If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Chemical reactions introduction

Reactants and products in reversible and irreversible chemical reactions.

Want to join the conversation?

  • mr pink red style avatar for user Monte` Rainwater
    Aren't hydrogen and oxygen both flammable? Why isn't water?
    (114 votes)
    Default Khan Academy avatar avatar for user
    • boggle blue style avatar for user Davin V Jones
      Oxygen is not flammable. A flame is what occurs when a fuel, like hydrogen, is rapidly oxidized, producing thermal energy. Oxygen cannot burn on its own. Water is basically the ash of hydrogen. It is already oxidized and can't burn any further without first breaking down the molecule back into hydrogen and oxygen.
      (259 votes)
  • leafers tree style avatar for user Nabil Shamee
    Why can't humans just chemically react molecules of hydrogen and oxygen to create water?
    (26 votes)
    Default Khan Academy avatar avatar for user
    • piceratops seed style avatar for user RogerP
      They can! This is easy to do if you have a cylinder of oxygen and a cylinder of hydrogen. Mix one part oxygen with two parts hydrogen, stand well back and ignite. There is an explosive reaction which produces water.

      Nonetheless, this is not a practical way of solving the droughts that affect many countries around the world.
      (95 votes)
  • duskpin ultimate style avatar for user kaya v
    if there is a tank filled with oxygen and hydrogen how come it doesn't just turn into water? are they put separately? why and how is hydrogen and oxygen liquid?
    (9 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user Hrishikesh Vinod
      Hydrogen, the fuel for the main engines of rockets, is the lightest element and normally exists as a gas. Gases – especially lightweight hydrogen – are low-density, which means a little of it takes up a lot of space. To have enough to power a large combustion reaction would require an incredibly large tank to hold it – the opposite of what’s needed for an aerodynamically designed launch vehicle.

      To get around this problem, turn the hydrogen gas into a liquid, which is denser than a gas. This means cooling the hydrogen to a temperature of ‑423 degrees Fahrenheit (‑253 degrees Celsius). Seriously cold.

      Oxygen Although it’s denser than hydrogen, oxygen also needs to be compressed into a liquid to fit in a smaller, lighter tank. To transform oxygen into its liquid state, it is cooled to a temperature of ‑297 degrees Fahrenheit (‑183 degrees Celsius). While that’s balmy compared to LH2, both propellant ingredients need special handling at these temperatures. What’s more, the cryogenic LH2 and LOX evaporate quickly at ambient pressure and temperature, meaning the rocket can’t be loaded with propellant until a few hours before launch.

      Once in the tanks and with the launch countdown nearing zero, the LH2 and LOX are pumped into the combustion chamber of each engine. When the propellant is ignited, the hydrogen reacts explosively with oxygen to form: water! Elementary!

      2H2 + O2 = 2H2O + Energy

      This “green” reaction releases massive amounts of energy along with super heated water (steam). The hydrogen-oxygen reaction generates tremendous heat, causing the water vapor to expand and exit the engine nozzles at speeds of 10,000 miles per hour! All that fast-moving steam creates the thrust that propels the rocket from Earth.

      I hope you understood it friend.
      Please do leave an upvote if you do understood it.
      (82 votes)
  • blobby green style avatar for user Payton Johnson
    Is there a limit to how many elements can be involved in a chemical equation?
    (12 votes)
    Default Khan Academy avatar avatar for user
    • old spice man green style avatar for user Matt B
      No limit. in fact, you might even be able to fit 100 elements in a super long equation (although nobody in their right mind would have good reasons to react 100 different elements together other than maybe curiosity)
      (25 votes)
  • blobby green style avatar for user Mohammad HMD
    is it possible to get water H2O from just one molecular H2 and O2 ??
    (0 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user Jason Bai
    Is the reason that H2 is not a compound is because H is only 1 molecule, making it single instead of a compound?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Rodrigo
    I don't understand what's the difference between saying "hydrogen" and "molecular hydrogen".
    (3 votes)
    Default Khan Academy avatar avatar for user
    • leaf grey style avatar for user Alex
      Hydrogen is generally referring to H, whether it's in H2O, NaHCO3, or just the element in general. "Molecular hydrogen" refers specifically to the hydrogen molecule H2. Let me know if this helps.
      (14 votes)
  • hopper cool style avatar for user Shiv Sah👨‍🚀🚀🌕
    What is "temperature change" physical or chemical reaction?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • leafers ultimate style avatar for user Neeraj Bannatwala
    H2 and O2 are gases and so highly reactive then how do they combine to form a less reactive compound
    (3 votes)
    Default Khan Academy avatar avatar for user
    • starky ultimate style avatar for user Alex H
      Think of it this way:

      All elements have electrons, and most are reactive. How an element can become stable? Well, they must fill up their outer electron shells by getting rid of or gaining electrons. Now to the example. A hydrogen has one electron but can hold two, and an oxygen has 6 electrons(outer shell) but can hold 8. Well, when they combine to make water, they "share" electrons to complete each other's outer shells so everyone is happy.
      In a diagram: H-O-H(dashes are shared electrons)
      (3 votes)
  • leaf blue style avatar for user ozyo the unfunny
    so if i got a bunch of hydrogen atoms, and a bunch of oxygen, and put them in say, a mason jar, i would have made water? (lets see if i can get my hands on some atoms)
    (2 votes)
    Default Khan Academy avatar avatar for user
    • leaf red style avatar for user Richard
      If you just had the correct reactants and you mixed them together, not much will happen. You also need energy. Specifically, you need enough energy for the reactants to overcome the activation energy and actually react with each other. Otherwise without the energy they would remain unreacted in the jar.

      As a side note, I wouldn’t recommend trying to make water like that inside something like glass mason jar since the reaction is highly exothermic and releases a lot of energy very quickly. Having a quick and large release of energy in a small, brittle container is a recipe for an explosion where the glass shards of the container get shot everywhere. Otherwise known as a bomb.

      Hope that helps.
      (4 votes)

Video transcript

- [Voiceover] Let's talk a little bit about chemical reactions. And chemical reactions are a very big deal. Without chemical reactions, you or I would not exist. In your body right now, there are countless chemical reactions going on every second. Without chemical reactions, we would have no life, we would not even have the universe as we know it. So what are chemical reactions. Well, they're any time that you have bonds being formed or broken between atoms or molecules. So what are we talking about there? Well this is maybe one of the most fundamental chemical reactions. Once again if one never occurred, we'd be in trouble, we would not have, we would not have any water. But let's think about what it is actually describing. So over here on the left-hand side we have the reactants. Let me write that down. So here we have the reactants. These are the molecules that are going to react. And then we have an arrow that moves us to the product. So let me do that in a different color. So we have an arrow that moves us to the product, or we could say the products. And so what are the reactants here? Well we have molecular hydrogen and we have molecular oxygen. Now why did I say molecular hydrogen? Because molecular hydrogen, which is the state that you would typically find hydrogen in if you just have it by itself, it is actually made up of two hydrogen atoms. You see it right over here, one, two hydrogen atoms. And what we have in order to have this reaction, you don't just need one molecular hydrogen and one, or one molecule of hydrogen and one molecule of oxygen. For every, for this reaction to happen, you actually have two molecules of molecular hydrogen. So this is actually made up of four hydrogen atoms. So let me make this clear. So this right over here, this is two molecules of molecular hydrogen. And that's why we have the two right out front of the H sub-two. This little subscript two tells us there's two of the hydrogen atoms in this molecule. And then this big, this big white two that we have right over here, that tells us that we're dealing with two of those molecules for this reaction to happen, that we need two of these molecules for every, for every molecule of molecular oxygen. And molecular oxygen, once again, this is composed of two oxygen atoms. One, two. So under the right conditions, so you need a little bit of energy to make this happen. If under the right conditions these two things are going to react. And actually it's very, very reactive, molecular hydrogen and molecular oxygen. So much so that it's actually used for rocket fuel. You are going to produce two molecules of water. We see that right over here. And look, I did not create or destroy any atoms. I had one, I had one, I had one oxygen atom here. It was part of the oxygen molecule right here, then I have the second one right over here now. Now they are part of separate molecules. I had, I had a, I had one two, three, four hydrogens. I now have one, two, three, four hydrogens, just like that. And actually this produces a... So we could say some energy, and I'm being inexact right over here. Some energy and then we could say a lot of energy. A lot of energy. So this is a reaction that you just give it a little bit of a kick-start and it really wants to happen. A lot, a lot of energy. So one thing that you might wonder, and this is something that I first wondered when I learned about reactions, well how do, how does this happen? You know, is this a very organized thing? You know, do these molecules somehow know to react with each other? And the answer's no. Chemistry is a incredibly messy thing. You have these things bouncing around, they have energy. They're bouncing around all over the place and actually when you provide energy, they're gonna bounce around even more rigorously, enough so that they collide in the right ways so that they break their old bonds and then they form these new bonds. So whenever you see these reactions in biology or chemistry class, keep that in mind. It looks all neat and organized but in a real system, these are all of these things just bouncing around in all different crazy ways. And that's why energy's an important thing here. Because the more energy you apply to the system, the more that they're going to bounce around, the more that they're going to interact with each other. The more reactants you put in, the more chance they're going to bounce around and be able to react with each other. Now I'm gonna introduce another word that you're gonna see in chemistry a lot. This water, these two... We see we have two water molecules here. We could call them molecules, but since they are actually made up of two or more different elements, we could also call this a compound. So water, water is, you could call it a molecule, or you could call it a compound. So this is a molecule or compound, while this molecular hydrogen, you would not call this a compound. And this molecular oxygen, of course it's a molecule, but you would not call it a compound either. And just to get an appreciation of how much energy this produces, let me just show you this picture right over here. That's the space shuttle and this, this big tank right over here, let me... This big tank contains a bunch of liquid oxygen and hydrogen. And to create this incredible amount of energy, it actually just... You mix the two together with a little bit of energy and then you produce a ton of energy that makes the rocket, that makes the space shuttle. Well, space shuttle's been discontinued now, but back when they did it, to make it get it's necessary, it's necessary velocity. Now let's talk about the idea. So, you know, this reaction, strongly goes in this, in the direction of going to water. But it can actually go the other way, but it's very, very hard for it to go the other way. So in general we would consider this to be an irreversible reaction, even though it is. You know irreversible sounds like, hey you can't go the other way. It just really means that it's very unlikely to go the other way. You have to supply a lot of energy to go the other way. To make this reaction go the other way, you would have to do something called electrolysis, you provide energy, etcetera, etcetera. But in general, the way that this is written, because the arrow is only pointing in one direction, this is implying that it is irreversible. Irreversible. Irreversible. Which probably makes you think, well what about reversible reactions? And I have an example of a reversible reaction, right over here. I have a one bicarbonate ion. And the word ion, that's just used to describe any molecule or atom that has either, has an imbalance of electrons or protons that cause it to have a net charge. So this makes this an ion, and actually right over here, this is a hydrogen, this is a hydrogen ion right over here. Both of these are charged. One has a positive charge, one has a negative charge. But they are both, they are both ions. And this reactions right over here, you have the bicarbonate ion that looks something like this. This is just my hand-drawing of it. Reacting with a hydrogen ion, it's really a hydrogen atom that has lost it's electron, so some people would even say this is a proton right over here. This is an equilibrium reaction, where it can form carbonic acid. And notice all that's happening is this hydrogen is attaching to one of the oxygens over here. And this is an equilibrium because if in an actual, in an actual solution, it's going back and forth. If you actually provide more reactants, you're gonna go more in that direction. If you provide more of the products over here, then you're gonna go in that direction. And so in an actual, in an actual environment, in an actual system, it's constantly going back and forth between these two things. And different reversible reactions might tend to one side or the other. If you provide more of the stuff on one side, it might go more in the other directions because these are gonna, they're gonna be more likely to interact, Or if you provide more of this, it might go in the other direction because these might more likely react with their surroundings or disassociate in some way. Now just to get a sense of, you know, it's nice to kind of, you know, are these just some random letters that I wrote here? Carbonic acid is actually an incredibly important molecule, or we could call it a compound because it's made up of two or, two or more elements, in living systems and in fact, you know, even in the environment. And even when you go out to get some fast food. When you have carbonated drinks, it has carbonic acid in it that disassociates into carbon dioxide and that carbon dioxide is what you see bubbling up. Carbonic acid is incredibly important in how your body deals with excess carbon dioxide in its bloodstream. Carbonic acid is involved in the oceans taking up carbon dioxide from the atmosphere. So when you're studying chemistry, especially in the context of biology, these aren't just, you know, interesting things that seem very academic. These are effecting your real life and your body and your environment.