3.1.1 Conservation of mass and balanced chemical equations
Law of conservation of mass
- No atoms are lost or made during a chemical reaction
- So mass of products = mass of reactants
- Total mass before = 127.6 + 126.86 = 254.46
- Total mass after = 153.09 + 101.37 = 243.46
- So mass of products = mass of reactants
- CO2 is a gas
- Which escapes during the reaction
- So the mass at the end of the experiment is less expected as mass has been lost
Explain why an unbalanced chemical equation cannot correctly describe a chemical reaction (2)
- Must end up with the same no of atoms as at the start
- Otherwise matter is shown to be lost / gained
- Won’t show correct amount of each element / compound
3.1.2 Relative formula mass
Relative formula mass (Mr)
- 207 + 2 × [14 + (3 × 16)] = 331
- In a balanced chemical equationSum of Mr of reactants in the quantities = sum of Mr of products in the quantities
- Why is there a change in mass?A reactant or product is a gas & its mass has not been taken into account
- Percentage composition
- 3.1.3 Mass changes when a reactant or product is a gas
When a metal reacts with O2
- Mass of the oxide produced is greater than the mass of the metal
- In thermal decompositions of metal carbonates
- CO2is produced
- Which escapes into the atmosphere
- Leaving the metal oxide as the only solid product
- The equation for the reaction is 2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2 (g). Explain why there is a loss in mass in this investigation (2)
- A gas is produced
- Which escapes from the flask
3.1.4 Chemical measurements
(Do practice questions)