Mass changes in open chemical reactions
Quantitative chemistry • Measurements and conservation of mass
Flashcards
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Key concepts
What you'll likely be quizzed about
Definition of non-enclosed (open) system
A non-enclosed system allows matter to move between the reaction vessel and the surroundings. Mass measurements of the vessel change when particles leave as gases or when gases from the surroundings enter. A closed system prevents particle exchange so the total mass of reactants and products measured remains constant.
Conservation of mass vs measured mass
Conservation of mass states that atoms are neither created nor destroyed during a chemical reaction; the total mass of all atoms involved remains constant. Measured mass can change in an open system because some atoms become part of gaseous molecules that escape from the container. The balanced symbol equation shows the atoms and molecules present before and after the reaction, identifying any gaseous products that can leave and change the measured mass.
Balanced symbol equations and prediction of mass change
Balanced equations list all reactants and products, including physical states. Presence of (g) for a product indicates a gas forms. Cause → effect: formation of a gaseous product (cause) → particles escape the vessel (effect) → measured mass decreases. If a gas from the surroundings reacts and becomes part of the products, measured mass can increase because additional particles enter the vessel and remain as products.
Particle model explanation
The particle model treats matter as discrete atoms or molecules. Cause → effect: rearrangement of atoms during the reaction (cause) → formation of gaseous molecules with kinetic energy allowing escape (effect) → fewer particles remain in the vessel, lowering the measured mass. Particle conservation remains true: the escaped atoms still exist but no longer contribute to the mass measured for the original container.
Common examples and their balanced equations
Magnesium reacting with dilute hydrochloric acid produces hydrogen gas: Mg + 2HCl -> MgCl2 + H2. Cause → effect: H2 gas formation (cause) → hydrogen escapes (effect) → measured mass decreases. Thermal decomposition of calcium carbonate produces carbon dioxide gas: CaCO3 -> CaO + CO2. Cause → effect: CO2 formation (cause) → CO2 escapes (effect) → measured mass decreases. Complete equations identify gaseous products and predict mass changes.
Limiting factors and experimental considerations
Cause → effect: incomplete containment or slow gas release (cause) → measured mass changes may be small or hard to detect (effect). Sources of error include leaks, dissolved gas that later escapes, gas that dissolves in the liquid rather than escaping immediately, and inaccurate state symbols in the equation. Accurate prediction requires correct balanced equations with physical states and proper experimental controls (e.g., closed system for conserved mass measurements).
Key notes
Important points to keep in mind