Atoms in Chemical Reactions
A chemical reaction is a process where atoms are rearranged to form entirely new substances. The core principle is that atoms are conserved—none are made or lost—which means the total mass before and after the reaction is always the same.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Chemical reactions produce new substances with different properties from the starting materials.
- Atoms are only rearranged during a reaction; they are not created, destroyed, or changed into other elements.
- The identity of atomic nuclei remains unchanged. Reactions involve the electrons surrounding the nuclei.
- The total mass of all reactants equals the total mass of all products. This is the Law of Conservation of Mass.
- If the mass of a solid appears to decrease during a reaction in an open container, it is because a gaseous product has been formed and has escaped.
Formulae
Total Mass of Reactants = Total Mass of Products To calculate the unknown mass of a substance in a reaction, assuming all other masses are known.
Definitions
- Chemical Reaction
- A process that involves the rearrangement of the atomic and molecular structure of substances, as opposed to a physical change which does not alter their fundamental composition.
- Conservation of Mass
- The principle that matter cannot be created or destroyed in a chemical reaction. The total mass of the products must equal the total mass of the reactants.
Worked example
A 5.0 g strip of magnesium ribbon is burned in the air, reacting with oxygen to form a white solid, magnesium oxide. The final mass of the white solid is measured to be 8.3 g. What mass of oxygen from the air must have reacted?
- 1
Identify the reactants (magnesium and oxygen) and the product (magnesium oxide).
- 2
List the known masses:
Mass of Magnesium = 5.0 g.
Mass of Magnesium Oxide = 8.3 g.
- 3
Apply the principle of conservation of mass:
Mass of Reactants = Mass of Products.
- 4
Write the equation with the known values:
Mass of Magnesium + Mass of Oxygen = Mass of Magnesium Oxide.
- 5
Substitute the values:
5.0 g + Mass of Oxygen = 8.3 g.
- 6
Solve for the unknown mass:
Mass of Oxygen = 8.3 g - 5.0 g = 3.3 g.
Answer: 3.3 g
Common mistakes
- ×Mistaking a decrease in solid mass for the destruction of matter. Mass is often 'lost' from an open system as an unweighed gas.
- ×Mistaking an increase in solid mass for the creation of matter. Mass is often gained when a substance reacts with a gas from the surroundings, like oxygen.
- ×Confusing a chemical change (e.g., burning) with a physical change (e.g., melting). A physical change does not create a new substance.
No-calculator tips
- ✓Conservation of mass problems are usually just simple addition or subtraction. Focus on identifying which substances are reactants and which are products.
- ✓For subtractions like 8.3 - 5.0, you can ignore the decimal for a moment (83 - 50 = 33) and then replace it (3.3).
- ✓If a question involves a reaction seeming to lose or gain mass, immediately think about what invisible substance (usually a gas) might be a reactant or product.