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Chemical Reactions And Equations. Chemical Change - objectives 1. List the common indicators of a chemical change and use them to identify when a chemical.

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Presentation on theme: "Chemical Reactions And Equations. Chemical Change - objectives 1. List the common indicators of a chemical change and use them to identify when a chemical."— Presentation transcript:

1 Chemical Reactions And Equations

2 Chemical Change - objectives 1. List the common indicators of a chemical change and use them to identify when a chemical reaction has probably occurred. They are: color change, change of state (gas bubbles, precipitate, etc), temperature change, pressure change in a closed container, and change in odor. Note the emphasis on change

3 Indications of a Chemical Reaction There are some easily observed changes that usually indicate a chemical reaction 1.Color change 2.Change of state (gas bubbles, precipitate, etc), 3.Temperature change 4.Pressure change in a closed container 5.Change in odor

4 1. Color change

5 2. Change of state: gas bubbles  Release of gas bubbles when two substances mixed is often evidence of chemical reaction Antacid in water

6 2. Change of state: formation of precipitate  Many reactions happen in solution  If solid appears after two solutions mixed, a reaction has probably happened  Example: Pb(NO 3 ) 2 + KI  A bright yellow solid PbI 2 appears as the proof that the reaction has happened

7 2. Change of state: formation of precipitate  The solid that appears is called precipitate  Precipitate  a solid that is made as a result of a chemical reaction in solution and that separates from the solution  PbI 2 precipitate

8 3. Temperature change Note: Heat by itself NOT necessarily sign of chemical change Reaction between vinegar and baking soda is accompanied by drop of temperature Reaction between glycerin C 3 H 5 (OH) 3 and potassium permanganate KMnO 4 produces extensive heat (burning)

9 4. Pressure change in a closed container If reaction is going in the gas phase and new gases are formed, the pressure in the closed container will change

10 5. Odor change Odor Changes It only takes one experience with a rotten egg to learn that they smell different that fresh eggs. When eggs and food spoil, they undergo a chemical change. The change in odor is a clue to the chemical change

11 Understanding Check Is it a chemical or physical change? 1.“Dry ice”, solid carbon dioxide, is sublimated into gaseous CO 2 at room temperature 2.Salt is dissolved in water 3.Iron rusts in a damp environment 4.Gasoline burns in the presence of oxygen 5.Liquid hydrogen peroxide (H 2 O 2 ) decomposes to water and oxygen 6.Rotting of eggs

12 Chemical Equations - objective 2. Given a word equation, write a chemical equation (this requires good formula writing skills)

13  Chemical reaction is a process by which one or more substances are changed into one or more different substances  Chemical equations represent chemical reaction with symbols and formulas. Chemical equations show the identities, physical phases and relative amounts of the reactants and products in a chemical reaction Reactants (state) → Products (state)  Reactants are compounds being brought together to react  Products are the new compounds that are made in the reaction Chemical Reactions and Equations

14 Physical Phase Solid (s) Liquid (l) Gas (g) Aqueous solution (aq) – solution in water Examples: Na (s); O 2 (g); HCl (l); Ca(NO 3 ) 2 (aq)

15 Word Equations  First step in writing chemical equation is to identify facts to be represented  Word equation  “equation” in which reactants and products are represented by words  Does not give quantities of reactants/products Reactants==>Products chemicals that react==>chemicals that are produced hydrogen + oxygen==>water hydrogen peroxide==>water + oxygen magnesium + oxygen==>magnesium oxide sodium chloride==>sodium + chlorine

16 Formula Equations Word equation: hydrogen (gas) + oxygen (gas)  water (liquid) Formula equation: H 2 (g) + O 2 (g)  H 2 O (l) magnesium (solid) + oxygen (gas)  magnesium oxide (solid) Mg (s) + O 2 (g)  MgO (s)

17 Characteristics of Chemical Equations 1.The equation must contain correct formulas for the reactants and products. 2.The Law of Conservation of Mass must be satisfied: mass is neither created nor destroyed in any ordinary chemical reaction. The quantity of substances produced (products) by a chemical reaction is always equal to the quantity of the reacting substances (reactants). The “quantity” here means mass or the number of atoms for each element participating in reaction

18 Balancing a Chemical Equation 1.Write a formula equation H 2 O(l)  H 2 (g) + O 2 (g) 2.Create a table showing the number of atoms for each element: This equation is not balanced! The number of atoms ReactantsProducts H22 O12

19 Balancing Equations 3.Correct the number of O by placing coefficient 2 in front of H2O 2H 2 O(l)  H 2 (g) + O 2 (g) 4.Correct the table: Oxygen is now balance with 2 on left and 2 on right. H is not! 5.Now balance hydrogen – 4 on left, 2 on right. Add coefficient 2 to H 2 2H 2 O(l)  2H 2 (g) + O 2 (g) balanced! The number of atoms ReactantsProducts H42 O22 The number of atoms ReactantsProducts H44 O22

20 Guidelines for Balancing Equations  Create a table of elements for reactant and product side of the equation  Balance the different types of atoms one at a time.  First balance the atoms of elements that are combined and that appear only once on each side of the equation.  Balance polyatomic ions that appear on both sides as single units.  Balance lone atoms, especially H and O atoms (or any), last.

21 DO NOT!!!..... DO NOT WRITE INCORRECT FORMULAS, THIS WILL MESS UP YOUR BALANCING DO NOT CHANGE SUBSCRIPTS IN FORMULAS TO BALANCE THE EQUATION! H 2 O(l)  H 2 (g) + O 2 (g) H 2 O(l)  H 2 (g) + O(g)

22 DO!..... When you think you have balanced the equation, COUNT THE NUMBERS OF EACH TYPE OF ATOM ON EITHER SIDE OF THE EQUATION 2H 2 O(l)  2H 2 (g) + O 2 (g) Reactant sideProduct side H44 O22

23 Sample Problem 1 Write balanced chemical equations for the following reaction: Solid sodium combines with chlorine gas to produce solid sodium chloride. Word reaction: Sodium (solid) + chlorine (gas)  sodium chloride (s) Formula reaction: Na(s) + Cl 2 (g) → NaCl(s) Balanced reaction 2Na(s) + Cl 2 (g) → 2NaCl(s)

24 Balance Reaction Na(s) + Cl 2 (g) → NaCl(s) 1.Count atoms: 1.Na (in compound): 1 on the left, 1 on the right - balanced 2.Cl (lone element): 2 on the left, 1 on the right – not balanced! 2.Adjust Cl: Na(s) + Cl 2 (g) → 2 NaCl(s) Count atoms: 1.Na: 1 on the left, 2 on the right – not balanced! 2.Cl: 2 on the left, 2 on the right – balanced 3.Adjust Na: 2 Na(s) + Cl 2 (g) → 2 NaCl(s) Count atoms – all balanced! 1.Na: 2 on the left, 2 on the right 2.Cl: 2 on the left, 2 on the right

25 Sample Problem 2 Write word, formula, and balanced chemical equations for magnesium and hydrochloric acid (HCl) react to produce magnesium chloride and hydrogen.  Word equation: Magnesium (solid) + hydrochloric acid (liquid)  magnesium chloride (solid) + hydrogen (gas)  Formula equation: Mg (s) + HCl (l)  MgCl 2 (s) + H 2 (g)  Count atoms: 1.Mg (in compound): 1 on the left, 1 on the right 2.Cl (in compound): 1 on the left, 2 on the right 3.H (lone element): 1 on the left, 2 on the right  Adjust HCl: Mg + 2HCl  MgCl 2 + H 2  Recount the atoms – all balanced!

26  Word equation: Methane (gas) + oxygen (gas)  carbon dioxide (gas) + water (gas)  Formula equation: CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(g) Sample Problem 3 ReactantsProductsBalanced? 1. C (in compounds in both sides)11Yes 2. H (in compound)42No 3. O (in compound and a lone element) 23No

27 Adjust H: CH 4 (g) + O 2 (g)  CO 2 (g) + 2H 2 O(g) Adjust O: CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) Sample Problem 3 (cont.) ReactantsProductsBalanced? 1. C (in compounds in both sides)11Yes 2. H (in compound)44Yes 3. O (in compound and a lone element) 44Yes ReactantsProductsBalanced? 1. C (in compounds in both sides)11Yes 2. H (in compound)44Yes 3. O (in compound and a lone element) 24No

28 Practice Problems Fe + Cl 2 = FeCl 3 2Fe + 3Cl 2 = 2FeCl 3 Zn + HCl  ZnCl 2 + H 2 Zn + 2HCl  ZnCl 2 + H 2 Al + O 2  Al 2 O 3 4Al + 3O 2  2Al 2 O 3


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