant online participant, but...
  2. Mohmmad Maaitah

    Distributed weight of inclined beam.

    I'm having problem in drawing the distributed load (weight per foot) for the inclined beam as it shows. should it be rectangular? if so shouldn't the distributed load be vertical down as the resultant weight at the centroid (W) please help me understand how to draw the free body diagram for this...
  3. Nova_Chr0n0

    Engineering Statics - Determine the Reactions on this bent bar levering between two surfaces

    The figure is shown below: Here is my FBD for the figure with assign +x and +y directions I started off by summing up the forces in the x-direction: Next is the summing up of the forces in the y-direction: After this, I solved for the moment at point A: assuming that counter-clockwise is +...
  4. rosyposy

    Question on Hydrates and Reactions

    Hey! I am currently working on a project where I have to react copper (II) chloride dihydrate (dissolved in water) and aluminum. When writing the equation and the ionic equation for this, would you include the dihydrate in the formula since it’s dissolved in water and no longer in its...
  5. Graham87

    Troubleshooting Nuclear Reactions: Decay, Gamma Rays & More

    The problem comes with solutions. However, I dont get the 3 steps in the solutions. Why do they calculate decay for 120min in step 3? And why is only the daughter nuclide relevant and no granddaughter? There might be something lacking in my knowledge about nuclear reactions. Also, I don't know...
  6. Andrea Vironda

    Calculating constraint reactions

    I would be interested in calculating the constraint reactions on the 6 pads in yellow in the figure, about 300mm apart among them and loaded with F=12500 kN in blue. Since the system is highly hyperstatic, I don't know how to calculate the constraints. Can you give me a hand? I've made a FEM...
  7. benswitala

    I Can Neutron-X Fusion Reactions Overcome the Repulsion Problem in Fusion Energy?

    Hi, I was reading about fusion reactions recently, and it was pointed out that the problem seems to be that positively charged nuclei repel each other. If that is the problem, why don't scientists try to go with another fusion reaction? Would it be possible to have fusion if neutrons fuse...
  8. A

    Chemistry Reactions With Dilute Hydrochloric Acid

    One of the alternatives was NaBr My resolution was as follows: NaBr + HCl forms NaCl and HBr I researched and saw that HBr would be a gas and would be denser than air because its molar mass is greater. However, the answer was not that. My error is in which step? In the reaction? What would...
  9. P

    Chemistry Draw the mechanism of the reactions of steps 1 and 2

    Hi everyone! There is a task that I can't solve. Can you explain me how I have to draw the full mechanism? Draw the mechanism of the reactions of step 1 and 2. Indicate the product before step 3. (in step 2 also TMG =Tetramethylguanidine is given and instead of the protecting group DMT was...
  10. G

    Reactions at the beam supports

    Figure: My attempt at a solution: We know that ##Q=A_T## We calculate ##Q##: $$Q=\dfrac{3\cdot 480}{2}+\dfrac{600\cdot 6}{2}+600\cdot 2=3720\, \textrm{lb}$$ Then we look for the point ##\overline{x}## of the centre of gravity: $$\overline{x_1}=1\, \textrm{ft},\quad...
  11. P

    A Antiproton reactions with water

    This started out with interest on antiproton radiotherapy but as it turns out there's little to no publication on the reaction equations that's happening. So this is just an attempt to have some equations at hand. The reactions are for antiproton with water. For simplicity everything is...
  12. T

    B Why can't nuclear reactions burn the atmosphere

    When the US first developed nuclear weapons against the Nazis and Japan, their primary concern was whether the nuclear reactions can trigger the atmospheric nitrogen to fuse and burn the whole atmosphere. However this claim was instantly debunked by physicists. Their reasoning is that in order...
  13. samy4408

    Integrated Rate Law for 2nd Order Reactions

    hello i have a question about kinetics : to have the integrated rate law for second order reaction the professor write the following why we don't write the rate like this : rate = -1/2(d[1]/dt) ? why we ignore the stoichiometric coefficient ?
  14. samy4408

    Medical Question about reactions in biochemistry

    we assume that we have an exergonic reaction , a substrate going to a product , is the negative ΔG′° the principal reason that the equilibrium favors the product (there is more product than substrate at equilibrium )?, or it is not and that depends on other factors? , thanks .
  15. A

    B Higgs field and nuclear reactions

    What happens to the higgs field when say a fusion reaction occurs. Like if mass is converted into energy and the higgs field gives a particle mass what happens to higgs field. I doubt this, but is the higgs field the mechanism that converts mass into gamma rays. Go easy on me I only have a high...
  16. S

    I Which direction do the loads and reactions go in a Ferris wheel?

    Considering a ferris wheel with the spokes being pretensioned cables. Which directions do the loads vs reactions go?
  17. barryj

    Chemistry What is the purpose of redox reactions?

    I am trying to understand redox reactions. I have read, and learned about oxidation numbers and even viewed examples of balancing redox equations. However, nowhere have I seen an explanation of why I care about redox equations. In contrast, I understand that a chemical equation must be balanced...
  18. M

    Engineering Statics question about FBD of a backhoe (w/ pin reactions)

    So, for the link "AB"...they didn't directly list the pin reactions from pin "A" here, but I want to be sure that I understand what those reactions are. So, from what I see in this image, the member is pushing at the pin "A" (red arrow) because otherwise, the bucket would want to rotate...
  19. Yash Agrawal

    Thermodynamics of chemical reactions

    In chemical reactions generally ΔG < 0 , but if we were to consider a reversible path between pure reactants and products at 1 bar pressure , shouldn't the ΔG = 0 for every reaction ? and if it is due to non-pv work , I don't see any non pv work being done in reactions happing in a closed...
  20. N

    Engineering Find the support reactions on a beam

    Hello! in this good afternoon ;). I need some help for this beam and his support reactions. The problem it is done, but my result its different from the teacher`s result. I'm sure my answer is correct, as even the "SkyCivbeam" online software also gives the same results. it appears that the...
  21. H

    Chemistry Meso Syn Reaction Steriochemistry: Is Product B Meso?

    reaction steriochemistry is syn? the product of reaction b is meso?
  22. hedera

    Statics problem, suport reactions and pins, IDK where to go

    So far I got the simplified forces of 12, 48 and 36 kN, at 1, 5 and 9.5m (from A) respectively, but I'm confused as what to do next, can you help me?
  23. S

    I Nuclear knockout reactions - why does a proton knockout a neutron?

    I am studying the synthesis of FDG for a job interview. The process of creating the F-18 used in FDG involves the collision of O-18 with an accelerated proton -- the proton kicks out a neutron and everything adds up. I am just wondering why a neutron is specifically knocked out, why wouldn't...
  24. I

    Chemistry Electrolysis Problem -- Half reactions and gasses produced

    The solution for a) is 2H2O+2e- → H2 +2OH-, and the solution for b) is O2, because the half reaction at the anode is 2H2O→O2 +4H+ + 4e-. I don't understand why the electrolysis does not involve Na2SO4 being reduced or oxidized, and only considers water. Thanks.
  25. T

    B How does the Gamow window change for different reactions?

    I am learning about the gamow energy and saw a graph explaining the gamow peak - but i am not quite sure i understand the relationship about the overwall gamow window. From what i think i understand so far for fusion if the gamow energy peak is greater the gamow window becomes smaller is this...
  26. I

    Memorizing Organic Chemistry: Structures, Reactions & Tips

    For organic chemistry, what is important to memorize when I am reading a textbook? Should I memorize the structures and names of different organic compounds when they are described as examples? Or is it more important to memorize the organic reactions? Also, does anyone have any tips on how to...
  27. G

    Chemistry How should I proceed with the reactions?

    The question: How will the first reaction take place for instance? Is CH3S- the nucleophile? Will it be added to the oxygen and the other hydrogen(H+) get attached to the other oxygen, with the double bond shifting to the center(C2-C3)? If this is right then where will CN- will attack?
  28. A

    Chemical reactions to depict Le Chatelier's Principle (Temperature)

    If an endothermic reaction has a lower temperature, since the forward reaction rate decreases more, the reaction should produce more energy to compensate for the decreased energy and raise the rate of the forward reaction until it reaches equilibrium with the reverse reaction. Is this saying...
  29. E

    B Can a high energy kinetic impact stimulate nuclear reactions?

    I apologize in advance if this is in the wrong forum, I have no idea where this question would really sit. A number of proposed theoretical weapons, and a concept often used in science fiction, describe a satellite launched, or rather dropped, rod that would gain large amounts of kinetic energy...
  30. Zachrz

    Safe Endothermic reactions for skin

    Hi, Its my first time here in PF, I was wondering what could be a fast endothermic reaction(with water) that does not burn the skin/ cause severe allergic reactions when/after touching or in contact with the reaction. (With high rate of cooling and hopefully no bubbles or gas forming). E.g to be...
  31. R

    What are animal reactions to death?

    This might be a little unsettling for some and I don't know exactly where to post this but here we go... What would the reaction of say a mother cat be in the moment if you where to take one of it's baby and kill it in front of them? Would they attack you? Would they show some signs of sadness...
  32. S

    Correct statement regarding nuclear reactions

    Option A is wrong because the reaction is still possible but just can not happen spontaneously? Option B is correct Option C is wrong because it should be binding energy per nucleon Option D is wrong because it should be nuclear reaction instead of chemical reaction? Thanks
  33. Adrian Tudini

    Equipment for performing chemical reactions

    Hi I was wondering what equipment is used to measure the actual yield of a chemical reaction when a voltage is applied?Thanks.Adrian
  34. Sonim

    Can Calorimeters Measure Enthalpy Variation of Fast Reactions?

    First of all this is a question that I had while reading some concepts of my book, so this isn't a homework question. I have started reading the thermochemistry chapter of my book, and it shows the story of Hess' Law and says that it was created is because a calorimeter can't be used to measure...
  35. A

    Allowed or forbidden reactions

    Hello everybody! I have a doubt about a reaction. $$ p + n \rightarrow \Lambda + \Sigma^+ $$ I have to establish if it is allowed or not. Charge is conserved (1 -> 1) Baryon number is conserved (2 -> 2) Strangeness is not conserved (0 -> -2) Third component of the isospin is not conserved (0...
  36. Dimitris Catzis

    MCNP4c2: Fission Reactions in a Spherical Subcritical Reactor

    Hi, i am new to simulation and for my thesis i have to make a simple simulation by using mcnp4c2. Is anybody familiar with this version of MCNP? I need to calculate the fission reactions per second in a geometry of a spherical sub critical reactor of Uranium with low percentage of U 235 with...
  37. brotherbobby

    Normal reactions at the bases of two light supports

    For equilibrium, using ##\Sigma \vec F = 0##, we get ##n_1 + n_2 = 300\; \text{N}##. Taking the system as a whole and applying ##\Sigma \vec \tau = 0## about the hinge (pin) at the top from where the load is hung, we get ##n_1 \times (0.8) \times 4 = n_2 \times (0.6) \times 3##, by taking...
  38. R

    Reverse reactions and Le Chateliers principle

    In acid-base reactions, the reaction never goes 100% toward the product, right? A reverse reaction will occur and eventually reaction toward product and reactant will reach equilibrium. In lab/industry conditions where yield is important, how does they push the reaction toward product? I am...
  39. TytoAlba95

    Lowered activation energy in enzymatic reactions

    I was stuck with a question; how does enzyme lower the activation energy of a reaction. After several internet searches, I found a post in biostackexchange which partially clarified my question. I'll write my current understanding before I proceed to my actual question. Activation energy is the...
  40. jim mcnamara

    How Dihalogens Catalyze Michael Addition Reactions. - NCBI - piectron?

    How Dihalogens Catalyze Michael Addition Reactions. - NCBI What Michael Addition is about: https://en.wikipedia.org/wiki/Michael_reaction I do not know what the definition of 'piectron' is. In the abstract of the paper, I assume it is a misspell. Which makes me wonder? I learned about...
  41. Decimal

    I Critical radius for nuclear reactions

    Hello! I am taking a course in nuclear physics using the book An Intro to the Physics of Nuclei & Particles by Dunlap. I am a little confused by an explanation related to the critical radius for nuclear reactions. The author first defines a value ##vq## as the average number of neutrons...
  42. Akash47

    Are all substitution reactions reversible?

    Consider a reaction: H2+CuCl2= Cu+2HCl This is a substitution reaction.But is this may not be a reversible reaction since Cu is less active than .So Cu can't substitute H from HCl and make a backward reaction.Is my thinking right?
  43. J

    I Ab Initio methods for chemical reactions?

    Well, I know this is not the "Chemistry" subforum but the question is all about quantum physics. When you study the Schrödinger equation you can compute the time evolution of the wave function, see what energy levels are possible etc. You can calculate the spectra of atoms describing them as a...
  44. M

    Help with Chemical reactions that make water unstable....

    Directly without any introduction _ I really want some help from chemical engineers I was just looking for some chemical reactions that can make water unstable like when we put it in the water the water will move will not be stable or in an equilibrium state thank you very much
  45. C

    Enzymatic surface for multistep reactions

    Is it possible to create a surface with immobilised enzymes (or any other technique) that will perform a multistep reaction?
  46. S

    Producing other metals with thermite reactions

    Hello all, I am curious as to whether it is possible to create aluminium from a thermite reaction. Thermite, as standard, uses aluminium and iron oxide, which react together when burnt to create a blob of molten iron. I would like to achieve a similar result, but with a blob of molten...
  47. K

    Investigating the Stoichiometry of Reactions 1 & 2

    Cl2(g) + 2I–(aq) -->I2(aq) + 2Cl–(aq) (reaction 1) [2S2O3^(2-)(aq) + I2(aq) --> S4O6(2-)(aq) + 2I–(aq) (reaction 2) Why isn't n(I2)(reaction 2)=2n(S2O3^(2-))(reaction 2). I don't see why this is wrong. <edit: remove size and italics>
  48. somasimple

    RedOx Reactions & Phase Changes

    Hi, In electro-chemical cells (galvanic, concentration, etc) or more generally when some RedOx reactions occur in solutes, is there always a phase change? I mean is there always a change in the state of matter from liquid to solid or gas?
  49. nomadreid

    Hibbing: physiological reactions ~ political orientation.... ?

    Dr. John R. Hibbing of the University of Nebraska-Lincoln, USA, has published several papers which basically say that there is a correlation between certain physiological reactions of a person and her political ideology (or at least her position in the American political spectrum, with Hibbing's...