What is Chemical reactions: Definition and 77 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. 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...
  2. A

    Balancing Chemical Equations - Is RHS=LHS true always?

    Below are some examples of Chemical Reactions/Equations. 2H2 + O2 = 2H2O CU+ H2SO4=CUSO4 + H2 2MG + O2 =2MGO 2NA + CL2 =2NACL and so on ... I mean is RHS=LHS always true for all Chemical Equations or only for specific chemical equations or not true at all? 2H2O=2H2 + O2 CUSO4+H2=CU+H2SO4...
  3. 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...
  4. 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...
  5. 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...
  6. 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
  7. 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...
  8. 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
  9. I

    Spontaneous "back-and-forth" chemical reactions

    I was wondering if there is a good example of a chemical reaction cascade that reverses itself, or does a "loop" in nature outside of life? First thing that came to mind was water evaporating and coming back down from clouds, but that is not really a chemical reaction. Or would it be considered...
  10. I

    Spontaneous uncoded chemical reactions in living systems?

    How easy (or not) is it for spontaneous chemical reactions to occur in a living cell, but particularly reactions that are not a result of and between coded molecules? For example, when we eat food, in the process while the essential molecules are being taken to be integrated into "coded...
  11. yrjosmiel

    Why do chemical reactions release energy?

    If I have a container filled with oxygen and hydrogen and heat it up, it would react and create water and heat. Why was heat released? What happens in chemical reactions at a microscopic level that makes it release energy in the form of heat?
  12. G

    Software to simulate an atom and chemical reactions?

    Hi, I am trying to simulate in 3D a bunch of chemical reactions. Some of them are complex and I would like to be able to just spawn the elements and molecules and watch them collide to see what happens. Is their any free software to do this?
  13. M

    Reactions to produce electricity

    I am looking for reactions that can power a small car. I'm currently working on a college project where we have to make a car powered by a chemical reaction. Could anyone guide me in the right direction?
  14. B

    Predicting precipitates in a solution

    Let's say we have a solution supersaturated with several species of ions, and there are a few potential precipitates predicted to be formed that all have one contain one species in common (therefore they are competing for that common ionic species). How can we predict which species will...
  15. physea

    Role of pressure and temperature in chemical reactions

    Hello! I have a feeling and I would like to know if it is true. I feel that if a reaction between two gases, takes place in high temperature only (perhaps with the presence of a catalyst), then if we increase the pressure high enough and keep the temperature low, the reaction may still take...
  16. S

    Entropy vs. enthelpy in chemical reactions.

    Hello, I am learning about using Free energy change /delta G to determine if a chemical reaction will occur spontaneously. /delta G = /delta H + T*/delta S. Now, enthalpy change can drive a reaction which leads to a decrease in entropy (multiple reactants => single product). My Question...
  17. A

    State of matter in chemical reaction

    Does anybody know where I could find worksheet to practice naming the state of matter in chemical reactions (with answers in the worksheet to check) as I have problems with that. Especially deciding when it is aqueous. I know (well at least i am right) that when you have acid as reactants it has...
  18. R

    Overall rate of reactions of decomposition of dimethyl ether

    Hi Guys, I'm solving this for almost a week, this is a part of our probset in chemical kinetics: The thermal decomposition of dimethyl ether: CH3OCH3 ---K1--> CH3' + OCH3' CH3' + CH3OCH3 ---K2--> CH4 + CH2OCH3' CH2OCH3' + --k3---> CH3' + HCHO CH3' + CH2OCH3' ---k4--> C2H5OCH3 using steady-state...
  19. 1

    Balance Chemical Reactions

    Homework Statement Write the balanced chemical reactions in ionic and neutral forms for: i) Enargite leaching by sodium bisulfide in basic solution to form chalcocite and thioarsenate. ii) Oxygen pressure oxidation of As2S5 in acid solution to form scorodite. Assume all sulfide sulfur forms...
  20. EternusVia

    Positrons, electrons, and chemical reactions

    Hello all, In chemistry class we recently began the subject of nuclear chemistry. I'm sure you all know that nuclear chemistry unleashes a swarm of new particles. My teacher mentioned the Positron, a particle few of my classmates recognized. To help us understand its nature we were told to...
  21. P

    How does selectivity work in chemical reactions again?

    I got a bit of a question that needs answering. See, I'm still not understanding how basic selectivity in organic chemistry works. I'm not looking to know the mechanism or anything. Just one simple thing. I'll give the description given from some textbook first: Homework Statement The...
  22. U

    Entropy generation in chemical reactions

    So from the first law for a closed system, dU=dQ-dW=dQ-PdV From the second law, dS=dQ/T + Sgenerated (i.e. the entropy generated) Putting expression of dQ from second law into first law, dU=T*dS-T*Sgen-PdV If s and v are constant, dU= -T*Sgen>0 Hence dU<0 This is a...
  23. L

    The electrical energy produced from chemical reactions

    Hi there, new member here (**TL,DR is available at the bottom of this post**) As you may or may not know, if you were to put in a plate of zinc at one end and a plate of copper at the other, put them in an acid solution, electricity will be produced. More of that stuff here...
  24. M

    Another: Simple Displacement Reactions & Balancing Chemical Reactions

    Homework Statement Complete the simple displacement reactions and balance any chemical reactions. Homework Equations Pb + NaCl → __________ + ____________ The Attempt at a Solution Pb + NaCl → PbCl + Na But I don't know which charge of lead to use: Pb2+ or Pb4+ to...
  25. M

    Simple Displacement Reactions & Balancing Chemical Reactions

    Homework Statement Complete the simple displacement reactions and balance any chemical reactions. Homework Equations copper + silver nitrate → __________ + ____________ The Attempt at a Solution Since copper on the periodic table can be Cu2+ or Cu+, my solution was copper + silver...
  26. A

    Looking for a better understanding of chemical reactions

    This is for self study. 1. Why don't some molecules react? For example, NH3 + CaCO3 2. Where can I find a good list of standart state enthalpy and entropy values for molecules? My Chem I teacher gave us all a pretty good handout, but that's been a while ago, and I don't know where it is...
  27. I

    Doubts about potential energy, bonding and chemical reactions

    Hello everybody, First, sorry for the bad english, it is not my language. I have a few theoretical problems and I would be very thankful if you helped me. 1º: I see the graphic about covalent bonds and potential energy a lot of times and they always say that potential energy...
  28. D

    Gaseous Chemical Reactions and Volumes

    (Hi everyone! Apologising for the trivial(and likely boring) question in advance. Sadly, it has me boggled for some reason). Homework Statement In a certain temperature and under a pressure, 500 mL of H2, and 100 mL of O2 are poured into a container. A Chemical reaction occurs as follows...
  29. B

    Mass loss in common chemical reactions?

    In any common chemical reaction that releases energy, say the reaction 2H2 + O2 = 2H2O, what mass is converted to energy via the E = MC2 equation? What sub-atomic particles are converted to energy during ordinary chemical reactions? I was taught us in HS and under-grad chemistry classes that...
  30. S

    Chemical Reactions of Substances & Professors' Advice

    [1] Good afternoon anyone! I am a Petroleum Engineering student at Palawan State University and conducting a research. I have seen a topic of research, and it is: The Production of Petrol from Air and Electricity, and I need to find some reaction equations for my topic, here are some: Air...
  31. M

    Describing chemical reactions

    For the reaction, 2SO2(g) +O2 (g) ->2SO3. Why is it wrong to say that the reaction must involve a collision between one O2 and two SO2 molecules? Don't they form S2O6, which is equivalent to the product, 2SO3? What should I say then, about the reactants in forming 2SO3? Thank you so much!
  32. C

    Can chemical reactions take place at absolute zero?

    Title says it all. I've been wondering. The argument is, is there a minimal temperature required for any chemical reaction to reach its activation threshold? Or would the energylessness of the atoms prevent bonds from forming (or dissolving, I guess)? Conrad.
  33. J

    Which chemical reactions are possible?

    The question in the attachment is from an old entrance exam for medical school. The correct answer is B. I realize that if I assign a 'point value' to the levels of reactivity for each of the letters i.e. the more reactive the higher the number T = 1 Q = 2 M = 3 J = 4 and that if...
  34. S

    Help in chemical reactions

    hi i was wondering if a carbonate will react with carbonic acid. such as calcium carbonate with carbonic acid: CaCO3+H2CO3CaCO3+CO2+H2O. Thanks for the help!
  35. H

    (at the most fundamental level) why does energy release upon chemical reactions?

    Please don't be bothered by my question, i know it sounds real elementary and it could easily be researched, but after doing so, i haven't been able to find anything on how or why the natural phenomenon of energy release takes place as chemicals react with one another. If you think about it...
  36. J

    P.Chem: Rate of chemical reactions; Half-life

    A second- order reaction of the type A + B -->P was carried out in a solution that was initially 0.075 mol dm^-3 in A and 0.050 mol dm^-3 in B. After 1.0 h the concentration of A had fallen to 0.020 mol dm^-3. a) Calculate the Rate constant. b) Solve for the half- life of each of the reactants...
  37. A

    How to Determine the Products of Chemical Reactions

    I'm in an intro chem class right now and I honestly have no clue where I was when they talked about this BUT here goes... I have several spot tests that I have to come up with the formulas for, I have the reactants just have no clue what the products are. I understand how to balance formulas I...
  38. L

    Comparing Cl 35 and Cl 37 Chemical Reactions

    what difference,if any are expected between the chemical reactions of Cl 35 and Cl 37? I'm not sure how to attempt this question =/
  39. W

    Are Chemical Reactions Quantumly Indeterminate?

    When one atom chemically combines to another atom is it totally determinate? I mean if one atom gradually approaches the other would the reaction occur at exactly the same place or does HUP play a part and give it some randomness? I think there is a known answer to this but I cannot find it...
  40. S

    Why chemical reactions happens?

    We do know that in the process of chemical reaction, atoms or molecules come closer, form bonds and get stabilized by sharing or donating electrons, thus filling the valance shell of atoms. But why do they want to get stabilized?.
  41. F

    Chemical Reactions Producing Light and Heat

    Hi there, I was just wondering about this point, why do some chemical reactions produce bright light (and even sound) whilst others do not? For example: in the rapid oxidation of Magnesium in air after given the necessary activation energy, a bright white light is emitted. However, in many...
  42. L

    Chemical Reactions: Stronger Bonds, Reactive Elements

    The more reactive an element, the stronger bonds it forms, because the same energy must be put into that bond to separate them that was put out due to the reaction. Most of the elements we are made of, in pure form, are reactive correct? Carbon and oxygen have very high electronegativities, and...
  43. J

    Fictional iron chemical reactions

    I am a mechanical engineer who is writing a science fiction novel where a means has been discovered to temporarily reduce the mass of the nucleus of certain 56Fe atoms. When a certain field is applied to the iron, it gets lighter. When the field is removed, the iron regains its original mass...
  44. O

    Chemical reactions according to equation

    Homework Statement 12.8 gr of O Flowed over 100 gr hot Al. and i got this reactions equation: 4Al(s)+3O2(g)--->2Al2O3(g) a)how many gr Aluminum oxide Al203 Created through a chemical process ? i know i need to find the number of moles of oxygen which flowed. and the number of...
  45. J

    Explaining Bonding & Mass in Chemical Reactions

    This at first may sound like a question based in chemistry, but I feel it is more of a particle physics question. When a chemical reaction occurs, it can be exothermic or endothermic. It is my understanding that if bonds are formed, energy is released, and when bonds are broken, energy is...
  46. W

    Are there any chemical reactions that can generate radio waves?

    I know that chemical reactions can emit light and heat, and these are part of the electromagnetic spectrum. But are there any chemical reactions that can directly produce electromagnetic emissions in the radio part of the spectrum?
  47. J

    Why Do Some Chemical Reactions Take Time?

    Why some of the chemical reactions are slow? Is it directly proportional to the size and shape of the molecules - since the complex shape makes 2 molecules less probable coming in contact with the appropriate alignment for the chemical reaction to happen? I'm trying to find an answer within...
  48. J

    Mass lost in chemical reactions?

    Homework Statement In chemical reactions, when energy is released, mass is lost according to Einstein’s formula just as it is for nuclear energy (though the amounts lost are so small that for centuries scientists thought that mass was conserved). When a kilogram of coal is burned, it...
  49. T

    Kinetics in chemical reactions

    Can a catalyzer in a reaction be an intermediary as well? Im not sure, i don't think because it would have to consume it self...
  50. E

    Chemical reactions to silver?

    I'm writing a book but my chemical knowledge is near non-existent and I need a TRUE scientific reason as to why warewolves would be 'allergic' to silver. I know I could make something up but I would like to have as much fact as I can when it comes to stuff like this. It'll help me stand out.