The Law of Conservation of Mass: Balancing from Scratch 1

Antoine-Laurent de Lavoisier The Law of Conservation of Mass states that the sum of the reactant’s masses is equal to the sum of the products’ masses This is how we can solve for the mass of an unknown and why we balance chemical equations. Antoine-Laurent de Lavoisier The Father of Modern Chemistry

Make sure you place the numbers with the appropriate substance. Solving for Mass When you are solving for masses, the most important thing to pay a attention to is the placement of the given masses. Sloppiness and laziness will make an easy question turn into a wrong answer. Na + Cl2 NaCl If 11.5 grams of sodium are mixed with chlorine gas to make 29.9 grams of sodium chloride, how much chlorine gas was used. 11.5 g ? g 29.9 g Make sure you place the numbers with the appropriate substance. 29.9 g – 11.5 g = 18.4 grams

Solve for the mass of the substances on your notes. ? g 64.0 g 88.0 g 14.0 g 6.0 g ? g 4

No, this equation is not balanced. Understanding Coefficients and Balancing Determining if an equation is balanced or not is a very simple task, but once again carelessness will cause an incorrect answer. Do NOT pretend to understand better than you really do. Make a table and get it RIGHT! Is the following equation balanced? N2 + 3O2  N2O5 Reactants side Products side ___ N ___ 2 2 ___ O ___ 6 5 No, this equation is not balanced.

Balancing from Scratch N2 + O2  N2O5 ___ N ___ 2 2 ___ O ___ 2 5 Now that we fully understand what it means to be balanced, we will learn how to balance an equation from the beginning. Do NOT panic. You will see that while it can take some time, the steps are not hard. Balance the following equation. N2 + O2  N2O5 Reactants side Products side ___ N ___ 2 2 ___ O ___ 2 5 Step 1: Determine how many of each element are present.

Balancing from Scratch N2 + O2  N2O5 ___ N ___ 2 2 ___ O ___ 2 5 Now that we fully understand what it means to be balanced, we will learn how to balance an equation from the beginning. Do NOT panic. You will see that while it can take some time, the steps are not hard. Balance the following equation. N2 + O2  N2O5 Reactants side Products side ___ N ___ 2 2 ___ O ___ 2 5 Step 2: Identify which element (if any) is keeping the equation from being balanced.

Balancing from Scratch N2 + O2  N2O5 5 2 ___ N ___ 2 2 ___ O ___ 2 5 Now that we fully understand what it means to be balanced, we will learn how to balance an equation from the beginning. Do NOT panic. You will see that while it can take some time, the steps are not hard. Balance the following equation. N2 + O2  N2O5 5 2 Reactants side Products side ___ N ___ 2 2 ___ O ___ 2 5 Step 3: Add coefficients to make this element balanced.

Balancing from Scratch Now that we fully understand what it means to be balanced, we will learn how to balance an equation from the beginning. Do NOT panic. You will see that while it can take some time, the steps are not hard. Balance the following equation. N2 + O2  N2O5 5 2 Reactants side Products side ___ N ___ 2 4 ___ O ___ 10 10 Step 4: Recalculate how many of EVERY element you now have in the equation.

Balancing from Scratch Now that we fully understand what it means to be balanced, we will learn how to balance an equation from the beginning. Do NOT panic. You will see that while it can take some time, the steps are not hard. Balance the following equation. N2 + O2  N2O5 2 5 2 Reactants side Products side ___ N ___ 2 4 4 4 ___ O ___ 10 10 10 10 Step 5: Add coefficients to make any other element that has been unbalanced correct and recalculate.

Balancing from Scratch Now that we fully understand what it means to be balanced, we will learn how to balance an equation from the beginning. Do NOT panic. You will see that while it can take some time, the steps are not hard. Balance the following equation. N2 + O2  N2O5 2 5 2 Reactants side Products side ___ N ___ 4 4 ___ O ___ 10 10 Step 6: STOP ADDING COEFFICENTS once it is balanced. Many students find this to be the hardest step.

Balancing from Scratch Now lets go through another one together. Balance the following equation. Pb(NO3)2 + NaI  NaNO3 + PbI2 Reactants side Products side ___ Pb ___ 1 1 ___ I ___ 1 2 ___ Na ___ 1 1 ___ NO3 ___ 2 1 Step 1: Determine how many of each element are present. (Remember that it is often easier to think of polyatomic ions as 1 substance.)

Balancing from Scratch Now lets go through another one together. Balance the following equation. Balance the following equation. Pb(NO3)2 + NaI  NaNO3 + PbI2 Reactants side Products side ___ Pb ___ 1 1 ___ I ___ 1 2 ___ Na ___ 1 1 ___ NO3 ___ 2 1 Step 2: Identify an element that is keeping the equation from being balanced. Just do one at a time.

Balancing from Scratch Now lets go through another one together. Balance the following equation. Balance the following equation. Pb(NO3)2 + NaI  NaNO3 + PbI2 2 Reactants side Products side ___ Pb ___ 1 1 ___ I ___ 1 2 ___ Na ___ 1 1 ___ NO3 ___ 2 1 Step 3: Add coefficient(s) to make this element balanced.

Balancing from Scratch Now lets go through another one together. Balance the following equation. Balance the following equation. Pb(NO3)2 + NaI  NaNO3 + PbI2 2 Reactants side Products side ___ Pb ___ 1 1 ___ I ___ 2 2 ___ Na ___ 2 1 ___ NO3 ___ 2 1 Step 4: Recalculate how many of EVERY element you now have in the equation.

Balancing from Scratch Now lets go through another one together. Balance the following equation. Balance the following equation. Pb(NO3)2 + NaI  NaNO3 + PbI2 2 2 Reactants side Products side ___ Pb ___ 1 1 1 1 ___ I ___ 2 2 2 2 ___ Na ___ 2 2 2 1 ___ NO3 ___ 2 2 2 1 Step 5: Add coefficients to make any other element that has been unbalanced correct and recalculate.

Balancing from Scratch Now lets go through another one together. Balance the following equation. Balance the following equation. Pb(NO3)2 + NaI  NaNO3 + PbI2 2 2 Reactants side Products side ___ Pb ___ 1 1 ___ I ___ 2 2 ___ Na ___ 2 2 ___ NO3 ___ 2 2 Step 6: STOP ADDING COEFFICENTS once it is balanced. It is very important that you know when to stop.

Balancing from Scratch When you are asked this on a multiple choice test, you will most likely be asked in the following way: Al + HCl  AlCl3 + H2 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 It is important that you understand that you can never know just one coefficient. You must make sure that the entire equation is balanced before you can know the answer.

Balancing from Scratch When you are asked this on a multiple choice test, you will most likely be asked in the following way: Al + HCl  AlCl3 + H2 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 ___ Al ___ 1 1 ___ H ___ 1 2 ___ Cl ___ 1 3 Step 1: Determine how many of each element are present. (Remember that it is often easier to think of polyatomic ions as 1 substance.)

Balancing from Scratch Hydrogen and chlorine are unbalanced! When you are asked this on a multiple choice test, you will most likely be asked in the following way: Al + HCl  AlCl3 + H2 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 ___ Al ___ 1 1 Hydrogen and chlorine are unbalanced! ___ H ___ 1 2 ___ Cl ___ 1 3 Step 2: Identify an element that is keeping the equation from being balanced. Just do one at a time.

Balancing from Scratch When you are asked this on a multiple choice test, you will most likely be asked in the following way: Al + HCl  AlCl3 + H2 2 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 ___ Al ___ 1 1 1 1 ___ H ___ 1 2 2 2 ___ Cl ___ 2 1 3 3 Step 3: Add coefficient(s) to make this element balanced.

Balancing from Scratch When you are asked this on a multiple choice test, you will most likely be asked in the following way: Al + HCl  AlCl3 + H2 2 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 Notice that there is no coefficient that multiplies by 3 to equal 2. Cl cannot be balanced this way ;the first coefficient we added was wrong! ___ Al ___ 1 1 ___ H ___ 2 2 ___ Cl ___ 2 3

Balancing from Scratch When you are asked this on a multiple choice test, you will most likely be asked in the following way: Al + HCl  AlCl3 + H2 2 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 Balancing Tip: When you are experiencing troubles like this, it often makes things easier if you place a 2 in front of the compound containing the odd number of atoms. ___ Al ___ 1 1 1 2 ___ H ___ 1 1 2 2 ___ Cl ___ 1 1 6 3 Repeat Step 3: It is okay to make a mistake and start over. This will happen many times as you first begin balancing.

Balancing from Scratch When you are asked this on a multiple choice test, you will most likely be asked in the following way: Al + HCl  AlCl3 + H2 6 2 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 ___ Al ___ 1 1 2 2 ___ H ___ 1 6 2 2 ___ Cl ___ 6 1 6 6 Steps 4 & 5: Recalculate how many of EVERY element you now have in the equation and continue to add coefficients until it is balanced.

Balancing from Scratch When you are asked this on a multiple choice test, you will most likely be asked in the following way: Al + HCl  AlCl3 + H2 6 2 3 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 ___ Al ___ 1 1 2 2 ___ H ___ 6 6 6 2 ___ Cl ___ 6 6 6 6 Steps 4 & 5: Recalculate how many of EVERY element you now have in the equation and continue to add coefficients until it is balanced.

Balancing from Scratch When you are asked this on a multiple choice test, you will most likely be asked in the following way: 2 Al + HCl  AlCl3 + H2 6 2 3 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 ___ Al ___ 2 1 2 2 ___ H ___ 6 6 6 6 ___ Cl ___ 6 6 6 6 Steps 4 & 5: Recalculate how many of EVERY element you now have in the equation and continue to add coefficients until it is balanced.

Balancing from Scratch Don’t forget to answer the question! When you are asked this on a multiple choice test, you will most likely be asked in the following way: 2 Al + HCl  AlCl3 + H2 6 2 3 When the above equation is balanced, the coefficient for aluminum chloride is – A 1 B 2 C 3 D 4 ___ Al ___ 2 2 Don’t forget to answer the question! ___ H ___ 6 6 ___ Cl ___ 6 6 Step 6: STOP ADDING AND CHANGING COEFFICENTS once it is balanced, and check your answers when you are done!

Balancing from Scratch Sometimes you might get coefficients that balance, but are not the correct set. 4 2 LiI + Pb(NO3)2  LiNO3 + PbI2 2 4 2 2 ___ Pb ___ 1 2 1 2 These coefficients are NOT correct because even though they are balanced, they are not the smallest numbers that can be used. Notice that they can all be reduced by a factor of 2. ___ I ___ 4 2 4 2 ___ Li ___ 2 4 4 2 ___ NO3 ___ 2 4 2 4 Now the chemical equation is balanced with the correct set of coefficients. Always make sure that your coefficients are in their most reduced form.

Balance the chemical equations on your notes. 1 Balance the chemical equations on your notes. 1 1 1 1 2 1 1 1 11 2 1 1’s will be placed in the blanks on the screen, but it is not necessary that you write 1’s. 1 4 29