Balancing chemical equations | Chemical reactions | High school chemistry | Khan Academy

Summary

This script explains the concept of balancing chemical equations in chemistry. It uses the example of aluminum reacting with dioxygen to form aluminum oxide, demonstrating how to ensure equal numbers of each atom on both sides of the equation. The process involves multiplying the reactants and products by whole numbers to achieve balance, avoiding fractions and emphasizing the importance of whole number molecules.

Takeaways

• 🔬 Balancing chemical equations is a fundamental concept in chemistry that can be understood with a methodical approach.
• ⚗️ A chemical equation represents a chemical reaction, describing the reactants and products involved.
• 🧪 In the given example, aluminum reacts with dioxygen (O2) to form aluminum oxide (Al2O3).
• ⚖️ Balancing a chemical equation ensures the same number of each type of atom on both sides of the equation.
• 🔄 Initially, the unbalanced equation shows one aluminum atom on the reactant side and two on the product side.
• 🧮 To balance the aluminums, the coefficient in front of aluminum on the reactant side is adjusted to 2.
• 🌐 The oxygens are initially unbalanced with two atoms on the reactant side and three on the product side.
• ➗ Multiplying by 1.5 balances the oxygens, but fractional molecules are not preferred in chemical equations.
• ✖️ Multiplying all coefficients by 2 converts fractional coefficients to whole numbers, resulting in 4 Al + 3 O2 → 2 Al2O3.
• 🔍 The final balanced equation has four aluminum atoms and six oxygen atoms on both sides, confirming its balance.

Q & A

What is a chemical equation?

A chemical equation is a symbolic representation of a chemical reaction, showing the reactants on the left and the products on the right, with the respective chemical formulas and coefficients indicating the quantities involved.

Why is it necessary to balance chemical equations?

Balancing chemical equations is necessary to ensure that the number of atoms of each element is the same on both sides of the equation, adhering to the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.

What is the initial imbalance in the example of aluminum reacting with dioxygen?

Initially, the imbalance is that there is one aluminum atom on the left side and two aluminum atoms on the right side in the aluminum oxide molecule, and two oxygen atoms on the left side as a dioxygen molecule versus three oxygen atoms on the right side in the aluminum oxide molecule.

How can you start balancing the aluminum atoms in the given chemical equation?

You can start by placing a coefficient of 2 in front of the aluminum on the left side of the equation to match the two aluminum atoms in the aluminum oxide molecule on the right side.

What is the issue with simply multiplying the dioxygen molecule by 1.5 to balance the oxygen atoms?

The issue is that in chemistry, we prefer whole number molecules rather than fractions or decimals, as it is not practical to have a half molecule in a chemical reaction.

How did the speaker resolve the issue of having 1.5 molecules of dioxygen?

The speaker resolved the issue by multiplying both sides of the equation by 2, which is the smallest whole number that can eliminate the fraction and result in whole number molecules.

What is the final balanced chemical equation for the reaction between aluminum and dioxygen?

The final balanced chemical equation is 4Al + 3O2 → 2Al2O3, indicating that four aluminum atoms react with three dioxygen molecules to produce two molecules of aluminum oxide.

What is the significance of using whole number coefficients in chemical equations?

Using whole number coefficients ensures that the law of conservation of mass is followed, as it represents the actual physical quantities of molecules that participate in the reaction.

Can you have a fraction or decimal as a coefficient in a balanced chemical equation?

Ideally, no. Fractions or decimals as coefficients are not preferred because they do not represent the discrete nature of molecules in a chemical reaction.

What is the role of the implicit '1' in the chemical equation before multiplying by two?

The implicit '1' represents the stoichiometric coefficient for the reactants and products that are not explicitly shown with a coefficient. It is assumed to be '1' unless otherwise stated.

How can you verify that a chemical equation is balanced?

You can verify that a chemical equation is balanced by counting the number of atoms of each element on both sides of the equation and ensuring they are equal.

Outlines

00:00

🧪 Balancing Chemical Equations: An Introduction

This paragraph introduces the concept of balancing chemical equations, a fundamental yet often confusing aspect of chemistry. It explains that a chemical equation represents a reaction, using the example of aluminum reacting with dioxygen to form aluminum oxide. The paragraph emphasizes the importance of having the same number of each type of atom on both sides of the equation, highlighting the imbalance in the initial example. It then explores the process of balancing by adjusting the coefficients in front of the chemical species to ensure an equal number of aluminum and oxygen atoms on both sides of the equation.

Keywords

💡Balancing chemical equations

Balancing chemical equations is the process of ensuring that the number of atoms of each element on the reactant side of a chemical equation is equal to the number of atoms of that element on the product side. This is a fundamental principle in chemistry, reflecting the law of conservation of mass. In the video, the concept is introduced and demonstrated through the example of the reaction between aluminum and oxygen to form aluminum oxide, emphasizing the importance of having equal numbers of each type of atom on both sides of the equation.

💡Chemical equation

A chemical equation is a symbolic representation of a chemical reaction, showing the reactants on the left and the products on the right, with an arrow in between indicating the transformation. The video script uses the chemical equation as a starting point to discuss the concept of balancing, highlighting the need for a balanced number of atoms for each element involved in the reaction.

💡Aluminum

Aluminum is a chemical element with the symbol Al and atomic number 13. It is a silvery-white, soft, nonmagnetic, and ductile metal in the boron group. In the script, aluminum is used as one of the reactants in the example chemical equation, where it reacts with oxygen to form aluminum oxide, illustrating the process of balancing the equation by adjusting the coefficients in front of the chemical species.

💡Dioxygen molecule

A dioxygen molecule refers to an O2 molecule, which consists of two oxygen atoms bonded together. It is a key component in the Earth's atmosphere and is essential for aerobic respiration in most living organisms. In the context of the video, a dioxygen molecule is combined with aluminum to form aluminum oxide, demonstrating the need to balance the number of oxygen atoms in the reaction.

💡Aluminum oxide

Aluminum oxide, with the chemical formula Al2O3, is a common compound of aluminum and oxygen. It is a white solid that is often used as an abrasive and as a refractory material. The video script describes the formation of aluminum oxide from aluminum and dioxygen, emphasizing the stoichiometry involved in balancing the chemical equation.

💡Stoichiometry

Stoichiometry is the quantitative relationship between the amounts of reactants and products in a chemical reaction. It is used to determine the balanced equation and the amounts of substances that can be produced or consumed in a reaction. The video script discusses stoichiometry in the context of balancing the chemical equation for the reaction between aluminum and oxygen.

💡Reactants

Reactants are the substances that are consumed in a chemical reaction to produce other substances, known as products. In the video, aluminum and dioxygen are the reactants in the formation of aluminum oxide, and the script explains how to balance the equation by ensuring the correct number of atoms of each element is present on both sides.

💡Products

Products are the substances produced in a chemical reaction. In the context of the video, aluminum oxide is the product of the reaction between aluminum and dioxygen. The script explains how to achieve a balanced equation by ensuring an equal number of atoms of each element in the reactants and products.

💡Coefficients

In a chemical equation, coefficients are the numbers placed in front of the chemical formulas to indicate the number of molecules or atoms of a substance involved in the reaction. The video script demonstrates how to adjust coefficients to balance the chemical equation, showing that multiplying the reactants and products by the same whole number can achieve this balance.

💡Conservation of mass

The law of conservation of mass states that mass in a closed system cannot be created or destroyed by chemical reactions. It is a fundamental principle in chemistry that underlies the need for balancing chemical equations. The video script explains that the number of atoms of each element must be the same on both sides of the equation to adhere to this law.

💡Whole number molecules

The concept of whole number molecules refers to the preference in chemistry for using whole numbers when balancing chemical equations, rather than fractions or decimals, to represent the number of molecules of a substance. The video script discusses the avoidance of fractional coefficients and the process of multiplying by a whole number to achieve whole number molecules, as seen in the example of balancing the aluminum and oxygen reaction.

Highlights

Balancing chemical equations is a fundamental concept in chemistry that can be confusing but is manageable with a methodical approach.

A chemical equation describes a reaction, such as the reaction between aluminum and dioxygen to form aluminum oxide.

The importance of having the same number of each atom on both sides of a chemical equation for it to be balanced.

The initial imbalance in the number of aluminum and oxygen atoms in the example equation.

The process of balancing the aluminum atoms by doubling the number on the reactant side.

The challenge of balancing oxygen atoms when they are not in whole number ratios.

The unconventional approach of multiplying by 1.5 to balance the oxygen atoms, which is not preferred in chemistry.

The convention of using whole number molecules in chemical equations, avoiding fractions or decimals.

The method of multiplying both sides of the equation by the same number to eliminate fractions and achieve balance.

The step-by-step process of multiplying the reactants and products by two to balance the oxygen atoms.

The verification of the balanced chemical equation with equal numbers of aluminum and oxygen atoms on both sides.

The final balanced chemical equation: 4Al + 3O2 yields 2Al2O3.

The art of balancing chemical equations as a skill that can be mastered with practice and understanding.

The analogy between balancing chemical equations and solving algebraic equations by using whole numbers.

The importance of understanding the principles of conservation of mass in chemical reactions.

The demonstration of the methodical process of balancing a chemical equation through an example.

The emphasis on the need for careful observation and adjustment to achieve a balanced chemical equation.

The explanation of how to handle molecules with more than one atom to ensure the equation is balanced.

The final verification step to ensure that the chemical equation is balanced and accurate.

Transcripts

- Balancing chemical equations is one of those

concepts in chemistry that often confuses people.

But I think we'll see that if we work through this

carefully and methodically, and we also appreciate

the art of balancing chemical equations,

that it's actually not too bad.

So first of all, what is a chemical equation?

Well this is a chemical equation right over here.

It's describing a reaction.

So if I take an atom of aluminum and I add it

to a dioxygen molecule, so a molecule that

has two oxygens with it, under the appropriate conditions

they will react to form aluminum oxide.

And the aluminum oxide molecule has

two aluminum atoms and three oxygen atoms.

And so you might say, "Okay, well what's

"the balancing business all about?

"I have a chemical reaction.

"What do I have to balance?"

Well if you look carefully, you might notice that

you don't have the same number of each atom on both sides.

For example, right over here on the left-hand side,

how many aluminums do we have?

Well on the left-hand side, we have one aluminum.

How many do we have on the right-hand side?

Well on the right-hand side, we have two aluminums.

And so aluminum just can't appear out of thin air

by virtue of some magical reaction.

You have to have the same amount of aluminums on both sides,

and the same thing is true for the oxygens.

Over here on the left-hand side, we have two oxygens.

They form one dioxygen molecule that has two oxygen atoms.

And then over here in the aluminum oxide molecule,

we have three.

We have three oxygen atoms.

So once again, we can't just have miraculously

an oxygen atom appear out of nowhere.

So we have to balance the number of aluminums on both sides,

this number and this number should be the same,

and we have to balance the number of oxygens,

this number and that number should be the same.

So how do we do that?

Well one thing might be to say, "Okay, if I've got

"two aluminums here and I have one aluminum here,

"well why don't I just double the number

"of aluminums right over here?"

I could just write a two in front of it,

so now this has two aluminums,

so I no longer have one aluminum here.

I now have two aluminums, and so it looks like

the aluminums are balanced, and they are indeed balanced.

But still we have an issue with the oxygens.

Over here I have two oxygens.

Over here I have three oxygens.

So one thing that you might say is,

"Okay, well how do I go from two to three?

"I could multiply by 1.5."

So I could multiply by 1.5,

and if I multiply 1.5 times two,

that's going to be three.

So now I have three oxygen atoms on this side

and three oxygen atoms on this side.

But the convention is that we don't like

writing "1.5 molecules."

We don't like having this notion of a half molecule,

which is kind of this bizarre notion.

We want whole number molecules.

So what can we do?

Well, you can imagine that this makes it very similar

to what you did in algebra, an algebraic equation.

We just can multiply both sides by the same number

that gets rid of having this fraction or this decimal here.

So if we multiplied both sides

by two, we're going to do that.

This is going to be a four, this is going to be a three,

this is going to be a two right over here.

So let me do that.

Let me multiply both sides by two.

So instead of two aluminum atoms,

let me have four aluminum...

Actually, let me just write the chemical equation first

in the form that it was before.

So I had aluminum plus dioxygen,

a molecule of two oxygens, yielding in the reaction --

these are the reactants, this is the product --

aluminum, aluminum, aluminum oxide.

So what I'm saying here is to get rid of this 1.5,

to turn it into a whole number, let's multiply all of these,

all of the number of molecules by two.

And here, there's implicitly a one...

Let me do this in a different color.

There is implicitly a one right over here.

So let's multiply all of these by two.

So two times two is...

Let me do that same color.

Two times two is four...

That's not the same color.

Two times two is four.

1.5 times two is three.

And then one times two,

one times two is two.

And now you can verify how many aluminums

do we have on each side?

Well I have four aluminum atoms on the left-hand side,

and how many do I have on the right-hand side?

I have four aluminum atoms.

How many oxygens do I have on the left-hand side?

I have three molecules of dioxygen.

Each molecule has two oxygen atoms,

so I have six oxygens on the left,

and I have two times three oxygens

on the right, or I have six oxygens.

So my chemical equation is now balanced.