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Balancing Chemical Equations SCIENCE 10 MS. MCGRATH

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Counting Atoms How many of each type of atom are present in the compounds below? ◦H 2 SO 4 ◦CaCl 2 ◦4NaF ◦2KNO 3 ◦3K 2 SO 4 ◦2Mg 3 (PO 4 ) 2 Type of Atom# of Atoms

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Law of Conservation of Mass Law of conservation of mass: mass of products = mass of reactants Video: http://www.youtube.com/watch?v=dExpJAECSL8http://www.youtube.com/watch?v=dExpJAECSL8 ◦the # of atoms of each type in the reactants is the same as the # of atoms of each type in the products ◦Matter can’t be created or destroyed: atoms can’t be gained or lost during a chemical reaction A + B → C + D Reactants Products “produces/ gives” “reacts with”

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Balancing Chemical Equations Skeleton equation: CH 4(g) + O 2(g) CO 2(g) + H 2 O ( l ) The skeleton equation needs coefficients to balance both sides. Balanced equation: CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O ( l ) State of matter (aq) = aqueous/dissolved in water (s) = solid ( l ) = liquid (g) = gas Coefficients - indicate the ratio of compounds in the reaction - here, there is twice as much NO and NO 2 than there is O 2 Subscripts - show the # of atoms *YOU CANNOT CHANGE THESE TO BALANCE AN EQUATION

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Types of Chemical Equations Word Equation (words only, no formulas): Methane + oxygen carbon dioxide + water Skeleton Equation (formulas, but not balanced): CH 4(g) + O 2(g) CO 2(g) + H 2 O (l) Balanced Equation:

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Remember: You can only change the coefficients (the number in front) to balance You can not have coefficients that are fractions Double check to make sure coefficients can’t be reduced

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Examples 1.Hg + O 2 HgO 2.Aluminum reacts with oxygen to form aluminum oxide. 3.CaO + H 2 O Ca(OH) 2

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Strategies for Balancing Equations Balance chemical equations by following these steps: ◦Trial and error will work, but can be very inefficient ◦Balance compounds first, elements last ◦Balance one compound at a time ◦Only add coefficients; NEVER change subscripts! ◦If H and O appear, attempt to balance them LAST ◦Polyatomic ions (such as SO 4 2– ) can be balanced as a whole group. They don’t break apart! ◦Always double-check after you think you are finished!

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More examples Balance the following: 1.__ N 2 (g) + ___ H 2 (g) → ___ NH 3 (g) __ N 2 (g) + _3_ H 2 (g) → _2_ NH 3 (g) 2.__ Fe (s) + ___ H 2 SO 4 (aq) → ___ H 2 (g) + Fe 2 (SO 4 ) 3 (aq) 2_ Fe (s) + _3_ H 2 SO 4 (aq) → _3_ H 2 (g) + Fe 2 (SO 4 ) 3 (aq)

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Balancing Word Equations 1.Write the skeleton equation. Remember to ensure ionic compounds have a neutral charge (+ and – charges must add to zero) 2.Then balance 3.Check coefficients for common factors. If yes, reduce them.

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Try It! 1.sodium + chlorine sodium chloride 2.iron (II) oxide iron + oxygen 3.methane + oxygen carbon dioxide + water 4.copper (I) oxide copper + oxygen

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Practice Fe + Br 2 FeBr 3 CaCl 2 + Na NaCl + Ca Sn(NO 2 ) 4 + K 3 PO 4 KNO 2 + Sn 3 (PO 4 ) 4 C 2 H 6 + O 2 CO 2 + H 2 O

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To Do: Text: ◦p.133 #13, 18 ◦p.137 #8, 9, 11-18 ◦p.134 #11-13

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