Chemical Equations and Reactions

Transcription

1 Chemical Equations and Reactions 8.5D recognize that chemical formulas are used to identify substances and determine the number of atoms of each element in chemical formulas containing substances; 8.5F recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass

2 Pages you need for this lesson:

3 What are Chemical Formulas Again? A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of the substance For example: (choose two to write down) H 2 O shows that a water molecule has 2 hydrogen and 1 oxygen atom CO 2 carbon dioxide 1 carbon and 2 oxygen atoms C 2 H 6 ethane (natural gas) 2 carbon and 6 hydrogen atoms

4 What they don t show: If we look at NH 3, it does show how many atoms and elements and the ratio that make up that molecule. What a formula doesn t show is the structure or shape of the substance You had a bit of practice with this idea during the stations lab

5 So why do we use the formula? There are many reasons, but the best reason is to demonstrate on paper, chemical reactions! Chemical reactions are shown with chemical equations. Learning how to read one is very important in chemistry.

6 Chemical Equations A chemical equation is a symbolic representation of a chemical reaction Elemental Symbol Number of them

7 Reading Chemical Equations (3 slides) Each side of an equation represents a combination of chemicals The combination is written as a set of chemical formulas, separated by + symbols. CH O 2 CO H 2 O

8 Reading Chemical Equations The two sides of the equation are separated by an arrow On the left side is the combination of chemicals before the reaction The right side indicates the combination of chemicals after the reaction

9 Language of Chemical Equations Yields 4Na + O 2 2Na 2 O Reactants Products Reactants Arrow (yields) Products In this reaction, sodium (Na) and oxygen (O 2 ) react to produce a single molecule, Na 2 O (this together is known as sodium oxide

10 Conservation of Mass and Balancing Equations 8.5F recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass

11 Why balance equations? In chemistry, a balanced equation accounts for every atom involved in a chemical change. Atoms have mass and the act of balancing the numbers of atoms of each element is an expression of a certain scientific law

12 Law of Conservation This law states that during a chemical reaction, matter cannot be created nor destroyed. The balancing of a chemical reaction show the atoms that are rearranged in space, making new substances that did not previously exist until these particular atoms interacted. Balancing equations just manipulates the coefficients in front of the formulas, until the numbers of atoms on both sides are equal.

13 Conservation of Mass and/or Matter What this means: 1. Matter is conserved type of atoms does not change (in the entire equation) Nothing is created or destroyed 2. Mass is conserved amount of atoms cannot change (in the entire equation) Nothing is created or destroyed

14 Law of Conservation in Action Here is an equation: 2 KNO 3 + H 2 CO 3 K 2 CO HNO 3 Just FYI this is potassium nitrate combined with Carbonic acid to make potassium carbonate and nitric acid Matter is conserved from the left to the right side how do I know? Look at the elements! Are they different elements or the same ones?

15 Law of Conservation in Action Again: 2 KNO 3 + H 2 CO 3 K 2 CO HNO 3 Mass is conserved from the left to the right side how do I know? Count the NUMBER of elements! Are they different amounts or the same?

16 So IN A CHEMICAL REACTION: Elements can combine to form new substances Substances can be broken down into simpler substances (down to the elemental level) Atoms are re-arranged within the equation; NOT created or destroyed

17 Balanced equations examples: 6 CO H 2 O C 6 H 12 O O 2 (photosynthesis) SnO H 2 Sn + 2 H 2 O (tin oxide) 8 CO + 17 H 2 C 8 H H 2 O (octane) How do we know they are balanced? Count the number of atoms for each element, on BOTH sides of the equation!

18 Are the following balanced? Let s look at the number of atoms on each side of the reaction and then decide if the equation is balanced.

19 Now, in your notes, there is a space to write examples and to explain why the examples are balanced or not. BALANCED or NOT BALANCED? Cu + O2 2Cu + O S + O2 Mg + O2 2Mg + O2 CuO 2CuO SO2 MgO 2MgO Not Balanced Not Balanced Balanced Not Balanced Balanced

20 Homework due next class:

21 CHEMICAL REACTIONS 8.5E Investigate how evidence of a chemical reaction indicate that a new substance with different properties has formed

22 Properties and Changes in Matter Matter can undergo both physical change and chemical change.

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24 Chemical Change vs. Reaction Let s make it clear In a chemical reaction - A chemical change produces one or more new substances. There is a change in the composition of one or more substances. The (chemical) reaction is what causes the (chemical) change!!

25 WHAT ARE CHEMICAL CHANGES? A change that produces new substances Properties of the original substances change

26 Be sure you have this page too:

27 Signs (Evidence) that a Chemical Change has occurred: a. Gas is formed/released usually seen as bubbles b. Change in odor c. Change in temperature rising or falling 1. A chemical reaction that gives off heat is called exothermic (the temperature goes up) 2. A reaction that needs heat is called endothermic the temperature goes down (during the reaction at least)

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29 In graph form:

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31 In graph form:

32 More Signs (Evidence) that a Chemical Change has occurred: d. A precipitate forms a solid forms from mixing 2 or more liquids together Ex. SOLID is formed- lead nitrate + potassium iodide = lead iodide (a precipitate)

33 More Signs (Evidence) that a Chemical Change has occurred: e. Light is produced (ex. flame, bioluminescence, glow sticks, explosions) f. Change in color (unexpected, ex. rust) g. Sound (firework boom, crackling sounds)

34 Common Misconception Alert! Temperature change due to adding or removing heat does not represent a chemical change. For example, melting or freezing is a change of state which is a physical change. Is this still chocolate?

35 State of Matter Change Label on your page: Sublimation: the transition of a substance directly from the solid to the gas phase without passing through the liquid phase.

36 The difference between physical and chemical changes And that chemical reactions are a change in energy and that there is evidence of them occurring But what TRIGGERS chemical reactions? Now you know

37 Chemical Reactions A few examples:

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40 Digestion

41 Elephant s Toothpaste Gas is Formed

42 Gas Production Flammable Water Water = H2O. The batteries split the H2 and the O. H2 = hydrogen.

43 Steve Spangler Color Change Change in Color

44 Rotten Egg Lab Change in Odor

45 Precipitation Reaction A Precipitate is Formed

46 Surprize Fire Light is Produced