E20: BALANCING EQUATIONS

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1 E20: BALANCING EQUATIONS Before we can talk about balancing equations, we need to know how to READ an equation. We've already talked about symbols (like H or Na), and you should know how to read a formula (like CO 2, which contains 1 carbon atom and 2 oxygen atoms). An equation describes a REACTION. The most basic parts to know are: REACTANTS yield PRODUCTS (what you start with) (what you end up with)

2 In E19, we reacted solid copper wire with dissolved ("aqueous") silver nitrate. We ended up with solid silver & blue copper nitrate solution. This can be written as: Cu (s) +2 AgNO 3(aq) 2 Ag (s) + Cu(NO 3 ) 2(aq) The letters in parentheses after the formulas tell the physical state of the substance. (s)= solid (aq) = aqueous (dissolved) (l) = liquid (g) = gas

3 WHY BALANCE? Newton's First Law states that "matter cannot be created nor destroyed" That means that, while the atoms in a chemical reaction can rearrange themselves by breaking bonds or creating new bonds (switching partners), they CANNOT magically appear or disappear. The numbers of each type of atom should be the same on each side of an equation ("balanced"). For example: In the equation H + O H O, we start with 2 hydrogen atoms and 2 oxygen atoms, but we produce 2 hydrogen & only one oxygen. This is impossible! Oxygen atoms do not disappear. To balance it, we can add "Co-efficients," like so: 2 H 2 + O 2 2 H 2 O Now we have 4H and 2 O on each side!

4 REACTANTS yield PRODUCTS Cu (s) +2 AgNO 3(aq) 2 Ag (s) + Cu(NO 3 ) 2(aq) Co-efficient BIG numbers ("mole ratio") THIS is what you can change or add to balance an equation. All NUMBERS in an equation Subscript (little numbers) Only applies to the element directly in front of it, or to contents of parentheses. DO NOT CHANGE THESE to BALANCE are MULTIPLIERS. Ex: 2AgNO 3 has Co-efficients multiply entire formulas, 2 silver, 2 nitrogen while subscripts multiply specific parts. and 3x2= 6 oxygen

5 BALANCE BY INSPECTION (trial and error method) PAGE 96 in WB You are NOT allowed to change the subscripts in any formulas! 1) Start with an element that appears in only one form on each side of the equation; Any element that appears in more than one compound should be balanced last. 2) Any polyatomic ion (NH + ; SO4 +2 ; NO3-1 ; OH -1 ) with subscripts can be balanced 4 easily by using the same number as the coefficient with it on the opposite side of the equation; Remember that subscripts after parentheses multiply everything inside them by that number. These will travel together- treat them like a package deal. Example: Al + H 2 SO 4 Al 2 (SO 4 ) 3 + H 2 start with this! 3) If an element has an odd number of atoms on one side, and an even number on the other, it may help to multiply the odd-numbered one by 2. Even out your odds! Example: P + H 2 PH 3 start by trying a 2 in front of PH 3! 4) It may sometimes help to think of water, H O, as HOH and balance water last. 2

6 Synthesis reactions 1. H 2 + Cl 2 HCl 2. H 2 + N 2 NH 3 3. P + O 2 P 2 O 5 4. As + O 2 As 2 O 3 HOMEWORK: WB pg 96- Practice Problems #5-12 ALL & Double Replacement ODDS (#13,15, 17, 19)

7 4 main types of reactions: SYNTHESIS: A + B AB 2 substances bond together to make a new compound. DECOMPOSITION: AB A + B A compound breaks down into simpler parts. Opposite of synthesis. SINGLE REPLACEMENT: A + BC B + AC One (more reactive) element kicks out another (less reactive) element. Stealing the partner- like a love triangle! DOUBLE REPLACEMENT: AB + CD AD + CB Switching partners: 2 elements switch places forming new products.

8 Decomposition reactions 5. HClO 3 HCl + O 2 6. HgO Hg + O 2 Single replacement reactions 7. Cl 2 + KBr KCl + Br 2 PRACTICE TEST TODAY (due tomorrow) E20 TEST TOMORROW! 8. Al + H 2 SO 4 Al 2 (SO 4 ) 3 + H 2 9. NH 3 + O 2 N 2 + HOH 10. Al + H 3 PO 4 H 2 + AlPO Ca + AlCl 3 CaCl 2 + Al 12. Li + H 2 SO 4 Li 2 SO 4 + H 2 Double replacement reactions 13. Al 2 (SO 4 ) 3 + Ca(OH) 2 Al(OH) 3 + CaSO Mg 3 N 2 + HOH Mg(OH) 2 + NH Bi 2 (SO 4 ) 3 + NH 4 OH Bi(OH) 3 + (NH 4 ) 2 SO Fe 3 O 4 + HNO 3 Fe(NO 3 ) 3 + NO 2

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10 Now: work on the E20 Practice Test (due tomorrow) You may work with a partner, but make sure you and your partner are helping eachother UNDERSTAND the concept! You EACH should complete the worksheet, which I will collect before the E20 test tomorrow. You may check your answers at the front desk in class, or on my blog at home. These notes and links to some interactive practice websites will also be found on my blog!