Bonding and Chemical Reactions

Transcription

1 Bonding and Chemical Reactions

2 Part I Bonding

3 I. What is a Compound? A. Classification of Matter Review Elements = the simplest form of a pure substance o by Cannot be broken down physical or chemical means Examples: Sodium, Chlorine, anything on the PT Compounds = 2 or more different elements chemically bonded in a fixed proportion o or Can be broken down separated by chemical means only Examples: H 2 O, CO 2, CH 4

4 B. Compound Formulas Subscripts = tell you the # of each type of atom present in the molecule Example: CO 2 # carbon atoms # oxygen atoms o If there are parentheses around two or more atoms, the subscript applies to all atoms within the parentheses Example: Al(CN) 3 # aluminum atoms # carbon atoms # nitrogen atoms o If one of the atoms within the parentheses has a subscript, you multiply this number by the number outside of the parentheses Example: Fe 2 (SO 4 ) # iron atoms # sulfur atoms 1 # oxygen atoms

5 Coefficients = the # in front that tells you how many total molecules you have Example: 3 BaCl 2 means that I have 3 molecules of BaCl 2. Therefore it contains: 3 6 # barium atoms (Ba) # chlorine atoms (Cl) Hydrates = a molecule with water surrounding it Example: CuSO 4 5 H 2 O o Note the dot in the hydrate formula does NOT mean multiplication! It means weakly bonded # Cu atoms # S atoms # O atoms # H atoms

6 Checks for Understanding Count the number of each atom in the compounds below a. 3 H 2 O 6 3 # H # O b. Na 2 (PO 4 ) # Na # P # O c. 2 Ca(NO 3 ) 2 # Ca 2 # N 4 # O 12 d. CaCO 3 # Ca # C # O 1 1 3

7 II. Gram-Formula Mass Gram-Formula Mass (GFM) = Mass of a compound To find the GFM, you add up the atomic masses of all of the elements in the compound o To find the atomic mass of an element, go to Periodic table Examples: a. H 2 O b. K 2 CO 3 2 (1g) + 1 (16g) = 18 g 2 (39 g) + 1 (12 g) + 3(16 g) = 138 g To find the GFM of a hydrate, follow the same steps, being sure to include the total mass of the water molecules Examples: a. CuSO 4 5 H 2 O b. BaCl 2 2 H 2 O 1 (64g) + 1 (32g) + 4 (16g) + 10 (1g) + 5 (16g) = 250 g 1 (137g) + 2 (35g) + 4 (1g) + 2 (16g) = 243 g

8 III. Percent Composition Think about this How would you find the percentage of boys in the classroom? # of boys (part) Total # of students (whole) Calculating the percent composition of elements in a compound is the same idea except it looks at percentage in terms of mass Percent Composition (by mass) = the percent of an element in a compound, in terms of mass o Equation: Table T x 100% % Composition = mass of part x 100 by mass mass of whole

9 Examples: 1. A g sample contains 3.83 g of iron and g of bromine. What is the percent composition of bromine? mass of part x 100 = g Br x 100 = % mass of whole g Sample 2. Find the percent by mass of nitrogen in NH 4 NO 3. Step 1: Calculate the part = add up the total masses of the N atoms 2 N atoms (14 g) = 28 g of N Step 2: Calculate the whole = calculate GFM 2 (14 g) + 4 (1 g) + 3 (16 g) = 80 g Step 3: Plug into equation % Composition = mass of part * 100 = 28 g N x 100 = 35 % by mass mass of whole 80 g

10 To find the percent by mass of water in a hydrate, follow the same steps, using the total mass of the water molecules as your part Examples: 1. Find the percent by mass of water in BaCl 2 2H 2 O Step 1: Calculate the part = add up the total mass of the water molecules 4 (1g) + 2 (16g) = 36 g of H 2 O Step 2: Calculate the whole = calculate GFM of hydrate 1 (137g) + 2 (35g) + 4 (1g) + 2 (16g) = 243 g of hydrate Step 3: Plug into equation mass of part * 100 = 36 g H 2 O x 100 = 14.8% mass of whole 243 g

11 g of BaCl 2 (s) hydrate was heated in a crucible. After heating for 10 minutes, 4.64g of anhydrous BaCl 2 remains. What is the % by mass of water in the original sample of hydrate? *Note anhydrous/anhydrate means the substance without water Step 1: Calculate the part = mass of the water (mass of hydrate mass of anhydrate) 5.40g 4.64g = 0.76g of H 2 O Step 2: Find the whole = the mass of the hydrate Step 3: Plug into equation 5.40g % Composition = mass of part * 100 = 0.76 g H 2 O x 100 = 14.1% by mass mass of whole 5.40 g

12 Checks for Understanding 1. Which species contains the greatest percent by mass of hydrogen? a. OH b. H 2 O c. H 3 O - d. H 2 O 2 A) 1/17 = 6 % B) 2/18 = 11 % C) 3/19 = 16 % D) 2/34 = 6 % g of CuSO 4 (s) hydrate was heated in a crucible. After heating for 10 minutes, 5.38 g of anhydrous CuSO 4 remains. What is the percent composition by mass of water in the hydrate? 36% H 2 O

13 IV. Introduction to Bonding Recall: A compound is defined as 2 or more different elements chemically bonded together in a fixed proportion. So what is a chemical bond and how and why do bonds form? Chemical bond = a force that holds elements together to form compounds o Force due the attraction between positive nuclei and negative electrons Bonds form due to the sharing or transfer of valence electrons between atoms in order to achieve a full outermost shell and become stable o Recall: only lone, unpaired valence electrons are involved in bonding Example: *Carbon has 4 unpaired valence electrons so it can form 4 bonds

14 Checks for Understanding Draw the Lewis Dot Structures for the following elements and identify how many bonds they could form N O Ca Cl Ar

15 V. Types of Bonds There are 3 main types of bonds 1. Metallic 2. Ionic 3. Covalent The type of bond formed between atoms depends on two things: o The type of atoms involved (metals, nonmetals) o The behavior of electrons

16 1. Metallic Bonds Type of atoms involved: two or more metals (usually two of the same metal i.e. ) Cu-Cu How the bond forms: metals share/make a sea of mobile valence electrons o The force of attraction between freely moving electrons and positively charged nuclei is what keeps the atoms together Why are the electrons freely moving? Recall: metals want to lose electrons this means they don t hold onto e- as tightly and the e- are able to move from one atom to another

17 Properties: 1. Good conductors of electricity and heat in solid state 2. Malleable and ductile can be manipulated without breaking 3. High melting and boiling points (see Table S) 4. insoluble (can t dissolve) in water Checks for Understanding 1. Conductivity in a metal results from the metal atoms having a. high electronegativity b. high I.E. c. highly mobile protons in the nucleus d. highly mobile electrons in the valence shell 2. Metallic bonding occurs between atoms of a. sulfur b. copper c. fluorine d. carbon

18 2. Ionic Bonds Type of atoms involved : metal and nonmetal How the bond forms: metal to transfers electrons the nonmetal o Results in the metal becoming a positive ion and the nonmetal becoming a negative ion. The force of attraction between the oppositely charged ions is what keeps the atoms together Why does it make sense that the e- would behave this way? Metals want to lose e- to become stable; nonmetals want to gain Salts are a type of ionic compound Example: NaCl = classic table salt

19 Properties: 1. Conduct electricity in liquid and aqueous state (when dissolved in water) 2. Solid (hard) at room temperature 3. High melting and boiling points 4. Soluble (dissolve in water) *think of the properties of table salt (NaCl)

20 Ionic bonds can also involve a polyatomic ion Polyatomic ion = A covalently bonded (sharing electrons) group of atoms that have a net electric charge ( fat chunks ) Found on Reference Table E Examples: SO 4-2 OH -1 NH 4 + Sulfate Hydroxide Ammonium Checks for Understanding 1. What occurs when potassium reacts with chlorine to form potassium chloride? a. Electrons are shared and the bonding is ionic. b. Electrons are shared and the bonding is covalent. c. Electrons are transferred and the bonding is ionic. d. Electrons are transferred and the bonding is covalent.

21 2. Which element reacts with oxygen to form ionic bonds? a. Calcium c. Chlorine b. Hydrogen d. Nitrogen 3. A solid substance is an excellent conductor of electricity. The chemical bonds in this substance are most likely a. ionic, because the valence electrons are shared between atoms b. ionic, because the valence electrons are mobile c. metallic, because the valence electrons are stationary d. metallic, because the valence electrons are mobile

22 3. Covalent Bonds Types of atoms involved: two or more nonmetals How the bond forms: nonmetals in share electrons order to achieve a stable arrangement of electrons o The sharing of electrons creates a bond between the atoms 2 electrons shared = one bond o Results in the formation of a molecule (also called molecular ) compounds Properties: 1. Does electricity not conduct in any phase 2. Soft 3. Low melting and boiling points 4. in Sometimes soluble water *SPLASH Soft, Poor Conductor, Low MP/BP and water is an example!

23 Checks for Understanding 1. Which formula represents a molecular substance? a. CaO b. CO c. Li 2 O d. Al 2 O 3 2. As a bond between a hydrogen atom and a sulfur atom is formed, electrons are a. shared to form an ionic bond b. shared to form a covalent bond c. transferred to form an ionic bond d. transferred to form a covalent bond 3. Magnesium nitrate Mg(NO 3 ) 2 contains chemical bonds that are a. covalent, only c. both covalent and ionic b. ionic, only d. neither covalent nor ionic

24 Electronegativity s Role in Bonding Recall: electronegativity is an atom s desire for electrons o It is on a scale from 0-4 o Metals have low electronegativity and nonmetals have high Think about this If ionic bonds exist between a metal and a nonmetal, why does it make sense, in terms of electronegativity, that metals transfer electrons to nonmetals? B/c nonmetals have a higher desire for electrons (higher electronegativity) so they pull electrons from the metals If covalent bonds exist between two nonmetals, why does it make sense, in terms of electronegativity, that the nonmetals share electrons? B/c both nonmetals would have a high desire for electrons so one wouldn t overpower the other Think of it like a tug-of-war for electrons!

25 VI. Lewis Dot Diagrams of Compounds The electron-dot diagram of a compound depends on the type of bonding involved Recall: Only single, unpaired electrons can bond! 1. Ionic Compounds Recall: Ionic bonds form as a result of the metal transferring electrons to the nonmetal. Steps for Drawing: 1. Draw the individual lewis dot diagrams for all the elements in the compound 2. Draw the lone electrons from the metal(s) transferring to pair up with the lone electrons of the nonmetal(s) (use an arrow to show the transfer) 3. Redraw the compound with brackets and appropriate new charges

26 *Hint it is just like drawing the lewis dot diagram of ions metals end up with 0 dots and a positive charge, nonmetals end up with 8 dots and a negative charge Example: NaCl

27 2. Covalent Compounds Recall: Covalent bonds form as a result of nonmetals sharing electrons Steps for Drawing: 1. Draw the individual lewis dot diagrams for all the elements in the compound. *If there are more than 2 elements, the element with the most unpaired valence electrons goes in the middle* 2. Draw lines between elements connecting all the lone electrons the goal is to match up all the unpaired electrons *one line=one bond=2 electrons Example: HBr

28 Checks for Understanding For each of the following compounds, first identify the type of bond present; then draw the appropriate lewis-dot diagram 1. CaO 2. CH 4 3. O 2 4. MgF 2

29 VII. Naming Compounds How you name a given compound depends on the type of bonding present in the compound 1. Ionic Compounds A) Chemical Formula Name If there are only 2 elements in the formula for the ionic compound, follow the steps below to name it Steps: 1. Write the name of the metal (*hint the metal is written first) 2. Write the name of the nonmetal, drop the ending, add ide. Examples: A) ZnO C) KCl Zinc Oxygen Zinc Oxide B) Ag 2 S D) Al 2 Br 3 Silver Sulfur Silver Sulfide Potassium Chlorine Potassium Chloride Aluminum Bromine Aluminum Bromide

30 If there are more than 2 elements in the formula for the ionic compound, it typically means there is a polyatomic ion involved. If that s the case, follow the steps below to name it Steps: 1. Write the name of the metal 2. Use Table E and write the name of the polyatomic ion as is Examples: A) Zn(OH) 2 C) Ga(PO 4 ) Zinc Hydroxide Gallium Phosphate B) AgCN D) Mg(ClO 3 ) 2 Silver Cyanide Magnesium Chlorate

31 B) Name Chemical Formula When writing the formula for ionic compounds, it is important to remember that compounds are electrically. neutral That means all the charges of the ions in an ionic compound must cancel out or. add up to zero To determine this, use the element s oxidation number ooxidation Number = the number of electrons lost or gained by an atom Found on Periodic Table in the top right corner

32 To ensure that the charges add up to zero, there is a trick called the criss-cross method o Criss-cross Method = a shortcut where you crisscross the oxidation numbers to determine how many atoms of each element must be in the compound to make the compound neutral. Helps determine the subscripts on each element so that the total sum of the charges = 0 Examples: A) Al +3 and S -2 B) Co 3+ and Sulfate (SO 4 ) 2 - Al 2 S 3 2(+3) + 3(-2) = 0 Co 2 (SO 4 ) 3 2(+3) + 3 (-2) = 0 This trick can be used to help with writing the formulas of ionic compounds

33 To write chemical formulas for ionic compounds, follow the steps below Steps: 1. Write the symbol of the metal with its oxidation number (remember, found in the upper righthand corner of PT) 2. Write the symbol of the nonmetal with its oxidation number (choose the 1 st one) 3. If the charges cancel each other (add up to zero) then simply combine the two symbols 4. If the two charges DO NOT cancel each other out, use the criss-cross method the charges cross down and become subscripts

34 Examples: A) Sodium Sulfide C) Aluminum Oxide Na 1+ S 2- Na 2 S Al 3+ O 2- Al 2 O 3 B) Magnesium Fluoride D) Calcium Oxide Mg 2+ F 1- MgF 2 Ca +2 O 2- CaO

35 2. Covalent (Molecular) Compounds With covalent compounds, prefixes are used to describe how many of each atom are present A) Chemical Formula Name 1. Name the first nonmetal. If there is more than one atom of it (if there is a subscript), put the correct prefix in front of it. The first element only gets a prefix if there is more than one 2. Name the second nonmetal, drop the ending, add ide. Place the correct prefix in front of it. The second element ALWAYS gets a prefix

36 Examples: A) CO B) CO 2 C) N 2 O 3 Carbon Monoxide Carbon dioxide Dinitrogen trioxide B) Name Chemical Formula 1. Write the chemical symbol of the elements 2. Use the prefixes to tell you how many there are of each element. Write the number as a subscript. *Remember, if there is no prefix on the first element, it s because there is only one and no subscript is needed. Examples: A) Diphosphorous pentoxide B) Phosphorous tetrafluoride P 2 O 5 PF 4

37 Part 2 Chemical Reactions

38 I. What is a Chemical Reaction? Review of Types of Changes Remember there are two types of changes that matter can undergo 1. Physical Change = a change that DOES NOT change the chemical nature of a substance; appearance changes but not chemical identity Examples: any phase change (boiling, melting, freezing), the act of dissolving 2. Chemical Change = a change that DOES change the chemical identity of a substance; new substances are formed with new chemical make-up Examples: burning, rusting, reacting

39 Chemical Reactions The main example of a chemical change is a chemical reaction ochemical Reaction = when one or more substances change into new substances oa chemical reaction involves substances called reactants forming new substances called products. The reactants react to produce the products. Example: Reactants yields Products 2H 2(g) + O 2(g) 2H 2 O (l) Physical state of substances How do you know if a chemical reaction happened? Remember, there are certain signs to look for: 1.Color change (an obvious one) 2. Bubbling/fizzing 3. Change in temperature 4. Formation of a precipitate (a solid falls out of a solution)

40 How do chemical reactions happen? There are certain conditions that must be met. Think of kicking a field goal. To get a field goal, 2 things must happen: 1. Proper placement 2. Enough power The same idea is true for a chemical reaction! o Collision Theory = theory that it is the that collisions between reactants allow a reaction to occur In order for a reaction to occur, the collisions must be effective. collisions They must have: 1. Proper orientation (positioning) If they don t collide in the correct way, no reaction will occur 2. Enough energy If the collisions are too slow, or the particles don t have enough force behind them, no reaction will occur

41 II. Rates of Chemical Reactions How can you speed up the rate of a chemical reaction, or in other words, what factors affect the rate of collisions? 1. Concentration (the number of particles per volume) If the concentration of the reactants, increases then the reaction will occur faster o More particles in a given space = more collisions *Hint think of adding more people to our classroom As concentration, increases number of collisions, increases and the reaction happens faster 2. Surface Area (how spread out the particles are) If the surface area of the reactants, increases then the reaction will occur faster *Hint think of crushed up alka seltzer vs. a tablet As surface area, increases the number of collisions, increases and the reaction happens faster

42 3. Temperature- If the temperature of the reactants, increases then the reaction will occur faster *Hint think of the alka seltzer in warmer water vs. colder water As temperature, increases the number of collisions increases, and the reaction happens faster 4. Catalyst If a catalyst is added to a reaction, then the reaction will occur faster o A catalyst speeds up a reaction by reducing the amount of energy it takes ; to start a reaction particles need less energy to react so they can react faster *Hint think of a catalyst as a short cut home from school When a catalyst is added, the energy it takes for a reaction to start decreases, so the reaction happens faster

43 Checks for Understanding 1. In order for a reaction to occur there needs to be effective collisions between reactant particles. What are the two things needed for a collision to be effective? a. Enough energy b. Proper orientation 2. When the amount of EFFECTIVE collisions increases, what happens to the rate of reaction? increases the reaction happens faster 3. Milk contains the sugar lactose. Over time, a chemical reaction occurs. Lactose changes to lactic acid, which makes the milk taste sour. Reaction: lactose + O 2 lactic acid Based on collision theory, why do we keep milk in the refrigerator? Because decreasing the temperature decreases the number of collisions, which makes the reaction happen slower How to Speed Up Chemical Reactions (and get a date) Video Clip

44 III. Balancing Chemical Reactions Subscripts and coefficients are also parts of a chemical reaction o Recall: subscripts tell you how many atoms of a particular element are in a molecule and coefficients tell you how many total atoms/molecules are present Subscripts and coefficients are significant because a chemical reaction must obey the Law of Conservation of Mass o Law of Conservation of Mass = matter/mass cannot be created or destroyed This means: 1. The total mass you started with must equal the total mass you end up with Example: Using the equation above, what is the mass of H 2 O produced when 320 grams of H 2 reacts completely with 40 grams of O 2? 360 grams of H 2 O

45 2.The total number of atoms of each element must be the same on both sides of the equation To accomplish this, we must make sure chemical reactions are balanced *Think of chemical reactions like recipes. The reactants are the ingredients and the products are the finished dish. When you add ¾ cups of chocolate chips to make chocolate chip cookies, there are still ¾ cups of chocolate chips in the cookies when the recipe is complete!*

46 How to balance chemical equations Coefficients are used to balance the number of atoms of each element on both sides of the reaction. o Must be a whole number o Must be as reduced as possible Important Note* Coefficients can be changed, subscripts cannot! Examples: 1) 2 HCl (aq) + 1 Zn (s) 1 H 2 (g) + 1 ZnCl 2 (aq) Reactants Elements Products H Cl Zn

47 2) 1 N H 2 2 NH 3 3) 2 Al 2 O 3 4 Al + 3 O 2 Elements

48 ) Al + Br 2 AlBr ) AlBr 3 + Cl 2 Al Cl 3 + Br 2