Chemical Reactions

1. Chemical Reactions 7th Grade Science Bowling Green Junior High

2. What are chemical reactions?

3. Chemical Reaction– a change that takes place when two or more substances (reactants) interact to form new substances (products) with new properties.

4. Matter made of two or more different elements chemically bonded. • Cannot be separated by physical means • Has properties that are different from the elements that make it up. Compounds

6. MORE COMMON THAN ELEMENTS DUE TO MANY ELEMENTS BEING REACTIVE WITH EACH OTHER • THE ELEMENTS THAT COMBINE MAKE A NEW SUBSTANCE WITH NEW PHYSICAL PROPERTIES • FOR A COMPOUND TO FORM OR BE BROKEN DOWN, A CHEMICAL REACTION MUST TAKE PLACE TABLE SALT NaCl = +

7. Respiration (breathing) • Photosynthesis • Grilling food • Starting a vehicle • Digestion • Rusting metal Everyday examples of chemical reactions

8. A new substance with new properties is formed How do you know when a chemical reaction has taken place?

9. Temperature Change (heat given off or required) • FIZZES OR BUBBLES • COLOR CHANGE • ODOR • LIGHT GIVEN OFF • NEW SUBSTANCE FORMED • Precipitate (solid) • Precipitate (gas bubbles) Signs of a chemical reaction

10. Reactants – Substances that start a chemical reaction (EX: chemicals on match head) Products – Substances produced in the reaction (EX: black material on match) Two parts of a chemical reaction

12. CHEMICAL EQUATIONS • Chemical equations are symbols used to describe the details of a chemical reaction. • Shows how the reactants changed into the product. • This involves indicating all the atoms involved in the reaction. Fe + O2 FeO2 Arrow: Means “yields” takes the place of an = sign Reactants: Iron and oxygen Product: Ferrous oxide (rust) Plus Sign: Shows substances combine Products are ALWAYS to the right of the arrow Reactants are ALWAYS to the left of the arrow

13. Combustion • Synthesis • Decomposition • Single replacement • Double replacement • Neutralization • Oxidation/Reduction • Hydrolysis • Endothermic/Exothermic Types of chemical reactions

14. What do you have to have to burn something?

15. When oxygen (O2) combines with another compound to form water and carbon dioxide. • Needs a fuel source • Takes place at high temperatures • Fast process that results in an increase of temperature and production of fire. Combustion reactions

16. Chemical reactions can be classified Combustion Reaction – always involves oxygen (O2) as a reactant. CH4 + 2O2 + CO2 2H2O Water Carbon Dioxide Methane Oxygen O O H H + C + H H O O

17. 4 types of reactions

18. Two or more substances react to form a new substance(s) • A + B  AB • S + O2  SO2 Synthesis reactions

19. Chemical reactions can be classified Synthesis Reaction – combines two or more simpler reactants to form new, more complex products. N2 2O2 + 2NO2 Nitrogen Dioxide Nitrogen Oxygen O O + N N O O Simple to complex

20. One substance breaks down into two or more simpler substances • AB  A + B • CaCO3  CaO + CO2 Decomposition reaction

21. Chemical reactions can be classified Decomposition reaction – breaks a reactant into two or more simpler products + 2H2O 2H2 O2 Water Oxygen Hydrogen H H + O Complex to simple H H O

22. One element replaces another element in a compound • AB + C  AC + B • Zn + 2HCl  H2 + ZnCl2 Single Replacement

23. Chemical reactions can be classified Replacement Reaction – elements switch places to form new compounds. 1) Single Replacement Zn 2HCl ZnCl2 H2 + + Zinc Hydrochloric Acid Zinc Chloride Hydrogen Cl Cl + Zn H H + Cl Cl H H

24. Elements from two different compounds switch places • AB + CD  AC + BD • HCl + NaOH  NaCl + H2O Double replacement

25. Chemical reactions can be classified Replacement Reaction – elements switch places to form new compounds. Double Replacement FeS 2HCl FeCl2 H2S + + Iron Sulphide Hydrochloric Acid Iron Chloride Hydrogen Sulfide Fe Cl + + H S Cl H

26. All chemical reactions are going to release (give off) energy or absorb (take in) energy. • Some will require energy to start the reaction (activation energy) • EX: before you use a new cell phone, what’s got to happen? • Activation energy=energy required to start a chemical reaction.

27. Endothermic vs. exothermic processes

28. Exothermic reactions are exactly the opposite. While they take some energy to get going, called the activation energy of reaction, these reactions give off heat during the reaction • Good examples of exothermic reactions are explosions like fireworks or combustion in engines. • Forming a chemical bond releases energy and is exothermic • Usually feel hot because it is giving heat to you Exothermic reactions or processes

29. Endothermic reactions are those which absorb heat during the reaction. They take in more energy than they give off, which leaves the surroundings cooler than the starting point • Evaporation of water by sunlight is a great example. The sun and the liquid water combine and the water absorbs energy and eventually becomes as gas. • Breaking a chemical bond requires energy and is endothermic • Usually feel cold because it is taking heat away from you Endothermic reactions or processes

31. Substance which speeds up a chemical reaction but is chemically unchanged at the end of the reaction. • The catalytic converter in a car contains platinum, which serves as a catalyst to change carbon monoxide, which is toxic, into carbon dioxide. • If you light a match in a room with hydrogen gas and oxygen gas, there will be an explosion and most of the hydrogen and oxygen will combine to create water molecules. Catalyst

32. A way of writing which type of atoms and how many of each there are in a compound.

33. Written as: C4H10 Butane Written as: CH4 Methane Chemical Formulas

34. = how many total molecules Subscripts= how many atoms

35. FeO2 • H2O • CO2 • MgBr2 • C6H12O6 • 3OH • 2H2O Counting Atoms

36. 2Na + MgF2 2NaF + Mg Counting atoms in chemical equations

37. 2K + Cl2 2KCl Counting atoms in chemical equations

38. 2Na2O  4Na + O2 Counting atoms in chemical equations

39. Law of Conservation of Matter Matter cannot be created or destroyed, it just changes forms. *The total mass of the reactants MUST EQUAL the total mass of the product.

41. Law of Conservation of Mass Alka-Seltzer and Water http://www.sky-web.net/science/balancing_chemical_equations_examples.htm

42. The number of atoms of the reactants must equal the number of atoms in the product. (Law of Conservation of Matter) Ex: 2Na + Cl2 -> 2NaCl 4P + 5O2 ->P4O10 Balancing Equations

43. Rules • Make sure that all atoms are equal on both sides. • You can only add coefficients. • Changing the subscripts will change the identity of the compound. • H2O & H2O2 EX: 2Na + Cl2 -> 2NaCl H2 + O2 -> 2H2O (Not balanced… So…) 2H2 + O2 -> 2H2O Balancing Equations

44. Balancing Chemical Equations Hg + O2 HgO H2 + Cl HCl Mg + O2 MgO O2 + H2 H2O CH4 + O2 CO2 + H2O Fe + Cl2 FeCl3

45. Hg + O2HgO

46. H2 + Cl HCl

47. Mg + O2 MgO

48. O2 + H2 H2O

49. Fe + Cl2 FeCl3

50. CH4 + O2 CO2 + H2O