1. Chemical Reactions Chapter 16 Physical Science Mrs. Nell

2. Chemical Reaction A well defined example of a chemical change. One or more substances are changed to new substances

3. Reactants In a chemical reaction, the substances that are about to react

4. Products In a chemical reaction, the new substances produced Reactants  products produce

5. Chemical Change A chemical change that you are familiar with is rust. In this chemical reaction, oxygen and iron, combine to form a product called iron oxide (rust). The reactants are the iron in the nail and oxygen in the air. The product is the rust on the nail.

6. Reactants & Products If we were to place a small amount of baking soda in a glass and add vinegar, we would observe bubbles and fizzing- evidence of a chemical reaction. In this reaction what are the two reactants? What is one of the products? Answer: Reactants: Vinegar, Baking Soda Products: CO 2 and H 2 O

7. Conservation of Mass Antoine Lavoisier French chemist Early 1700s Found that matter is neither created nor destroyed during a chemical change (the starting mass of the reactants equals the final mass of the products)

8. Symbols used in Chemical Equations SYMBOLMEANING  produces or forms ✚ plus (cr) crystalline solid (l) liquid (g) gas (aq) aqueous, a solid is dissolved in water heat  the reactants are heated light  the reactants are exposed to light elec.  an electric current is applied to the reactants

9. Chemical Equations An expression that describes a chemical reaction using chemical formulas and other symbols

10. Coefficients Represent the number of units of each substance taking part in a reaction

11. Quiz Identify the reactants and the products in the following chemical equation. 2B(cr) + 3I 2 (g)  2BI 3 (cr) Reactants Product

12. Quiz 2 What is the name and state of matter of each substance in the following reaction? 4Al(cr) + 3O 2 (g)  2Al 2 O 3 (cr) Al- aluminum, crystalline solid O 2 - oxygen, gas Al 2 O 3 - aluminum oxide, solid

13. Chemical Reactions in the Air A gas called ozone (0 3 ), found in the Earth’s upper atmosphere, helps keep some of the harmful ultraviolet (UV) radiation from reaching Earth’s surface.

14. Going to Antarctica? Bring Your Sunscreen! The ozone hole over Antarctica, which fluctuates in size, grew to the size of North America in September 2008. The fifth largest on record, the hole formed over Antarctica and covered 27 million kilometers. If efforts continue, the layer should start to recover by 2049.

15. Oh where, oh where has our ozone gone? Decreases in the level of ozone, particularly around the poles, are now being detected. The result- more UV radiation reaches Earth.

16. CFCs Chlorofluorocarbons (CFCs) are compounds that destroy ozone. CFCs are used in air-conditioner units, aerosol spray cans, styrofoam, and refrigerators.

17. Chemical Equations Remember- matter is never created or destroyed in a chemical reaction. A balanced chemical equation has the same number of atoms of each element on both sides of the equation.

18. Writing Balanced Chemical Equations Step 1: – Describe the reaction in words, putting the reactants on the left side and the products on the right side.

19. Step 2: – Write a chemical equation for the reaction using formulas and symbols.

20. Step 3: – Check the equation for atom balance. Set up a chart to help you (see below). Atoms in Unbalanced Equation Mg+O 2  MgO Kind of AtomNumber of atoms Mg11 O21 The magnesium atoms are balanced, but the oxygen atoms are not. Therefore, this equation isn’t balanced.

21. Step 4: – Choose coefficients that balance the equation. Remember, never change subscripts!

22. Practice #1 Balance the following equation: __ H 2 + __ O 2  __H 2 O Answer: 2H 2 + 1O 2  2H 2 O

23. Practice #2 Balance the following equation: __Fe + __Cl 2  __FeCl 3 Answer: 2Fe + 3Cl 2  2FeCl 3

24. Practice #3 Balance the following equation: __Al + __O 2  __Al 2 O 3 ANSWER: _4_Al + _3_O 2  _2_Al 2 O 3

25. Practice #4 Balance the following equation: __KNO 3  __KNO 2 + O 2 Answer: 2KNO 3  2KNO 2 + O 2

26. Practice #5 Balance the following equation: __O 2 + __CS 2  __CO 2 + __SO 2 Answer: 3O 2 + 1CS 2  1CO 2 + 2SO 2

27. Practice #6 Balance the following equation: __KClO 3  __KCl + __O 2 Answer: 2KClO 3  2KCl + 3O 2

28. Types of Chemical Reactions Classifying chemical reactions Synthesis Reactions Decomposition Reactions Single-Displacement Reactions Double-Displacement Reactions

29. Synthesis Reactions When 2 or more substances combine to form another substance General formula: A + B  AB ex. 2H 2 (g) + O 2 (g)  2H 2 O(g)

30. Decomposition Reaction When one substance breaks down or decomposes, into two or more simpler substances. Most require the use of heat, light, or electricity. General formula: AB  A + B ex. 2H 2 O(l)  2H 2 (g) + O 2 (g) elec.

31. Single-Displacement Reactions Occurs when one element replaces another in a compound. Two General formulas: A + BC  AC + B or D + BC  BD + C

32. Activity Series of Metals This table lists metals according to how active they are. A metal will replace any metal beneath it in the table. Most Reactive Least Reactive

33. Double-Displacement Reactions Occurs if a precipitate, water, or a gas forms when two ionic compounds in solution are combined. A precipitate is an insoluble compound formed during this type of reaction. The positive ion of one compound replaces the positive ion of the other compound to form two new compounds. The general formula is: AB + CD  AD + CB

34. Energy & Chemical Reactions In all chemical reactions, energy is either released or absorbed. In most chemical reactions chemical bonds in the reactants must be broken, which takes energy. In order for products to be produced, new bonds must be formed- which releases energy. Vapors of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride

35. Exothermic Reactions In exothermic reactions, some form of energy is given off by the reaction. Many chemical reactions release energy in the form of heat, light, or sound. The burning of wood and the explosion of dynamite are exothermic.

36. Catalysts Some reactions are very slow. To speed up these reactions a catalyst may be used. Enzymes act as catalysts in living organisms. For example an enzyme found in saliva breaks up food for digestion on contact. With out this catalyst it would take weeks for us to digest our food.

37. A Little Goes A Long Way There is just as much catalyst at the end of a reaction as there was at the beginning. The catalyst is used over and over again. Because catalysts work so rapidly and are used again and again, it is only necessary to have very small quantities of catalyst present to make a chemical reaction go faster.

38. Catalytic Converter You might have a "catalytic converter" in the exhaust pipe of your car. If the catalyst was platinum you might expect silly people to start stealing car exhaust pipes; but there is so little catalyst there that it would not be worthwhile for them. A little bit of catalyst goes a long way!

39. What is a Catalytic Converter? A catalytic converter is a ceramic structure used to reduce toxicity of emissions. It is coated with a metal catalyst, usually platinum, rhodium and/or palladium. The catalyst metals are extremely expensive. Some of the newest converters have even started to use gold- because it is cheaper!

40. Inhibitors Sometimes we need to prevent a reaction from occurring. Substances called inhibitors are used to combine with one of the reactants. Some food preservatives are inhibitors. They keep food from spoiling.

41. Endothermic Reactions At times, more energy is required to break bonds than to form new ones in a chemical reaction. These are endothermic, and energy has to be provided for the reaction to take place. This absorbs so much energy, that the container can feel cold to the touch. Endothermic reactions are used to obtain metal from its ore. Cold packs are also a result of an endothermic reaction. These ice packs usually contains water and a packet of ammonium chloride. The cold pack is activated by breaking the barrier separating the water and ammonium chloride, allowing them to mix.