Lesson Video: Conservation of Mass Chemistry

In this video, we will learn the definition of the law of conservation of mass, apply it to reactions, and learn some of its limitations.

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Video Transcript

In this video, we will learn the definition of the law of conservation of mass applied to reactions and learn some of its limitations. The law of conservation of mass says that the mass of a closed system cannot change. The law of conservation of mass is sometimes known as the principle of mass conservation or the law of indestructibility of matter. A closed system is a sample of matter or a regional space that does not exchange matter with its surroundings. Energy, like thermal energy or light, can still be exchanged. A sealed container or a defined set of chemicals are examples of closed systems.

Another way of putting this is that no matter how you arrange the mass, the atoms, ions, electrons, et cetera, you won’t change the overall amount. For chemistry, this is extremely useful to know because it tells us that, regardless of the reaction, the mass of the reactants and the mass of the products will be identical. The ideas behind the law of conservation of mass began in the 1600s. And it was explored in more detail over the following 200 years.

The most famous of the experiments was done by Antoine Lavoisier who showed that heating tin with air in a sealed glass flask did not change the total mass, despite a chemical reaction occurring between the tin and the oxygen in the air. It was Antoine Lavoisier in 1773 that made the modern definition popular.

We now trust that the law of conservation of mass applies to all chemical reactions and physical changes in all circumstances. However, the law of conservation of mass was stated before we understood that matter was made of atoms, made of protons, electrons, and neutrons. The truth, as we came to discover, is that matter is not indestructible. Sometimes, matter can be created or destroyed. So the law of conservation of mass breaks down when matter is converted to other forms of energy and energy is converted to matter.

A good example of this is nuclear fusion like what happens in the sun. Hydrogen fuses together to form helium. About 0.7 percent of the mass of the four hydrogens is converted to other forms of energy. This means while the conservation of mass applies to chemical reactions and physical processes, it does not apply to radioactive decay, nuclear reactions, or matter-antimatter reactions.

There are minor changes in mass energy from changes in chemical bonds. But these are small enough that we generally ignore them. While the conservation of mass is not always reliable, the conservation of energy is because mass is just another form of energy. The good news is-is the law of conservation of mass applies to the majority of chemistry. And we can get some really excellent results from it. Now, let’s have some practice.

Yeast converts glucose to ethanol and carbon dioxide by anaerobic fermentation, as represented by the equation glucose reacts to form ethanol plus carbon dioxide. In a particular fermentation process, 200.0 grams of glucose is fully converted. What is the total mass of ethanol and carbon dioxide at the end of the reaction.

Anaerobic means no oxygen. So glucose is our only reactant. And ethanol and carbon dioxide are the only products. We can ignore the yeast because the enzymes in the yeast are acting as a catalyst for this process. In this example, 200 grams of glucose is being converted completely into ethanol and carbon dioxide. What we have to do is figure out the total mass of the ethanol and carbon dioxide produced. To help us on our way, we can recall the law of conservation of mass, which says that the mass of a closed system cannot change.

For a chemical reaction, the mass of the reactants equals the mass of the products. The mass of the reactants refers to the mass of glucose. And the mass of the products refers to the mass of the ethanol and the carbon dioxide. This is what we’re looking for. We have 200.0 grams of glucose to begin with that was fully converted into ethanol and carbon dioxide. So the total mass of ethanol and carbon dioxide is 200.0 grams.

If the fermentation process were carried out in an open container, would the mass of the container and contents increase, decrease, or stay the same?

Let’s imagine an open container like a beaker without a lid with yeast and glucose mixed together inside. The yeast will convert the glucose to ethanol, a liquid, and carbon dioxide, a gas. The ethanol will stick around in the container. And the carbon dioxide will escape. Since the carbon dioxide escapes the system of the container and its contents, we can be sure that as the fermentation precedes the mass of the contents and the container decreases. Since we’re dealing with an open system, where mass is being lost, the law of conservation of mass does not apply because the law of conservation of mass only applies to a closed system.

If the reaction produces 97.7 grams of carbon dioxide, what mass of ethanol is produced?

What the question is saying is that when 200.0 grams of glucose is fully converted into ethanol and carbon dioxide, we have 97.7 grams of carbon dioxide. Our job is to work out the mass of the ethanol. Going back to the law of conservation of mass and part a), we know that the mass of glucose that is consumed in the reaction is equal to the mass of ethanol plus the mass of carbon dioxide produced by the reaction. So 200.0 grams is equal to the mass of ethanol plus 97.7 grams. The mass of ethanol is, therefore, 200.0 grams minus 97.7 grams which is equal to 102.3 grams. So the mass of ethanol produced when 200 grams of glucose is fully converted is 102.3 grams.

A 100-gram sample of tin undergoes a phase change from solid to liquid. Is the mass of liquid tin equal to, less than, or greater than the mass of tin in the original sample?

So what we have is 100 grams of solid tin where the atoms of tin are all locked in place. This tin has undergone a phase change from solid to liquid, so it’s melted. And the atoms are now free to move around one another. What we need to answer is whether the mass of the liquid tin has gained mass, lost mass, or kept the same mass as the original solid tin. The law of conservation of mass says the mass of a closed system cannot change. A phase change is an example of a physical process where there’s no chemical change. For a physical process like this, the mass before equals the mass after.

In this scenario, our system consists of all the tin atoms. After melting, the system still contains all the original atoms of tin. So the mass of the system is the same. So the mass of liquid tin is 100 grams. The mass of liquid tin is, therefore, equal to the mass of tin in the original sample.

So in this video, we’ve learnt that the law of conservation of mass means that the mass of a closed system cannot change and that, for chemical reactions, the mass of the reactants equals the mass of the products, while for physical processes the mass stays constant. This applies for cases where the system is the chemicals themselves. And finally, we learnt that the law of conservation of mass does not apply to radioactive decay, nuclear reactions, or matter-antimatter reactions, cases where there is an exchange between mass energy and other forms of energy.

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