1. Chapter 7 – Chemical Reactions

2. 7.1 – Describing Reactions

3. What is a chemical change? Chemical Change: occurs when a substance reacts and forms one or more new substances – EXAMPLES: baking a cake, leaves changing color in the fall, food digesting in your stomach, paper burning

4. Evidence of a Chemical Change Some clues that a chemical change has taken place: – change in color – production of gas – formation of precipitate Click HERE for video (production of a gas)HERE

5. Chemical Reactions When a substance undergoes a chemical change, a chemical reaction is said to have taken place. In order to understand chemical reactions, you must be able to describe them!

6. Describing Chemical Reactions Identify what is present before and after the change – Reactants: the substances that undergo change – Products: the new substances formed as a result of that change During a chemical change, the reactants change into products – Reactants  Products

7. Chemical Reaction

8. Example: Burning charcoal Word Equation: carbon + oxygen  carbon dioxide Chemical Equation: C + O 2  CO 2

9. Chemical Equation: a representation of a chemical reaction in which the reactants and products are expressed as formulas Read aloud as: carbon and oxygen react to form carbon dioxide, or the reaction of carbon and oxygen yields carbon dioxide

10. Mass is Conserved! As the charcoal burns, what happens to the mass of the piece of charcoal? Where does it go? It does not just disappear! Would the mass of the charcoal before you burn it equal the mass after it is burned?

11. Mass is Conserved If you were to measure the mass of the charcoal before the reactions and the mass of the CO 2 (carbon dioxide gas) what would you notice? – THEY WOULD BE EQUAL!!! – During a chemical reaction, the mass of the products is ALWAYS equal to the mass of the reactants. This is known as the Law of Conservation of Mass- mass is neither created nor destroyed in a chemical reaction.

12. Showing Mass is Conserved In order to ensure that the Law of Conservation of Mass is being obeyed, in other words, that mass is being conserved, you must write a balanced chemical equation. This means that the number of atoms of an element in the reactants MUST EQUAL the number of atoms of that element in the products

13. Rules to Balancing a Chemical Equation 1.Count the number of atoms of each element on each side of the equation a.Example: N 2 H 4 + O 2  N 2 + H 2 O b.N = 2 H = 4 O = 2  N=2 H=2 O=1 2.Change one or more coefficients until the equation is balanced a.Coefficient- the numbers that appear before the formula b.NEVER change the subscripts! Only the coefficients! c.Example: ____N 2 H 4 + ____O 2  ____N 2 + ____ H 2 O

14. Example of Balancing ____N 2 H 4 + ____O 2  ____N 2 + ____ H 2 O How would you correctly balance the problem above?

15. Correctly Balanced ____N 2 H 4 + ____O 2  ____N 2 + 2 H 2 O

16. 7.2 – Types of Reactions Many chemical reactions produce carbon dioxide: – Burning charcoal – Hydrochloric acid dropped on limestone – Heating limestone However, just because two reactions have the SAME product, you cannot assume they are the same type of reaction!

17. Classifying Reactions Reactions are often classified by the type of reactant or the number of reactants or products. Some general types of chemical reactions are: – Synthesis – Decomposition – Single-replacement – Double replacement – Combustion reactions

18. Classifying Reactions Each type describes a different way in which reactants interact to form products.

19. Synthesis Reaction A synthesis reaction is a reaction in which two or more substances react to form a single substance. The general equation for a synthesis reaction is: – A + B  AB – Ex: 2Na + Cl 2  2NaCl

20. In the cartoon, the skinny bird (reactant) and the worm (reactant) combine to make one product, a fat bird. Synthesis Reaction

21. Decomposition Reaction The opposite of a synthesis is decomposition. A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The general equation for a decomposition reaction is: – AB  A + B – Ex: 2H 2 O  2H 2 + O 2

22. Decomposition Reaction In this cartoon the egg (the reactant), which contained the turtle at one time, now has opened and the turtle (product) and egg shell (product) are now two separate substances.

23. Single-Replacement Reaction A single-replacement is a reaction in which one element takes the place of another element in a compound. The general equation for a single-replacement reaction is: – A + BC  B + AC – Ex: Cu + 2AgNO 3  Cu(NO 3 ) 2 + Ag

24. Single-Replacement Reaction Notice, the guy in the orange shirt steals the date of the other guy. So, a part of one of the reactants trades places and is in a different place among the products.

25. Double-Replacement Reaction A double-replacement reaction is one in which two different compounds exchange positive ions and form two new compounds. The general form of a double-replacement reaction is: – AB + CD  AD + CB – Ex: Pb(NO 3 ) 2 + 2KI  PbI 2 + 2KNO 3

26. Double-Replacement Reaction In this cartoon, the guys exchange hats.

27. Combustion Reaction A combustion reaction is one in which a substance reacts rapidly with oxygen, often producing heat & light. Ex: the burning of natural gas – CH 4 + O 2  CO 2 + 2H 2 O + heat & light

28. Reactions as Electron Transfers The discovery of subatomic particles enabled scientists to classify certain chemical reactions as transfers of electrons between atoms Oxidation-Reduction Reaction (redox reaction): A reaction in which electrons are transferred from one reactant to another

29. Oxidation Oxidation: Any process in which an element loses electrons during a chemical reaction – A reactant is oxidized if it loses electrons Example: 2Ca + O 2  2CaO – Ca  Ca 2+ + 2e -

30. Reduction Reduction: The process in which an element gains electrons during a chemical reaction – A reactant is said to be reduced if it gains electrons Example: 2Ca + O 2  2CaO – O + 2e -  O 2-

31. Oxidation-Reduction Reaction Oxidation and reduction always occur together! When one element loses electrons, another element must gain electrons Helpful way to remember: OIL RIG – Oxidation Is Loss & Reduction Is Gain

32. 7.3 – Energy Changes in Reactions

33. Chemical Bonds & Energy Some reactions yield more than just the products (elements). For example, when propane burns in the presence of oxygen, it creates carbon dioxide (CO 2 ) and water (H 2 O) – but what else??? – HEAT AND LIGHT! – The heat that is released in the reaction comes from the reactants.

34. Chemical Energy Chemical Energy: the energy stored in the chemical bonds of a substance.

35. Chemical Energy Each of the bonds in propane has stored energy. Energy changes in chemical reactions are determined by changes that occur in chemical bonding. In other words… – Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.

36. Chemical Energy Breaking chemical bonds REQUIRES ENERGY. – That’s why a grill requires an igniter to produce a spark -- to give enough energy for the bonds in reactants to break, starting the reaction. Forming bonds RELEASES ENERGY. – The heat and light that are given off by propane stoves are the result of the formation of new chemical bonds. – The bonds are formed as carbon, hydrogen, and oxygen atoms rearrange.

37. Energy Exchange Remember how physical changes deal with an exchange of energy? – Example of endothermic phase change: melting. – Example of exothermic phase change: freezing.

38. Endothermic Exothermic

39. Energy Exchange in Chemical Reactions Energy also flows into and out of chemical changes. During a chemical reaction energy is either released or absorbed. – Endothermic Reaction (feels cool) – Exothermic Reaction (feels warm)

40. Exothermic Reaction Exothermic reaction: chemical reaction that releases energy to its surrounding – Feels WARM – Energy released, as the products form is greater than the energy required to break the bonds in the reactants. – Example: combustion reactions (burning)

42. Endothermic Reaction Endothermic Reaction: a chemical reaction that absorbs energy from its surroundings – Feels COLD – More energy is required to break the bonds in the reactants than is released by the formation of the products – In other words, the energy of the products is greater than the energy of the reactants. – Example: decomposition reactions (breaking down)

43. Decomposition Reaction

45. No matter the reaction… energy is conserved! In both types of reactions, the amount of energy from the reactants side must equal the amount of energy from the products side. – Energy cannot be created nor destroyed! – You cannot end up with more energy than was put into the reaction, and vice versa. This is called the Law of Conservation of Energy.

46. 7.4 Reaction Rates

47. Reactions over Time Progress of a chemical reaction is measured over time and expressed as a rate. Reaction Rate: rate at which reactants change into products over time Reaction rates tell you how fast a reaction is going

48. Factors Affecting Reaction Rates Factors that affect reaction rates include: – Temperature – Surface Area – Concentration – Stirring – Catalysts.

49. Temperature An increase in temperature will increase the reaction rate A decrease in temperature will decrease the reaction rate – Cooking on stove top: turn up heat faster the food cooks – Store milk in refrigerator to slow down reaction that causes milk to spoil – Reason: Increased temperature = Increased particle motion = Higher number of particle collisions = Increased reaction rate

50. Surface Area The smaller the particle size of a given mass, the larger is its surface area Example: Using newspaper to cover the floor of a room – Keeping all the sections folded together versus separating all the pages. If you separate the newspaper into open pages you can cover a much larger are with the same mass of paper – Increase in surface area increases the exposure of reactants to one another – Reason: Increased surface area = Greater exposure = More collisions = More particles reacting = Increased reaction rate

51. LESS SURFACE AREA Surface Area GREATER SURFACE AREA LESS SURFACE AREA

52. Stirring By stirring you increase the exposure of reactants to each other Example: Washing your clothes in the washer – Leave clothes to just soak in tub of water and detergent reaction proceeds more slowly – Placing clothes and detergent in washer – reaction sped up by stirring the contents back and forth – Reason: Stirring =Collisions more likely to occur =Increase in reaction rate

53. Concentration Concentration: number of particles in a given volume – More reacting particles = Greater chance for particles to collide = Faster reaction rate

54. Pt Catalyst Catalyst: a substance that affects the reaction rate without being used up in the reaction – Use a catalyst to: Speed up a RXN or Enable a RXN to occur at a lower temperature Lower the amount of energy required for a reaction to occur – Catalyst is neither a reactant or a product, so it is written over the arrow of the chemical equation (it is NOT used up) Platinum is a catalyst for breaking hydrogen peroxide into water and oxygen Example: 2H 2 O 2 2H 2 O + O 2

55. Catalyst