About this video
The video explains what happens at the particle level in a chemical reaction. In physical changes the atoms keep the same arrangement, but in chemical reactions chemical bonds break and new bonds form, so atoms are rearranged into new substances. Substances before the reaction are reactants and after are products, and because no atoms are created or destroyed, the mass of the reactants equals the mass of the products.
You'll be able to
- explain that in a chemical reaction atoms are rearranged as bonds break and new bonds form.
- identify reactants and products in a reaction.
- explain why the total mass stays the same during a chemical reaction.
Key words
- chemical reaction
- A chemical change in which two or more substances combine to form a new substance.
- chemical bond
- The attraction that holds atoms together in a molecule.
- reactants
- The substances present before a chemical reaction.
- products
- The new substances present after a chemical reaction.
- molecule
- A group of atoms stuck together, like H2O.
Transcript
- [Instructor] When wood burns to ash, or when hydrogen and oxygen explode together to give you water, or when you mix these two liquids together to give you this beautiful yellow color, solid precipitate, in all these cases, we are producing new substances altogether, as we call them, chemical changes or chemical reactions.
But the big question is where do these new substances come from? What exactly is happening at the microscopic level? That's what we're gonna figure out in this video. So let's begin. But first, let's recall what physical changes are. If you take, for example, a crystal of NaCl, which is basically table salt, and let's say you hammer it and you break it into tiny pieces, what you get is still crystals of NaCl.
You haven't changed the identity or anything. You've just broken it down. Look, the ratios of the sodium and the chlorine, the ratios of all the atoms that make them up, they're all exactly the same. Similarly, if you consider boiling water, again, H2O molecules stay the same.
The water molecules don't change. The identity does not change. The ratio of hydrogen to oxygen, all of that stays the same. Physical changes. But now let's consider what happens when hydrogen reacts with oxygen. Are you ready to see what happens? Here we go. Ooh. Do you notice what just happened?
Let's look at it one more time. When hydrogen and oxygen react together, look at what happens. Well, if you bring back our hydrogen and oxygen, what you notice is that the arrangement of the atoms have changed. Earlier, these two were stuck together, these two were stuck together forming a molecule, and these two are stuck together forming a molecule.
This is due to their force of attraction between them. But then what happens is that that force of attraction breaks that bond, which we say chemical bond breaks and new chemical bonds are formed. Now this oxygen is stuck to these two hydrogen atoms, and similarly, this oxygen is stuck to these two hydrogen atoms.
This is how look, new stuff is created. The ratios change. This is what water is. Compare this to what we saw earlier, and it comes to physical changes. No new stuff is created. The arrangement of the atoms, they stay exactly the same as before. But over here, the arrangements have changed.
We have new arrangements all together. That's what happens in a chemical change, and that's how new substances are created. And so this is the reason why we define a chemical reaction as a chemical change that occurs when two or more substances combine to form a new substance all together.
This is a chemical reaction. Now, the substances that are present before the chemical reaction, like these hydrogen molecules or these oxygen molecules, we call them reactants, and the substances that are produced that are present after the chemical reaction, the new subset is produced.
We call them products. And here's a quick question. Which one do you think weighs more? The reactants or the products? What do you think? Well, look, there are no new atoms created. A chemical change is all about rearrangement of the same atoms. Since there are no new atoms created, we would expect them to stay way exactly the same.
And they do, reactants and products have the same mass. It's kind of like when you're playing with LEGOs, if you were to take a few bricks and then rearrange them, well, their mass doesn't change, right? Because it's the same bricks. You just arrange them differently.
Now, in some chemical reactions, the products can have gases which can escape, and here the weight of the remaining stuff will reduce because you're not weighing the gases that are escaping over here. But if you were to do this carefully, and if you could contain all the gases and then weigh them, you will definitely find that the weight earlier is the same.
And the weight before and weight after stays pretty much the same. So putting it all together, a chemical reaction is a chemical change that occurs when two or more substances combine to form a new substance. How does it happen? It's not that new atoms are being created.
No, no, no. Atoms are just rearranging. You'll get different arrangements of atoms. And substances that were present before the chemical reactions are called reactants, and the substances after the chemical reactions are called the products. And because no new atoms are created, it's just a rearrangement.
There is no matter or mass that is lost or gained during a chemical reaction.
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