Conservation of mass during a reaction
Problem — How does the mass of substances change during a chemical reaction?
- Understand the law of conservation of mass during a chemical reaction.
- Be able to explain why the total mass of reactants equals that of the products.
- Apply this concept to analyze examples of simple reactions.
- Use this knowledge to anticipate and solve problems related to the amount of substance.
Part 1: Mass, a fundamental quantity in chemistry
Mass is a measure of the amount of matter contained in an object or sample. It is measured in grams (g) or kilograms (kg) using a balance.
Before studying the conservation of mass during a chemical reaction, it is essential to fully understand the concept of mass. In chemistry, when observing a transformation, we measure the mass of substances before and after the reaction.
Measuring the mass of substances
- The mass of the reactants corresponds to the sum of the masses of all the initial substances before the reaction.
- The mass of the products corresponds to the sum of the masses of the new substances formed after the reaction.
Mass is an essential measurable quantity in chemistry. It allows us to quantify the matter present before and after a reaction, which is the basis for studying and understanding chemical transformations.
Part 2: The law of conservation of mass
The law of conservation of mass states that during a chemical reaction, the total mass of the reactants is equal to the total mass of the products. In other words, mass is conserved.
This law was demonstrated in the 18th century by the chemist Antoine Lavoisier. It is based on the principle that atoms are neither created nor destroyed in a chemical reaction: they simply rearrange to form new substances.
Illustration by a concrete example
Consider the combustion reaction of carbon:
C (s) + O2 (g) → CO2 (g)
If we weigh the reactants before the reaction, we measure the mass of solid carbon and gaseous oxygen. After the reaction, we weigh the resulting carbon dioxide. The mass of the carbon dioxide will be equal to the sum of the masses of the carbon and oxygen consumed.
For example:
- Mass of carbon: 12 g
- Mass of oxygen: 32 g
- Mass of carbon dioxide produced: 44 g
The law of conservation of mass is a fundamental principle that helps us understand that matter does not disappear or appear during a chemical reaction. This understanding is crucial for studying reactions and predicting their outcomes.
Part 3: Applications and experimental verification
It is possible to verify the conservation of mass by performing a simple experiment:
Example experiment
Dissolution reaction of an effervescent tablet (usually containing citric acid and sodium bicarbonate) in water:
- Weigh the empty container.
- Precisely add the tablet, then measure the total mass.
- Add water, then measure again.
- Observe the reaction (formation of carbon dioxide bubbles).
- After the reaction is complete, weigh the container with the mixture again.
Despite the gas being released, the total mass remains constant if the system is closed or if the gas is trapped in the container, demonstrating conservation of mass.
A closed system is one where matter cannot enter or leave. This allows us to observe conservation of mass in a chemical reaction.
The simple experiment shows that the total mass remains constant during a chemical transformation if the system is closed. This observation confirms the law of conservation of mass and helps to better understand the concept in practice.
Part 4: Practical implications in chemistry
Conservation of mass is essential for:
- Writing and balancing chemical equations, where the number and type of atoms are the same on both sides.
- Calculating the amounts of reactants needed or products obtained.
- Designing experiments and industrial chemical processes.
Concept of a balanced chemical equation
A balanced chemical equation follows the law of conservation of mass by having the same number of atoms of each element on both sides of the arrow.
For example, the balanced equation for methane combustion:
CH4 + 2 O2 → CO2 + 2 H2O
This equation shows that 1 molecule of methane reacts with 2 molecules of oxygen to produce 1 molecule of carbon dioxide and 2 molecules of water, conserving the number of C, H, and O atoms.
Conservation of mass is the foundation for correctly writing chemical equations and calculations in chemistry. It ensures that quantities of matter are consistent and allows predicting reaction results.
The law of conservation of mass is a fundamental principle in chemistry: during any reaction, the total mass of substances does not change because atoms rearrange but do not disappear. Understanding this law helps write chemical equations correctly, predict the amount of products formed, and interpret experimental observations. This simple yet rigorous principle is an essential foundation for studying all chemical transformations.