Law of Conservation of MassLaw of Conservation of Mass In a chemical reaction, mass cannot be In a chemical reaction, mass cannot be created created

or destroyedor destroyed Mass of the reactants is Mass of the reactants is always equalalways equal to the to the

mass of the productsmass of the products Number of Number of atomsatoms in the reactants is always in the reactants is always

equal to the number of atoms in the productsequal to the number of atoms in the products Therefore in a chemical equation, there must be Therefore in a chemical equation, there must be

the the samesame number of atoms of each element on number of atoms of each element on each sideeach side this is known as a “ this is known as a “balanced balanced chemical equationchemical equation””

Steps to help balancing chemical equations:Steps to help balancing chemical equations:

1.1. Get yourself an unbalanced chemical equationGet yourself an unbalanced chemical equation- This may or may not be given; you MUST be able to This may or may not be given; you MUST be able to

determine the correct formulas for ALL of the determine the correct formulas for ALL of the compounds involved compounds involved

- i.e.: i.e.: NaCl + BeF NaCl + BeF22 NaF + BeCl NaF + BeCl222.2. Figure out how many of each atom you have on Figure out how many of each atom you have on

your reactants side and your products sideyour reactants side and your products side- Reactants: Reactants: Na = 1, Cl = 1, Be = 1, F = 2Na = 1, Cl = 1, Be = 1, F = 2- Products: Products: Na = 1, Cl = 2, Be = 1, F = 1Na = 1, Cl = 2, Be = 1, F = 1- Notice how Cl and F are not equal? We need to balance the Notice how Cl and F are not equal? We need to balance the

equation.equation.- We want each atom to be equal on each side of the reactionWe want each atom to be equal on each side of the reaction

3.3. Draw boxes around all the chemical compoundsDraw boxes around all the chemical compounds- This is to remind you to This is to remind you to NEVERNEVER change anything change anything

inside the boxes (i.e.: subscripts). If you do, you will inside the boxes (i.e.: subscripts). If you do, you will ALWAYS get the wrong answerALWAYS get the wrong answer

- i.e.: i.e.: NaCl + BeF NaCl + BeF22 NaF + BeCl NaF + BeCl22

Steps to help balancing chemical equations:Steps to help balancing chemical equations:

5.5. Write coefficients (numbers) in front of the boxes Write coefficients (numbers) in front of the boxes to the left in order to make the numbers of atoms to the left in order to make the numbers of atoms to be equal on each side of the equation to be equal on each side of the equation

- i.e.: i.e.: NaCl + BeF NaCl + BeF22 NaF + BeCl NaF + BeCl22- The coefficients are multiplied to each atom in a The coefficients are multiplied to each atom in a

compoundcompound- i.e. – place a 2 in front of NaCl to get 2NaCl; this means that i.e. – place a 2 in front of NaCl to get 2NaCl; this means that

we now have we now have 2 Na’s and 2 Cl’s2 Na’s and 2 Cl’s- Atoms with subscripts have the subscript multipliedAtoms with subscripts have the subscript multiplied

- i.e. – if we placed a 2 in front of BeFi.e. – if we placed a 2 in front of BeF22 we would get 2 BeF we would get 2 BeF22; ; this means that we now have this means that we now have 2 Be’s and 4 F’s2 Be’s and 4 F’s

- NEVER CHANGE THE INITIAL SUBSCRIPTNEVER CHANGE THE INITIAL SUBSCRIPT- This will ALWAYS give you the wrong answerThis will ALWAYS give you the wrong answer- i.e. – Do not change i.e. – Do not change BeFBeF22 to BeF to BeF44

6.6. Use a pencil – you will have to use trial and error Use a pencil – you will have to use trial and error to get the final answer to get the final answer

2NaCl + BeF2NaCl + BeF22 2NaF + BeCl 2NaF + BeCl22

ExamplesExamples

__ FeCl__ FeCl33 + __ Be + __ Be33(PO(PO44))22 __ BeCl __ BeCl22 + __ Fe(PO + __ Fe(PO44))

__ Ag(NO__ Ag(NO33) + __ Li(OH) ) + __ Li(OH) __ Ag(OH) + __ Li(NO __ Ag(OH) + __ Li(NO33))

__CH__CH44 + __ O + __ O22 __ CO __ CO22 + __ H + __ H22OO

__ Mg + __Mn__ Mg + __Mn22OO33 __ MgO + __ Mn __ MgO + __ Mn

Examples - AnswersExamples - Answers

2 FeCl2 FeCl33 + Be + Be33(PO(PO44))22 3 BeCl 3 BeCl22 + 2 Fe(PO + 2 Fe(PO44))

Ag(NOAg(NO33) + Li(OH) ) + Li(OH) Ag(OH) + Li(NO Ag(OH) + Li(NO33))

CHCH44 + 2 O + 2 O22 CO CO22 + 2 H + 2 H22OO

3 Mg + Mn3 Mg + Mn22OO33 3 MgO + 2 Mn 3 MgO + 2 Mn

But here’s the best part…But here’s the best part… If you understand how to balance equations If you understand how to balance equations

instinctively, you don’t NEED to go through all instinctively, you don’t NEED to go through all the steps. The steps are just there to help you!the steps. The steps are just there to help you!

Honestly, sometimes the best strategy for Honestly, sometimes the best strategy for balancing equations is balancing equations is trial and errortrial and error

Keep practicing and working on questions and it Keep practicing and working on questions and it will get easier & easierwill get easier & easier

Practice: Try to balance all the equations from Practice: Try to balance all the equations from the word & skeleton equations sheetthe word & skeleton equations sheet