Law of Conservation of Mass

1. Law of Conservation of Mass • In a chemical reaction, mass cannot be created or destroyed • Mass of the reactants is always equal to the mass of the products • Number of atoms in the reactants is always equal to the number of atoms in the products • Therefore in a chemical equation, there must be the same number of atoms of each element on each side this is known as a “balanced chemical equation”

2. Steps to help balancing chemical equations: • Get yourself an unbalanced chemical equation • This may or may not be given; you MUST be able to determine the correct formulas for ALL of the compounds involved • i.e.: NaCl + BeF2 NaF + BeCl2 • Figure out how many of each atom you have on your reactants side and your products side • Reactants: Na = 1, Cl = 1, Be = 1, F = 2 • Products: Na = 1, Cl = 2, Be = 1, F = 1 • Notice how Cl and F are not equal? We need to balance the equation. • We want each atom to be equal on each side of the reaction • Draw boxes around all the chemical compounds • This is to remind you to NEVER change anything inside the boxes (i.e.: subscripts). If you do, you will ALWAYS get the wrong answer • i.e.: NaCl + BeF2 NaF + BeCl2

3. Steps to help balancing chemical equations: • Write coefficients (numbers) in front of the boxes to the left in order to make the numbers of atoms to be equal on each side of the equation • i.e.: NaCl + BeF2 NaF + BeCl2 • The coefficients are multiplied to each atom in a compound • i.e. – place a 2 in front of NaCl to get 2NaCl; this means that we now have 2 Na’s and 2 Cl’s • Atoms with subscripts have the subscript multiplied • i.e. – if we placed a 2 in front of BeF2 we would get 2 BeF2; this means that we now have 2 Be’s and 4 F’s • NEVER CHANGE THE INITIAL SUBSCRIPT • This will ALWAYS give you the wrong answer • i.e. – Do not change BeF2 to BeF4 6. Use a pencil – you will have to use trial and error to get the final answer • 2NaCl + BeF2 2NaF + BeCl2

4. Examples • __ FeCl3 + __ Be3(PO4)2  __ BeCl2 + __ Fe(PO4) • __ Ag(NO3) + __ Li(OH)  __ Ag(OH) + __ Li(NO3) • __CH4 + __ O2 __ CO2 + __ H2O • __ Mg + __Mn2O3  __ MgO + __ Mn

5. Examples - Answers • 2 FeCl3 + Be3(PO4)2  3 BeCl2 + 2 Fe(PO4) • Ag(NO3) + Li(OH)  Ag(OH) + Li(NO3) • CH4 + 2 O2 CO2 + 2 H2O • 3 Mg + Mn2O3  3 MgO + 2 Mn

6. But here’s the best part… • If you understand how to balance equations instinctively, you don’t NEED to go through all the steps. The steps are just there to help you! • Honestly, sometimes the best strategy for balancing equations is trial and error • Keep practicing and working on questions and it will get easier & easier • Practice: Try to balance all the equations from the word & skeleton equations sheet