Welcome! 11/13&14 TOC. 24 Baggie Lab 11/13&14 25 Conservation of Mass & 11/13&14 Balancing Equations 26 Rube-Goldberg Design 11/9

Transcription

1 Welcome! 11/13&14 TOC 24 Baggie Lab 11/13&14 25 Conservation of Mass & 11/13&14 Balancing Equations 26 Rube-Goldberg Design 11/9

2 Trial #1 Pre-Reaction Mass (g) Post-reaction mass (g)

3 Trial Pre-Mass (g) End-Mass (g) 1a b 2a b 3a b 4a b 5a b 6a b Period 2

4 Trial Starting Mass (g) Ending Mass (g)

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7 Physical and Chemical Changes How to tell the difference

8 Physical Change Substance may seem different, but the way the atoms link up is the same.

9 It s a physical change if It changes shape or size It dissolves.

10 It s a physical change if... It changes phase (freezes, boils, evaporates, condenses)

11 Chemical Change Changes the way the molecules link up Makes new substances

12 It s a chemical change if. It burns Temperature changes without heating/cooling

13 It s a chemical change if... It bubbles (makes a gas)

14 It s a chemical change if... It changes color It forms a precipitate

15 What kind of change is it if someone... Tears up paper? Physical change Mixes salt and water? Physical change

16 What kind of change is it if someone... Burns paper? Chemical change Evaporates salt water? Physical change

17 What kind of change is it if someone... Mixes vinegar and baking soda? Chemical change

18 Conservation of Matter

19 Mass is neither created nor destroyed during a chemical reaction

20 If a certain mass of chemicals are added to a baggie and a reaction occurs then

21 Period 6 Trial Pre Mass (g) Post Mass (g) 1a b 2a b 3a b 4a b 5a b a b a

22 Reactants Products Mass of Reactants = Mass of Products

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24 Chemical reactions are represented on paper by chemical equations.

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26 Subscript

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32 Works Cited s/matter-and-energy/masscons.html ich/equation/stoich01.htm

33 Balancing Chemical Equations What goes in must come out!

34 Balancing Chemical Equations Balancing a chemical equation is much like the work of an accountant who has to show every penny that comes in and where it has gone to.

35 Objectives Learn the steps to balancing chemical equations. Take notes to help you understand. Test yourself with a set of equations to balance. Enter your own equations to see if they balance.

36 Law of Conservation of Mass You need to remember this law! The Law of Conservation of Mass states: that mass is neither created nor destroyed in any chemical reaction. Therefore balancing of equations requires the same number of atoms on both sides of a chemical reaction. The number of atoms in the Reactants must equal the Number of atoms in the Products

37 Chemical Equations Because of the principle of the Conservation of Matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788

38 Law of Conservation of Mass The mass of all the reactants (the substances going into a reaction) must equal the mass of the products (the substances produced by the reaction). Reactant + Reactant = Product

39 A simple equation, such as the synthesis of Iron (II) sulfide, iron + sulfur Iron (II) sulfide Fe + S FeS Note that in a chemical equation, by convention, we use the arrow of the equals = ". " instead

40 The last stage is to put in state of matter symbols, (s, l, g, aq), as appropriate (solid, liquid, gas, aqueous or dissolved in water) Fe(s) + S(s) FeS(s)

41 Balancing Equations 2 3 Al(s) + Br 2 (l) ---> Al 2 Br 6 (s)

42 Steps to Balancing a Chemical Equation 1. Write all reactants on the left and all products on the right side of the equation arrow. Make sure you write the correct formula for each element 2. Use coefficients in front of each formula to balance the number of atoms on each side.

43 Steps to Balancing a Chemical Equation 3. Multiply the coefficient of each element by the subscript of the element to count the atoms. Then list the number of atoms of each element on each side. 4. It is often easiest to start balancing with an element that appears only once on each side of the arrow. These elements must have the same coefficient. Next balance elements that appear only once on each side but have different numbers of atoms. Finally balance elements that are in two formulas in the same side.

44 Re-cap of steps from rule 4: Balance elements that appear only once on each side of the arrow. Next balance elements that appear only once on each side but have different numbers of atoms. Finally balance elements that are in two formulas in the same side.

45 Double-check: 4NH3 + 5O2 4NO + 6H2O 4 N (nitrogen atoms) 12 H (hydrogen atoms) 10 O (oxygen atoms) 4 N (nitrogen atoms) 12 H (hydrogen atoms) 10 O (oxygen atoms) The equation is Balanced P1 stopped here

46 Balancing Practice For more help go to: ance.htm#part0 For some fun balancing equations go to: ncing_equations_tutorial.htm

47 Example NH3 + O2 Reactants NO + H2O Products N appears once on both sides in equal numbers, so the coefficient for NH3 is the same as for NO.

48 Example: NH3 + O2 NO + H2O Next look at H which appears only once on each side but has different numbers of atoms, 3 on the left and 2 on the right. The least common multiple of 3 and 2 is 6, so rewrite the equation to get 6 atoms of H on both sides: 2NH3 + O2 NO + 3H2O

49 Example: 2NH3 + O2 NO + 3H2O There are 2 oxygen atoms on the left and 5 on the right the least common multiple of 2 and 5 is 10, so rewrite the equation as: 2NH3 + 5O2 4NO + 6H2O

50 Now count the atoms on each side: 2NH3 + 5O2 4NO + 6H2O Write them out keeping them on the appropriate side of the chemical equation 2 N (nitrogen atoms) 4 N (nitrogen atoms) 6 H (hydrogen atoms) 12 H (hydrogen atoms) 10 O (oxygen atoms) 10 O (oxygen atoms) This shows the equation not to be balanced YET

51 Check the number again: If you double the N and H on the left the equation will be balanced: 4NH3 + 5O2 4NO + 6H2O

52 Balancing Chemical Equations An easier way

53 First you need an equation with the correct formulae. You ll probably be given this in the question Just like this one Mg + O 2 MgO Then all you do is list the atoms that are involved on each side of the arrow Mg + O 2 MgO Mg O Mg O

54 Then start balancing: [1] Just count up the atoms on each side Mg + O Mg O MgO [2] The numbers aren t balanced so then add BIG numbers to make up for any shortages And adjust totals Mg + O Mg O MgO 2 2

55 But the numbers still aren t equal, so add another BIG number 2 Mg + O Mg 2 O MgO 2 2 And adjust totals again NOW BOTH SIDES HAVE EQUAL NUMBERS OF ATOMS WE SAY THAT THE EQUATION IS BALANCED!!

56 Try to balance these equations using the same method: [1] Na + Cl 2 NaCl [2] CH 4 + O 2 CO 2 + H 2 O [3] Li + HNO 3 LiNO 3 + H 2 [4] Al + O 2 Al 2 O 3

57 How did you do?? Here are the answers: [1] 2 Na + Cl 2 2 NaCl [2] CH O 2 CO H 2 O [3] 2 Li + 2 HNO 3 2 LiNO 3 + H 2 [4] 4 Al + 3 O 2 2 Al 2 O 3 HOPE YOU VE GOT THE IDEA REMEMBER TO CHECK THAT YOU CAN DO ELECTROLYSIS EQUATIONS TOO