1. Unit 8 Chemical Equations Test: Tuesday, January 27, 2009 Chapter: 10

2. Chemical Reactions When a substance undergoes a chemical change, it takes part in a chemical reaction. –After it reacts, it no longer has the same chemical identity. What are the evidences that a chemical change has occurred? _______________

3. Chemical Reactions Example Remember the classic science fair project - the homemade volcano? It’s done by combining vinegar and baking soda (and some red food coloring for effect). These substances undergo a chemical reaction, evident by the overflow and bubbles. This reaction produces three new substances: water, carbon dioxide, and sodium acetate (used in hot packs) Reaction of baking soda and vinegar: http://www.metacafe.com/watch/706897/vinegar_grenade/

4. Chemical Equations –The simplest way to represent this reaction is by using words –Chemical equations replace chemical names with chemical formulas Reactants (LEFT) substances that undergo a reaction Products (RIGHT) new substances formed in a reaction Yield sign (MIDDLE) represents the change/reaction

5. Chemical Equations Practice What is the chemical equation for the following reactions? What are the reactants and products? 1) Water and sulfur trioxide combine to form sulfuric acid 2) Potassium chlorite decomposes to form potassium chloride and oxygen H 2 O + SO 3 → H 2 SO 4 reactants product KClO 2 → KCl + O 2 reactant products

6. Chemical Equations States of Matter Some chemical equations indicate the state of matter of the substances (i.e. liquid) –Solid (s) - baking soda –Liquid (l) - water –Gas (g) - carbon dioxide –Aqueous (aq) - vinegar and sodium acetate Aqueous means the substance is dissolved in water. For example, NaCl(aq) means table salt dissolved in water. This may be the only way the reaction can occur. 

7. Chemical Equations Energy Energy is either released or absorbed during a chemical reaction. Can be many forms of energy (light, heat, mechanical, chemical, etc). –Endothermic reactions absorb energy. –Energy is a reactant –Can feel cool to the touch

8. Chemical Equations Energy Energy is either released or absorbed during a chemical reaction. Can be many forms of energy (light, heat, mechanical, chemical, etc). –Exothermic reactions release energy. –Energy is a product –Can feel warm to the touch

9. Chemical Equations Energy Photosynthesis: plants consume carbon dioxide, water, and light energy (from the sun) and convert it into sugar (glucose), oxygen, and water. 6 CO 2 (g) + 12 H 2 O(l) + energy → C 6 H 12 O 6 (aq) + 6 O 2 (g) + 6 H 2 O(l) Cellular Respiration: animals consume sugar (glucose) and oxygen to produce carbon dioxide, water, and chemical energy (ATP) C 6 H 12 O 6 (aq) + 6O 2 (g) → 6CO 2 (g) + 6H 2 O(l) + energy These chemical equations are important principles of biology. Which is endothermic and which is exothermic?

10. Law of Conservation of Mass The LCM says matter is neither created nor destroyed –The mass of the reactants is always the same as the mass of the products. –The number of atoms of each element is always the same in the reactants and the products.

11. Law of Conservation of Mass Examples Hydrogen gas and oxygen gas combine to create water. 2H 2 + O 2  2H 2 O The large numbers, known as coefficients, indicate how many moles of each compound there are. This helps to satisfy the LCM! They apply to every element in the compound (2H 2 O means 4 H’s & 2 O’s) They are the coefficients should be in the lowest whole number ratio Particles are rearranged, not created or destroyed!

12. Law of Conservation of Mass Examples This should be a review - just count moles of atoms! How many carbon atoms are conserved? Hydrogen atoms? Oxygen atoms?

13. Law of Conservation of Mass Examples Calcium chloride and sodium sulfate combine to form calcium sulfate and sodium chloride. CaCl 2 + Na 2 SO 4  CaSO 4 + 2NaCl Before and After: Does the mass change? Do the number of atoms change? # atoms in the reactants # atoms in the products Ca11 Cl22 Na22 S11 O44 Total10

14. Review - Chemical Equations Label the following chemical equation: SnO 2 (s) + 2H 2 (g)  Sn(s) + 2H 2 O(g) productsreactants state of matter subscript coefficient yield sign

15. Balancing Chemical Equations Because the Law of Conservation of Mass (LCM) states that matter is not created or destroyed, the number of atoms of each element must be identical in the reactants and products. This means the chemical equation is balanced. Example 1: # atoms in the reactants # atoms in the products H22 C11 O33 Total66 This equation is balanced because it satisfies the LCM Writing coefficients is not necessary because each compound has a coefficient of 1

16. Balancing Chemical Equations Example 2: # atoms in the reactants # atoms in the products Na12 O34 H12 C11 Total69 This equation is not balanced because it does not satisfy the LCM Writing coefficients is necessary So we have to balance the equation…

17. Balancing Chemical Equations # atoms in the reactants # atoms in the products Na22 O44 H22 C11 Total99 Example 2 continued: The process of balancing chemical equations requires trial and error. You may not alter the subscripts because it changes the substance! You may only place coefficients in front of the compounds. These act as multipliers to balance the number of atoms. By placing a 2 in front of NaOH, the equation now satisfies the LCM and is balanced ***Tip: balance hydrogen and especially oxygen last - they often fall into place !

18. Balancing Chemical Equations Practice Write a balanced chemical equation for the following reactions. Place coefficients in the blanks. Chemical equations never actually write the coefficient “1”, but for this activity write a number in every blank. A) ___ K(s) + ___ H 2 O(l)  ___ H 2 (g) + ___ KOH(aq) B) ___ CaCl 2 (aq) + ___ Na 2 CO 3 (aq)  ___ CaCO 3 (s) + ___ NaCl(aq) C) ___ N 2 (g) + ___ H 2 (g)  ___ NH 3 (g)

19. Classifying Chemical Reactions Reactions are classified into several categories. –Synthesis A + B  AB –Decomposition AB  A + B –Single Replacement A + BC  AC + B –Double Replacement AB + CD  AD + CB –CombustionC x H y + O 2  CO 2 + H 2 O By recognizing these patterns, you can classify a reaction and make predictions about its products. Compare these reactions to dancing!

20. Combination Reaction Whenever two or more substances combine to form a single product, the reaction is called a synthesis reaction. When iron rusts, iron metal and oxygen gas combine to form one new substance, iron (III) oxide.

21. Decomposition Reaction Whenever a compound breaks down into two or more simpler substances, the reaction is called a decomposition reaction. When hydrogen peroxide spontaneously decomposes, it becomes water and oxygen gas. 2H 2 O 2 (aq)  2H 2 O(l) + O 2 (g)

22. Single-Displacement Reaction Whenever one element takes the place of another, the reaction is called a single-displacement reaction. This reaction will only occur if the more active metal will become part of a compound in the product. An iron nail soaking in copper (II) sulfate yields iron (II) sulfate and copper

23. Activity Series Li Rb K Ca Na Mg Al Mn Zn Fe Ni Sn Pb H 2 Cu Hg Ag Pt Au Most Active Least Active This feature is found on your handy helper. Use it to determine whether a single- displacement reaction will occur (react) or not. Active metals have a low electronegativity. They bond with nonmetals more readily because they are less stable by themselves. If a very active metal is competing with a less active metal to bond (with a nonmetal or negatively charged polyatomic ion), the more active metal will “win”. Think of it as a competing suitors who want to date the same girl!

24. Activity Series Example #1: Is this combination possible? Pb + MgSO 4  PbSO 4 + Mg Answer: No, because Mg is more active than Pb Example #2: Is this combination possible? NaNO 3 + K  Na + KNO 3 Answer: Yes, because K is more active than Na Li Rb K Ca Na Mg Al Mn Zn Fe Ni Sn Pb H 2 Cu Hg Ag Pt Au Most Active Least Active   

25. Double-Displacement Reaction Whenever two ionic compounds trade cations, the reaction is called a double-displacement reaction. One of the products will either be water, a gas, or most commonly a precipitate (insoluble in water). Lead (II) nitrate and potassium iodide form lead (II) iodide and potassium nitrate. Note the yellow precipitate is PbI 2.

26. Combustion Reaction Whenever hydrocarbons combine rapidly with oxygen (usually by fire), the reaction is called a combustion reaction. The products will be water and carbon dioxide. Burning methane (CH 4 ) yields water and carbon dioxide - this reaction contributes to our greenhouse gases and global warming.

27. Identifying Chemical Reactions Identify each of the following chemical equations as synthesis, decomposition, single-displacement, double- displacement, or combustion reaction. A) B) C)

28. Predicting Chemical Reactions Steps 1.Determine what type of reaction is being presented 2.Write the correct formulas for the product(s) 3.Balance the equation Example: CaCl 2 + Al(OH) 3  ? 1.Double-displacement reaction 2.__CaCl 2 + __Al(OH) 3  __Ca(OH) 2 + __AlCl 3 3.3CaCl 2 + 2Al(OH) 3  3Ca(OH) 2 + 2AlCl 3

29. Predicting Chemical Reactions Practice Predict the chemical reaction: NaCl + Ca(OH) 2  ? Predict the chemical reaction: Mg + CuSO 4  ?

30. End of Unit 8 Notes Study for your test!