Yes, mass is generally considered to be conserved in exothermic reactions, but with a very important caveat as described in the provided reference. Here's a breakdown:
Understanding Mass Conservation in Chemical Reactions
In typical chemical reactions, including exothermic ones, we often state that mass is conserved. This means the total mass of the reactants equals the total mass of the products. However, this is an approximation when we don't account for changes in energy.
Exothermic Reactions and Energy Release
Exothermic reactions release heat (thermal energy). This released heat represents a difference in the total energy of the reactants and the products, as stated in the reference: "An exothermic reaction gives off heat. The heat is the thermal energy which is the difference between the total energy of the products and the reactants."
The Role of Einstein's E=mc²
The key point, and the qualification to the conservation of mass, is highlighted by Einstein's famous equation: E=mc². This equation demonstrates the relationship between energy (E) and mass (m), with c being the speed of light squared, a massive constant.
- Mass-Energy Equivalence: The equation states that mass and energy are interchangeable. When energy is released (as in an exothermic reaction), there is, in theory, a corresponding decrease in mass.
- Magnitude of Mass Loss: The change in mass (Δm) due to the energy change (ΔE) is extremely small because the speed of light squared is such a large number. The reference states, "There is no loss of mass in the process unless you are referring to Einstein's equation E=mc²." So, while there is a slight loss of mass in theory, it is generally negligible in the context of normal chemical reactions that do not involve nuclear reactions.
Practical Implications
- In most chemical reactions, including those we conduct in the laboratory or industry, the mass change is so minuscule that it's undetectable with ordinary equipment. We often say the mass is practically conserved.
- However, in nuclear reactions, where enormous amounts of energy are released, the mass change is significant and measurable, validating the mass-energy equivalence concept.
- Therefore, in typical exothermic reactions, although a very small loss of mass occurs theoretically (due to energy release), mass is considered conserved because it's an immeasurable change.
Examples
- Combustion of Wood: When wood burns, it releases heat and light (exothermic), but the mass loss is very small from the energy being released. The total mass of the ash, smoke, and gaseous products would be nearly the same as the original wood and oxygen.
- Neutralization Reactions: Reactions between acids and bases release heat. But again, the mass change due to this heat release is negligible.
Conclusion
While technically, a small amount of mass is converted into energy during an exothermic reaction (following E=mc²), this mass loss is generally undetectable in normal chemical processes. Hence, we often approximate mass as being conserved in these reactions.
