1/9 Opener Don’t forget to turn in extra credit! If you have 2 moles of NH3 how many moles do you have of H? If you have 12 moles of H how many moles do you have of NH3? If you have 3 moles of Ca3(PO4)2 how many moles do you have of PO4? If you have 3 moles of Ca3(PO4)2 how many moles do you have of O?

Balancing Chemical Equations

Vocabulary! Coefficient: Word Equation: Formula Equation: is a small, whole number that appears IN FRONT of a chemical formula in a chemical equation. Example: 4 HCl Word Equation: an equation in which the reactants and products in a chemical reaction are represented by words. Example: hydrogen + oxygen yields water Formula Equation: represents the reactants and products of a chemical reaction by their symbols or formulas. Example: 2 H2 + O2  2 H2O

Why are we learning this? Coefficients of a chemical reaction indicate relative amounts of reactants and products. Coefficients are MOLE RATIOS Example: H2(g) + Cl2(g)  2HCl(g) 1 mol H2 : 1 mol Cl2 : 2 moles HCl The relative masses of the reactants and products can also be determined from the coefficients. Example: 1 mol H2 = 2.02 g H2 1 mol Cl2 = 70.90 g Cl2 2 mol HCl = 72.92 g HCl Therefore, 2.02g H2 will react with 70.90g Cl2 to form 72.92g of HCl.

The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction. So what? Because of this, chemical reactions can be read in any direction. Forward and Reverse.

Law of Conservation of Mass The law of conservation of mass states that matter is neither lost nor gained in chemical reactions. It simply changes form.

Dalton’s Law of Definite Proportions Atoms are indestructible and unchangeable, so compounds are formed when one atom chemically combines with other atoms. When elements react to form compounds, they react in defined, whole-number ratios. Reactions are not random events. They proceed according to precise and well-defined formulas.

Rules for Balancing Equations Write a word equation. Determine the reactants, the products, and the physical states involved. (once you feel comfortable you can do this step in your head!) Translate to Chemistry. Write the unbalanced equation using chemical formulas for names. Solid zinc and hydrochloric acid react to yield zinc chloride and hydrogen gas. Zn(s) + HCl  ZnCl2 + H2(g)

Rules for Balancing Equations Cont Satisfy the law of conservation of mass. Start with the most complicated molecule. (Do oxygen last!) Determine the coefficients necessary to satisfy the law of conservation of mass. RAP Tables can be helpful in this step Check your work. 3. Zn(s) + HCl  ZnCl2 + H2(g) __Zn(s) + __HCl  1 ZnCl2 + __H2(g) 1 Zn(s) + 2 HCl  1 ZnCl2 + __H2(g) 1 Zn(s) + 2 HCl  1 ZnCl2 + 1 H2(g) 4. 1 Zn, 2 H, 2 Cl  1 Zn, 2 H, 2 Cl

If a chemical equation does not obey the law of conservation of mass the equation is said to be what? NOT BALANCED So Let’s look at the steps we need to take to BALANCE chemical equations Let’s work with the following equation (write this down): Fe + O2  Fe2O3

Create a RAP table (what’s a RAP table ??) A RAP table shows us what atoms are present in this reaction, how many there are and are they reactants or products? For example: Fe + O2  Fe2O3 #R atom #P Fe 2 O 3 From this slide on write notes as needed. DO write down & solve chemical equations and RAP tables in your notes!

#R atom #P Fe 2 O 3 Fe + O2  Fe2O3

Polyatomics When an equation has Polyatomics in it, such as in this Balanced chemical equation 2AgNO3 + MgCl2  2AgCl + Mg(NO3)2 And the polyatomic appears on BOTH the reactant and product side of the equation Count the polyatomic as an “ATOM” So the above reactant atoms would be: #R atom #P 2 Ag 2 2 NO3 2 Mg 1 2 Cl 2 If the same polyatomic does not Appear on both sides break the Polyatomic down into atoms!

Opener 1/10 Complete Practice in Notes NaOH + CaBr2  Ca(OH)2 + NaBr C2H6 + O2  CO2 + H2O NH4OH + FeCl3  Fe(OH)3 + NH4Cl

Rule 2. Go to the first atom that’s not balanced and balance it! Since Fe atoms are not balanced what do we need to do to balance it? Right! Multiply it by 2 (Only multiply) #R atom #P 1 Fe 2 2 O 3 2x

Modify the equation to reflect the change In step 2 we balanced the number of Fe atoms by multiplying the reactant side by 2. This now becomes the new coefficient in the chemical equation. #R atom #P 1 Fe 2 2 O 3 2x Modify the equation to reflect the change 2Fe + O2  Fe2O3 Are all atoms balanced?

Is this equation balanced? NaOH + CaBr2  Ca(OH)2 + NaBr What atoms do we have in This equation? Count atoms & Start the RAP table #R atom #P 2) Do the #Reactant atoms = the # of Product atoms? 3) So pick the 1st unbalanced atom & begin balancing

We’ll start with balancing Hydroxide NaOH + CaBr2  Ca(OH)2 +NaBr How can we make both Hydroxides equal? #R atom #P Sure we’ll multiply #R OH by 2 Next step> rewrite the modified eqn. 2NaOH + CaBr2  Ca(OH)2 + NaBr Hydroxide is now balanced so let’s move to the next Unbalanced atom, which is? …

What can we do to balance the Bromine? Sure! Multiply the #P Bromine by 2 Now adjust the table to reflect The changes and then rewrite the Eqn. #R atom #P 2 Na 1 2 OH 2 1 Ca 1 2 Br 1 x2 2NaOH + CaBr2  Ca(OH)2 + 2NaBr

Ok Try Balancing this equation: C2H6 + O2  CO2 + H2O #R atom #P 2 C 1 6 H 2 2 O 3 Step 1. Total up the atoms Step 2. Balance the #P Carbon #R atom #P 2 C 2 6 H 2 2 O 5 & Re-write the equation C2H6 + O2  2CO2 + H2O Are we done?

Step 3. Carbons are balanced now but Hydrogen isn’t. So, balance Hydrogen atoms next #R atom #P 2 C 2 6 H 2 2 O 5 Multiply #P Hydrogen by 3 x3 Step 4. Re-write the eqn. & Retotal the number of atoms C2H6 + O2  2CO2 + 3H2O #R atom #P 2 C 2 6 H 6 2 O 7 Carbon and Hydrogen are now balanced but oxygen isn’t.

Step 5. To balance Oxygen multiply O by 3½ #R atom #P 2 C 2 6 H 6 2 O 7 Step 6. Re-write the eqn. & Retotal the number of atoms 3 ½ x C2H6 + 3½ O2  2CO2 + 3H2O It looks like we’re balanced. But, are we? No! We can’t have 3 ½ Oxygen molecules! Only whole Numbers are allowed. So what do we need to do to fix this?

Step 7. Let’s clean this up by Multiply everything by 2 C2H6 + 3½ O2  2CO2 + 3H2O x 2 2C2H6 + 7 O2  4CO2 + 6H2O #R atom #P 4 C 4 12 H 12 14 O 14 Step 8. Retotal #R and the #P atoms Are we balanced? YES!

Try this problem NH4OH + FeCl3  Fe(OH)3 + NH4Cl Start here. Recognize we Have polyatomics but they Appear on both sides of the Equation. #R atom #P 1 NH4 1 1 OH 3 1 Fe 1 3 Cl 1 OK … Now finish it up

Answer to previous problem 3NH4OH + FeCl3  Fe(OH)3 + 3NH4Cl