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Chemical Reactions Chapter 10. Part I: Counting Atoms How Many Atoms in a Molecule?

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Presentation on theme: "Chemical Reactions Chapter 10. Part I: Counting Atoms How Many Atoms in a Molecule?"— Presentation transcript:

1 Chemical Reactions Chapter 10

2 Part I: Counting Atoms How Many Atoms in a Molecule?

3 Counting Atoms Most substances that we encounter are compounds, not elements. A chemical compound is a pure substance formed from the combination of two or more different elements.   The properties of the compound may be completely unlike those of the elements that form it. The formula for a compound lists the symbols of the individual elements followed by subscripts which indicate the number of atoms of that element.   (If no subscript is given, it is understood to be “1.”) E.g., NaCl, H 2 O, C 12 H 22 O 11.

4 Counting Atoms A molecular formula gives the actual number of atoms of each element in a molecule of a compound.   Hydrogen peroxide H 2 O 2   Water H 2 O   Glucose C 6 H 12 O 6 A structural formula uses lines to represent covalent bonds, and shows how the atoms in a molecule are joined together:   H—O—O—H   H—O—H   O=C=O

5 Counting Atoms Example: How Many Atoms? Example: How Many Atoms? 2 H’s and 2 O’s = 4 atoms H2O2H2O2H2O2H2O2 2 C’s+ 5 H’s + 1 O’s + 1 H = 9 atoms C 2 H 5 OH 3 K’s+1 P’s + 4 O’s = 8 atoms K 3 PO 4 6 C’s + 12 H’s + 6 O’s = 24 atoms C 6 H 12 O 6

6 Counting Atoms with Polyatomic Ions - Al 2 (SO 4 ) 3 Al S O O O O S O O O O S O O O O = 17 ATOMS

7 Counting Atoms with Polyatomic Ions When counting atoms with polyatomic ions; When counting atoms with polyatomic ions;  Count number of atoms in one polyatomic ion o Ions inside the parentheses  Multiply by number of polyatomic groups in the molecule (number outside the parenthesis) Examples: Examples:  Al 2 (SO 4 ) Al’s + 3(1 S + 4 O’s) = 2 + 3(5) = 2+15 = 17 atoms  Mg(NO 3 ) 2 – 1 Mg + 2(1 N + 3 O’s) = 1 + 2(4) = 1+8 = 9 atoms

8 Hydrates Hydrates are ionic compounds which also contain a specific number of water molecules associated with each formula unit. The water molecules are called waters of hydration.   The formula for the ionic compound is followed by a raised dot and #H 2 O   Example: MgSO 47H 2 O. They are named as ionic compounds, followed by a counting prefix and the word “hydrate”   CuSO 45H 2 O copper(II) sulfate pentahydrate   BaCl 26H 2 O barium chloride hexahydrate   MgSO 47H 2 O magnesium sulfate heptahydrate (Epsom salts)

9 How Many Atoms in a Hydrate? When counting atoms in the hydrate, count the water atoms also. When counting atoms in the hydrate, count the water atoms also. Example: Example:   CuSO 45H 2 O o o 1 Cu + 1 S + 4 O’s + 5(2 H’s + 1 O’s) o o = (3) o o = 6+15 = 21 atoms   BaCl 26H 2 O o 1 Ba + 2 Cl + 6(2 H’s + 1 O’s) o = 1+2+6(3) o = = 21 Atoms

10 Part II: Conservation of Mass

11 Conservation of Mass In a normal chemical reaction, the mass of substances in a closed system will remain constant, no matter what processes are acting inside the system. In a normal chemical reaction, the mass of substances in a closed system will remain constant, no matter what processes are acting inside the system.  How ever many atoms a reaction starts with, ends with the same number.  Atoms don’t change their identity in a chemical reaction  Number of atoms for EACH ELEMENT STAYS THE SAME in a chemical reaction  The elements just rearrange their organization  The beginning MASS of the reaction EQUALS the ending MASS of the reaction

12 Conservation of Mass Total Mass stays the same in a chemical reaction Total Mass stays the same in a chemical reaction 2g H g O 2 yields 18g H 2 O Number and Identity of Atoms stays the same in a chemical reaction Number and Identity of Atoms stays the same in a chemical reaction  2  2 H O 2 yields 2 H 2 O H H H H O OH H O H H O

13 Part III: Writing Reactions How Do You Write a Chemical Reaction?

14 III. Chemical Reactions Definition – process by which the atoms of one or more substances are rearranged Definition – process by which the atoms of one or more substances are rearranged  KEY: new substances are formed  KEY: No Atoms are Gained or Lost A chemical reaction is the process by which atoms of one or more substances are rearranged into new substances A chemical reaction is the process by which atoms of one or more substances are rearranged into new substances  Chemical change occurs  How do you know?

15 III. Evidences of a Chemical Reaction 1) gas production 2) light production 3) temperature change (endo/exothermic) 4) precipitate formed (solid from 2 liquids) 5) permanent color change

16 Energy is stored in compounds as chemical potential energy Energy is stored in compounds as chemical potential energy  due to specific arrangements of atoms. A chemical reaction changes the potential energy present. A chemical reaction changes the potential energy present. III. Energy Changes

17 When energy is gained; heat is added for a reaction to occur. These are called ______________________,When energy is gained; heat is added for a reaction to occur. These are called ______________________, Energy Changes exothermic reaction endothermic reactions These reactions get hotter. When energy is lost as heat, it is called an __________________.When energy is lost as heat, it is called an __________________. These reactions get colder. Energy in a reaction is shown with: ΔH (heat) kJ Joules Heat energy

18 III. Chemical Reactions Representing Chemical Reactions: Representing Chemical Reactions:  Reactants – the ‘stuff’ you start with  An ‘arrow’ which means ‘yields’, or ‘becomes’  Products – the ‘stuff’ you end up with Principle of “Conservation of Mass” applies to chemical reactions. Principle of “Conservation of Mass” applies to chemical reactions.  Why?

19 III. Chemical Reactions Word Equations: Word Equations:  Reactant-A + Reactant-B yields Product-AB  Example: o Sodium(s) + Chlorine(g) → Sodium Chloride(s) o The small letters in paretheses () indicate the state of the reactant or product (solid, liquid, gas, or aqueous solution)  (s) = solid  (l) = liquid  (g) = gas  (aq) = aqueous = dissolved in water

20 Part IV: Balancing Equations Applying Conservation of Mass to Equations

21 VI. Chemical Equations Step 1: Write a Skeleton Equation Step 1: Write a Skeleton Equation Skeleton Equation uses chemical formulas and symbols instead of words: Skeleton Equation uses chemical formulas and symbols instead of words:  Words: Sodium + Chlorine gas yields Sodium Chloride  Symbols: Na(s) + Cl 2 (g) → NaCl  Skeleton Equations are not complete equations, but are the first step in writing a complete equation

22 IV. Chemical Equations Chemical Equation is BALANCED Chemical Equation is BALANCED  Balanced means that “conservation of mass” is upheld  All atoms in reactants are also in products o No more, no less o Just rearranged

23 IV. Chemical Equations Balancing Equations Balancing Equations  Use a number before the compound/element symbol to indicate how many of them are needed o Called a COEFFICIENT o Written in front of the atom/compound o KEY: Coefficient is a MULTIPLIER  Number of atoms per molecule is SUBSCRIPT  Change ONLY the COEFFICIENTS to balance the equation

24 IV. Chemical Reactions Steps to Balance Equations Steps to Balance Equations 1. Write the skeleton equation 2. Count the atoms of EACH element in the reactants 3. Count the atoms of EACH element in the products 4. Change the coefficients to make the number of atoms of each element equal on both sides of the equation 5. Write the coefficients in the lowest possible ratio 6. Check your work NEVER CHANGE A SUBSCRIPT NEVER CHANGE A SUBSCRIPT

25 IV. Chemical Equations 1.Write the skeleton equation: Al + O 2 → Al 2 O 3 2. Count Number of atoms for each element on both sides This is not balanced because the numbers don’t match 3. Multiply coefficients until they match – multiply the entire units 2 Al + O 2 → Al 2 O 3 Go to 6 Oxygens

26 IV. Balancing Equations Al + O 2 2 Al 2 O Al + 2 Al 2 O 3 O2O2 Multiply each atom by 2

27 IV. Balancing Equations 2 The work of balancing a chemical equation is in many ways a series of trials and errors. Consider the equation given below. Does this represent a balanced chemical equation? N 2 + H 2  NH 3

28 IV. Balancing Equations 3 To balance this reaction, it is best to choose one kind of atom to balance initially. Let's choose nitrogen in this case. 2 Nitrogen Atoms in Reactants requires 2 Ammonia molecules in Product to balance the nitrogen 2NH 3 H2H2 +N2N2

29 IV. Balancing Equations 2 Once we know what the molecules are (N2, H2, and NH3 in this case) we cannot change them (only how many of them there are). The nitrogen atoms are now balanced, but there are 6 atoms of hydrogen on the product side only 2 of them on the reactant side. The next step requires multiplying the number of reactant hydrogen molecules by three to give: N2N2 2NH 3 H2H2 + 3H 2

30 IV. Don’t Forget: Diatomic Elements Definition – 7 elements that NEVER occur as singular atoms (always paired with an the same or different element) Definition – 7 elements that NEVER occur as singular atoms (always paired with an the same or different element) H2 O2 F2 Br2 I2 N2 Cl2 Ex: 2 HCl + 2K  2 KCl + H 2

31 IV. Balancing Equations 3 1. Start with an unbalanced equation 2. Draw boxes around the compounds so you don’t mess with the groups Don’t be threatened by how complex it looks!

32 IV. Balancing Equations 2 3. Make an element inventory – count number of atoms for each element on each side of the equation

33 IV. Balancing Equations 3 4. Write coefficients in front of each of the boxes until the inventory for each element is the same both before and after the reaction Save Oxygen and Hydrogen for last, Treat Polyatomic like an atom. Let’s start with Sodium N Y Y N We have 2 in products, so I need 2 in reactants Multiply reactant with sodium by 2 and recount atoms H SO 4 O Na ProductReactantElement Balanced? 1 2 Y 3 4 N

34 IV. Balancing Equations 3 Inventory Shows: Reactant side has FOUR hydrogen atoms Product side has TWO hydrogen atoms Using your amazing powers of mathematics two hydrogen multiplied two becomes four hydrogen N Y N Y H SO 4 O Na ProductReactantElement Balanced? 1 2 Y 2 4 Y

35 Helpful Hints Balance hydrogen and oxygen last Balance hydrogen and oxygen last Balance polyatomic ions as a group if present on both reactants and products Balance polyatomic ions as a group if present on both reactants and products  You can consider a polyatomic ion as a single element If the balancing starts to get very complex: If the balancing starts to get very complex:  Stop  Start over  Select a different atom to balance first.

36 Example Using PolyAtomics Before Before MgCl 2 + NaOH  Mg(OH) 2 + NaCl 1 Mg 2 Cl1 Cl 1 Na 1 OH2 OH MgCl NaOH  Mg(OH) NaCl 1 Mg 2 Cl 2 Na 2 (OH) After After

37 Types of Chemical Reactions Part V

38 Classifying Chemical Reactions Synthesis Synthesis Decomposition Decomposition Single replacement Single replacement Double Replacement Double Replacement Combustion Combustion

39 Synthesis Definition – two or more substances react to form ONE product Definition – two or more substances react to form ONE product  Product is usually bigger or more complex than either reactant A + B  AB

40 Hey baby let’s get jiggy.

41 Synthesis reaction of two elements reaction of two elements ___ Al + ___ Cl 2  ___ AlCl 3 Al 3+ Cl

42 reaction of two compounds reaction of two compounds reaction of element and compound reaction of element and compound Synthesis CaO + H 2 O  Ca(OH) 2 SO 2 + O 2  SO 322

43 Decomposition definition – one substance breaks down into two or more simpler products definition – one substance breaks down into two or more simpler products AB  A + B

44 Break yoself fool!

45 Decomposition Example reaction: Example reaction: __ NaN 3 (s)  ___ Na (s)+ ___ N 2 (g) 223 __ CaO (s)  ___ Ca (s)+ ___ O 2 (g) 221

46 Single Replacement Reactions Definition – one element replaces another element in a compound to form new compound Definition – one element replaces another element in a compound to form new compound A + BX  AX + B

47 I’m gon’ dance with yo’ lady

48 Double Replacement Defn – exchange of cations between two ionic compounds Defn – exchange of cations between two ionic compounds A B + C D  AD + CB switch

49

50 3 possible products of double replacement reactions Precipitate Precipitate Gas Gas Water Water

51 Reactivity Series (or Activity Series) More active will replace less active More active will replace less active  Less active will NOT replace more active metals metals Li K Ca Na Mg Al Mn Zn Fe Ni Sn Pb Cu Ag Au most active least active F Cl Br I most activeleast active halogens halogens

52 examples aluminum + iron (III) oxide aluminum + iron (III) oxide Al Fe 2 O 3 + Stronger? Al 3+ O 2- Al 2 O 3 Fe + Fe 3 + O

53 examples silver + copper (I) nitrate silver + copper (I) nitrate Ag CuNO 3 + Stronger? Cu 1 + NO 3 1- NO RXN

54 examples fluorine gas + sodium bromide fluorine gas + sodium bromide F2F2 NaBr + Stronger? Br 2 NaF+ Na 1 + F

55 examples chlorine gas + hydrogen fluoride chlorine gas + hydrogen fluoride Cl 2 HF + Stronger? NO RXN

56 Example Problem lithium iodide and aqueous silver nitrate react lithium iodide and aqueous silver nitrate react Li 1+ I 1- Ag 1+ NO 3 1- Li I AgNO 3 LiNO 3 Ag I (s) + +

57 Combustion definition – compound reacts with O 2 definition – compound reacts with O 2 Hydrocarbon – compound w/ only carbon and hydrogen Hydrocarbon – compound w/ only carbon and hydrogen

58 Combustion Combustion of hydrocarbons ALWAYS produces CO2 and H2O C x H y + O 2 CO 2 + H 2 O

59 Ex problem show combustion of propane (C 3 H 8 ) gas show combustion of propane (C 3 H 8 ) gas C3H8C3H8 O2O2 CO 2 H2OH2O I sell propane and propane accessories!


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