Init: 12/14/2011 by Daniel R. Barnes WARNING: This presentation may contain images and other content that have been taken from the world wide web without permission of the owners of that content. Please do not copy or distribute this presentation. Its very existence may be illegal.

SWBAT balance chemical equations

Prentice Hall’s Chemistry textbook, page 326, says that the reaction pictured above violates the law of conservation of mass. Why is this?

The following stuff comes from Mr. Barnes’ Chemical Equation Balancing Practice Worksheet.

1.a. H 2 O O HH One water molecule

1.b. 2O 2 O Two oxygen gas molecules O O O

1.c. 3HCl Cl Three hydrogen chloride molecules H Cl H H

1.d. 8NaCl Eight sodium chloride formula units Cl - Na + Cl - Na + Cl - Na + Cl - Na + Cl - Na + Cl - Na + Cl - Na + Cl - Na +

1.e. 4CO 2 Four carbon dioxide molecules O C O O C O O C O O C O

1.f. 3Cl 2 Three chlorine gas molecules Cl

Think-pair-share: Compare and contrast 4O, 2O 2, and O 4. (If you haven’t seen this question recently.)

O OO O O OO O O O O O 4O Four separate, unbonded, monatomic oxygen atoms 2O 2 Two molecules made of two oxygen atoms each O 4 One molecule made of four oxygen atoms stuck together

5.a. H 2 + O 2  H 2 O Notice all the space in front of each formula. You’re going to need it sometimes, but you never know when, so you better always leave about an inch of space in front of each formula.

5.a. H 2 + O 2  H 2 O Once you’ve copied the equation, you need to do an initial atom count. The first step in doing that is to list the elements on both sides of the arrow, in the same order. H = O = H = O = Then, count the number of atoms of each element on the left and on the right Your initial atom count is now finished.

5.a. H 2 + O 2  H 2 O The whole point of balancing an equation is to make sure that the numbers on the left equal the numbers on the right. H = O = H = O =

1. Copy the equation onto your scratch paper. Leave plenty of space (about an inch) in front of each formula for coefficients. Yes, you might need about an inch because the first coefficient you put down might be wrong, and you’ll need to cross it out and put a new one to the left of it, repeating that process several times for some formulas. Don’t erase old coefficients. Cross them out. I want to see all your “footprints” on your scratch paper. 2. Break down the equation into its elements. List the elements in the same order on the left as you do on the right. Trust me. That’ll make it easier. 3. Do an atom count for each element on both sides of the arrow.

4. Is the equation balanced? In other words, are the atom counts on the left equal to the atom counts on the right? If so, you’re done. Put a happy face on it. If not, you’re going to have to do a bit of adjusting. 5. If the equation isn’t balanced yet, change one of the coefficients so that you end up with more of whatever element you need more of on the side of the arrow where you need it. 6. As soon as you change a coefficient, immediately update all the atom counts that were affected by that coefficient. Remember that a coefficient affects the amount of every element in the whole formula that the coefficient is in front of.

Let’s see those steps again, but more abbreviated.

1. Copy equation. 3. Atom count. 4. Equation balanced? 5. Change a coefficient. 6. Update atom count. 2. Elemental breakdown

Q1: What do you call the big numbers out in front of the formulas in a chemical equation? A: coefficients Q2: What do you call the little numbers found inside of a formula? A: subscripts Q3: What do coefficients tell you? A: how many molecules take part in a reaction Q4: What do subscripts tell you? A: how many atoms of an element are in each molecule of a substance

Q5: If a quantity does not change during a process, we say that the quantity is... A: “conserved”. Q6: What quantities are conserved during a chemical reaction? A: number of atoms (both overall and of each element), mass (amount of matter), electric charge. Q7: How do you know if a chemical equation is balanced or not? A: If the number of atoms of each element is the same on the left of the arrow as it is on the right of the arrow, the equation is balanced.

Q8: What is the only thing you are allowed to change about an equation when you balance it? A: the coefficients Q9: If there is no coefficient in front of a formula, how many molecules are indicated? A: one molecule Q10: Coeffcients usually have to be integers/whole numbers. Name one exception to this rule and a specific example of that exception. A: You many use fraction made of an odd number over two as a coefficient in front of a diatomic element. For instance, “½ O 2 ” is acceptable.

DIRECTIONS: Balance the following equations. Show your steps. 1. Ba + N 2  Ba 3 N 2 2. Sr + P  Sr 3 P 2 3. Al + O 2  Al 2 O 3 4. O 3  O 2 5. Na + F 2  NaF

DIRECTIONS: Balance the following equations. Show your steps. 6. H 2 + O 2  H 2 O 7. H 2 O 2  H 2 O + O 2 8. Ca(OH) 2 + HCl  CaCl 2 + H 2 O 9. B + Cl 2  BCl Fe(OH) 3 + HNO 3  Fe(NO 3 ) 3 + H 2 O

DIRECTIONS: Balance the following equations. Show your steps. 11. Na + O 2  Na 2 O 12. C 4 H 10 + O 2  CO 2 + H 2 O 13. C 6 H 14 + O 2  CO 2 + H 2 O 14. C 12 H 22 O 11 + O 2  CO 2 + H 2 O 15. CH 3 CH 2 OH + O 2  CO 2 + H 2 O

“The Monster” (from Steve Wallis) ___K 2 Mn 2 O 8 + ___H 2 SO 4 + ___H 2 C 2 O 4  ___CO 2 + ___MnSO 4 + ___K 2 SO 4 + ___H 2 O

NOTE TO INSTRUCTOR: The equation balancing quiz slides have been broken off to form a separate presentation so that this one might be postable on the web without compromising test security.