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Review from yesterday’s lesson We are learning how to balance equations because the conservation of mass says that the mass before a reaction must be the same as the mass after a reaction. Atoms cannot be created nor destroyed H2 + O2 ---> H2O Review from yesterday’s lesson We must know how to count the number of atoms in a substance before we can balance 6Na2O3 Coefficient = 6 Subscript Na= 2 Subscript O= 3 Total number of Na= 6 X 2= 12 Total number of O= 6 X 3= 18 Is the blue number a coefficient or a subscript? 3Li Na2CO3 4K2PO4 6H 0 2Mg(OH) 2 2 2H3PO4 6LiCl 4 Count the number of atoms 5H2CO3 How many H atoms? 5 X 2= 10 How many C atoms? 5 X 1= 5 How many O atoms 5 x 3= 15 Click here to go to balancing equations steps Balancing Equations Are you up for the challenge? Why do you need to balance equations? Let’s read page 80-81 The Law of Conservation of Mass says that the mass of the reactants must equal the mass of the products. In this example: H2 + O2 ---> H2O There are 2 hydrogen’s on the left and 2 Oxygen’s, but there is only 1 oxygen on the right. H2 + O2 ---> H2O The reactants do not equal the products! Where did the extra oxygen go? Atoms can’t be created nor destroyed. They didn’t go anywhere, you just need to add numbers so the sides become equal An equation is balanced by changing coefficients in a somewhat trial-and-error fashion Important Vocabulary- Coefficient Coefficient- Large number located in front of an element or compound in a chemical formula. 2H= 2 hydrogen's 4H20 Coefficient is 4 H2O H2O H2O H2O Important Vocabulary- Subscript Subscript- The small number that is in the lower right corner of an element H20 Subscript is 2 2 hydroden’s H H If there is no coefficient or subscriptassume it is 1 1 C1 S2 Coefficient=1 C’s subscript= 1 S’s subscript= 2 Let’s practice 2Na2 O 4 H 3PO 4 5H 2 O 2 3S 8 6 K20 Math Review Distributive Property- The coefficient times the subscript gives the total number of atoms. 4H201 H= 4 X 2= 8 Hydrogen’s O= 4 X 1= 4 Oxygen’s Math Review 4Mg(OH) 2 1 Mg= 4 X 1= 4 OH= 4 x 2= 8 3KOH How many K atoms do I have? Coefficient 3 Subscript 1 3 X1= 3 How many O atoms do I have? Coefficient 3 Subscript 1 3 X 1= 3 How many H atoms do I have? Coefficient 3 Subscript 1 3 X 1= 3 3K2O How many K atoms do I have? Coefficient 3 Subscript 2 3 X 2= 6 How many O atoms do I have? Coefficient 3 Subscript 1 3 X 1= 3 2H2O How many H atoms do I have? Coefficient= 2 Subscript= 2 2 X 2= 4 How many O atoms do I have? Coefficient= 2 Subscript= 1 2 X 1= 2 K3PO4 How many K atoms do I have? Coefficient= 1 Subscript= 3 1 X 3= 3 How many P atoms do I have? Coefficient= 1 Subscript= 1 1 X 1= 1 How many O atoms do I have? Coefficient= 1 Subscript= 4 1 X 4= 4 Mg3(PO4)2 How many Mg atoms do I have? Coefficient= 1 Subscript= 3 1 X 3= 3 How many P atoms do I have? Coefficient= 1 Subscript=1 X 2= 2 1 X 2= 2 How many O atoms do I have? Coefficient= 1 Subscript= 4 X 2=8 1 X 8= 8 Steps for balancing equations 1. Draw boxes around all the chemical formulas. Never, ever, change anything inside the boxes. Ever. Really. If you do, you're guaranteed to get the answer wrong. H2 + O2 ---> H2 O Steps for balancing equations 2. Make an element inventory. How are you going to know if the equation is balanced if you don't actually make a list of how many of each atom you have? You won't. You have to make an inventory of how many atoms of each element you have, and then you have to keep it current throughout the whole problem. See example on next slide H2 + O2 ---> H2O Reactants Products Hydrogen (H) 2 2 Oxygen (O) 2 1 But what if you have more than one formula on each side of the equation? Let’s practice with the REACTANTS! NaOH + H2CO3 Na2CO3 + H2O How many Na atoms do I have? 1 X 1 =1 How many O atoms do I have? 1 X 1= 1 1 X 3=3 1+3=4 How many H atoms do I have? 1 X 1= 1 1 X 2= 2 1 + 2= 3 How many C atoms do I have? 1 X 1= 1 Steps for balancing equations 3. Write numbers in front of each of the boxes until the inventory for each element is the same both before and after the reaction. Whenever you change a number, make sure to update the inventory - otherwise, you run the risk of balancing it incorrectly. When all the numbers in the inventory balance, then the equation can balance, 2 H2 + O2 ---> 2 H2O Reactants Products Hydrogen (H) 2 2 4 Oxygen (O) 2 1 2 4 Notice- I put the coefficient in front of the box- NOT inside! Steps for balancing equations 4. Find the elements which appear in the fewest numbers of molecules and balance these first. Continue in sequence until you balance the element which appears in the most molecules last. Tip: Start by balancing an element that appears in only one reactant and product. Step 1: Write formula and draw boxes 2 H2 + O2 2 H2O Step 2: Make an element inventory R P H 2 4 2 4 O 2 2 1 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! Rules You cannot change a subscript. You cannot place a coefficient in the middle of a formula. H2 + O2 ---> H22O Make sure that your final set of coefficients are all whole numbers with no common factors other than one. For example, this equation is balanced: 4 H2 + 2 O2 ---> 4 H2O However, all the coefficients have the common factor of two. Divide through to eliminate common factors like this. If you run into problems trying to figure out the answer… Sometimes, you will need to find the lowest common multiple in order to make an inventory balance! The lowest R P common multiple H 2 2 between 2 and 7 is 14 O 2 7 14 14 If you run into problems trying to figure out the answer… What happens when the only way you can get a problem to work out is to make one of the numbers a decimal or fraction? When this happens, find the largest molecule in the equation and stick a "2" in front of it. Then start the problem over. Will this work all the time? Well, no. But it will work sometimes, and give you a new strategy for hard problems. Step 1: Write formula and draw boxes 4 C + S8 4 CS2 Step 2: Make an element inventory R P C 1 4 1 4 S 8 8 2 How do I get the S’s in the products to equal 8? Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! Step 1: Write formula and draw boxes 4 Na + O2 2 Na2O Step 2: Make an element inventory R P Na 1 4 2 4 O 2 2 1 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! Step 1: Write formula and draw boxes 2 N2 +5 O2 2 N2O5 Step 2: Make an element inventory R P N 2 4 O 2 10 5 10 2 4 HINT: Find the lowest common multiple of the Oxygen molecules Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! Step 1: Write formula and draw boxes 2 Na + O2 Na2O2 Step 2: Make an element inventory R P Na 1 2 2 O 2 2 How do I get the Na’s in the reactants to equal 2? Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! Step 1: Write formula and draw boxes 6K 3 K2O +2 B + B2O3 Step 2: Make an element inventory K B O Reactants Products 1 2 6 6 1 2 2 3 1 3 HINT: Start on the products side- and start with the element with the lowest number of molecules Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! Step 1: Write formula and draw boxes N2 +3 H2 2 NH3 Step 2: Make an element inventory R P N 2 1 2 H 2 6 3 6 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! Step 1: Write formula and draw boxes 6 Cs + N2 2 Cs3N Step 2: Make an element inventory R P Cs 1 6 3 6 N 2 2 1 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! Step 1: Write formula and draw boxes 3 Li + AlCl3 3 LiCl ` + Al Step 2: Make an element inventory R Li 1 Al 1 3 Cl P 3 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! 1 3 1 1 3 Step 1: Write formula and draw boxes 2NaOH + H2CO3 Na2CO3 +2H2O Step 2: Make an element inventory Hint: We are going to start with the Na on the reactant side because it is not balanced and has the lowest number of molecules R Na O H C 1 4 3 P 2 5 4 1 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! 2 4 2 1 5 4 Step 1: Write formula and draw boxes 2 C2H6 +7 O2 4 CO2 +6H2O Step 2: Make an element inventory Hint: We are going to look for the common factor of H’s R P C 2 H 6 12 O 4 2 14 1 4 2 12 3 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! 8 14 Step 1: Write formula and draw boxes K3PO4 +3H2O H3PO4 +3 KOH Step 2: Make an element inventory Hint: We are going to look for the lowest number of molecules that are not balanced. R H P O K 4 P 6 1 2 6 1 5 7 5 1 3 3 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! 7 Step 1: Write formula and draw boxes 10 Na +2 NaNO 3 6 Na2O + N2 Step 2: Make an element inventory Hint: We are going to look for the next common factor of 1 and 3 R P Na 2 3 12 2 N 1 2 2 O 3 6 1 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go! 12 6 Step 1: Write formula and draw boxes 2H3PO4 3 6 + Mg(OH)2 Mg3(PO4)2 + H2O Step 2: Make an element inventory R H P O Mg P 5 912 2 1 2 2 6 1 1014 3 9 12 14 3 Step 3: Add coefficients to make the inventory balance. Don’t forget to change the inventory as you go!