Balancing Chemical Equations: Step-by-Step Guide

1. Don’t be the last to stand! Stand up whenever the total number of atoms equals 10. 2 Na2O2H 10

2. 2 Na2OH2 10

3. 3 NaCl 6

4. 3 Na2Cl 9

5. 5 Na2 10

6. 4 H2Cl 12

7. 2 H2Cl3 10

8. 2 H4O 10

9. 3 MgOH 9

10. 2 Mg2OH2 10

11. Balancing Chemical Equations STEP 1: Box around compounds STEP 2: Identify elements STEP 3: Count atoms STEP 4: Add Coefficients STEP 5: Update numbers

12. Balancing Chemical Equations 3 3 AlBr3 + K → KBr + Al Al = Br = K = Al = 1 Br = 3 K = 1 Al = 1 Br = 1 K = 1 Al = Br = K = 3 3 3 STEP 1: Box around compounds STEP 2: Identify elements STEP 3: Count atoms STEP 4: Add Coefficients STEP 5: Update numbers

13. P4 + O2 → P2O3 P4 + 3O2 → 2P2O3

14. N2 + H2 → NH3 N2 + 3H2 → 2NH3

15. Chapter 13 Test: Friday, Feb. 23rd • I can use prefixes in a compound to write out a chemical formula. • I can count atoms in a chemical formula using subscripts, coefficients, and parentheses. • I can explain the law of conservation of mass. • I can count and compare atoms in a chemical equation to determine if it represents the law of conservation of mass.

16. What we still need to know… • I will be able to identify chemical reactions (or chemical equations) as a synthesis reaction, decomposition reaction, single-displacement reaction, or double-displacement reaction. • I will be able to explain how the rate of a reaction can speed up or slow down. • I will be able to identify chemical reactions as endothermic or exothermic reactions.

17. Endothermic vs. Exothermic Reactions Let’s remember that there are physical and chemical changes… • A physical change doesn’t create something new. The composition (what it’s made of) doesn’t change. • A chemical change does create something new. The bonds get rearranged. We have signs to show this: change in color or smell, gives off light or heat, forms a solid…

18. Let’s Compete! Is it a physical or chemical change? Physical

19. Let’s Compete! Is it a physical or chemical change? Physical

20. Let’s Compete! Is it a physical or chemical change? Chemical

21. Let’s Compete! Is it a physical or chemical change? Physical

22. Let’s Compete! Is it a physical or chemical change? Chemical

23. Let’s Compete! Is it a physical or chemical change? Chemical

24. Let’s Compete! Is it a physical or chemical change? Physical

25. EXOTHERMIC REACTIONS • An exothermic reaction is a reaction that releases energy (or heat). • Example: Making ice cubes (physical) Rusting bike (chemical) A candle flame (chemical)

26. ENDOTHERMIC REACTIONS • An endothermic reaction is a reaction that absorbs (or uses) energy. • Example: Photosynthesis (chemical) Evaporating liquid water (physical) Melting a crayon (physical) Baking bread (chemical)

29. Around the room… • There are 13 questions around the room. • Walk around with your notes and a partner and answer all 13 questions. • You are only allowed to discuss answers with your partner. • Write your answers in your composition book and when you have finished, pick up a blank sheet of white paper. • Sit down in your assigned seat and QUIETLY create a foldable like the one taped to the teacher’s lab table. Synthesis Decomposition Single-Displacement Double-Displacement Fold the paper hamburger style