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Chemical Reactions. Empirical formula The simplest ratio between the atoms. The formulas for ionic compounds must be written as empirical formulas. For.

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Presentation on theme: "Chemical Reactions. Empirical formula The simplest ratio between the atoms. The formulas for ionic compounds must be written as empirical formulas. For."— Presentation transcript:

1 Chemical Reactions

2 Empirical formula The simplest ratio between the atoms. The formulas for ionic compounds must be written as empirical formulas. For molecular substances sometimes the ratio between the atoms in a molecule is not in the simplest ratio. When the molecular and empirical formula are different then the molecular formula is a multiple of the empirical formula.

3 DO NOW Which compound contains a greater % of Oxygen. Justify your answer with a calculation Li 2 O, CaO, PbO 2

4 All chemical reactions… have two parts: 1.Reactants = the substances you start with 2.Products = the substances you end up with The reactants will turn into the products. Reactants → Products

5 Reactants Products

6 How to Describe a Reaction A reaction can be described several ways: #1. In a sentence every item is a word Copper reacts with chlorine to form copper (II) chloride. #2. In a word equation some symbols used Copper + chlorine → copper (II) chloride #3. In a chemical equation only chemical equations are used Cu + Cl 2 → CuCl 2

7 Symbols in Equations (s) after the formula = solid: Fe (s) (g) after the formula = gas: CO 2(g) (l) after the formula = liquid: H 2 O (l) (aq) after the formula = dissolved in water, an aqueous solution: NaCl (aq) is a salt water solution

8 Symbols used in equations the arrow → separates the reactants from the products (arrow points to products) Read as: “reacts to form” or yields or produces The plus sign + means “and” ↑ used after a product indicates a gas has been produced: H 2 ↑ ↓ used after a product indicates a solid has been produced: PbI 2 ↓

9 Symbols used in equations ■ double arrow indicates a reversible reaction (more later) ■ shows that heat is supplied to the reaction ■ is used to indicate a catalyst is supplied (in this case, platinum is the catalyst)

10 What is a catalyst? A substance that speeds up a reaction, without being changed or used up by the reaction. Enzymes are biological or protein catalysts in your body.

11 The Skeleton Equation All chemical equations are a description of the reaction. A skeleton equation uses formulas and symbols to describe a reaction but doesn’t indicate how many; this means they are NOT balanced

12 Write a skeleton equation for: 1.Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. 2.Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

13 Write a skeleton equation for: 1.Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. FeS (s) + HCl (g) → FeCl 2(s) + H 2 S (g) 2.Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water. HNO 3(aq) + Na 2 CO 3(g) → CO 2(g) + H 2 O (l)

14 Now, read these equations: Fe (s) + O 2(g) → Fe 2 O 3(s) Cu (s) + AgNO 3(aq) → Ag (s) + Cu(NO 3 ) 2(aq) NO 2(g) N 2(g) + O 2(g)

15 ANSWERS TO WORKSHEET Crystal is a hydrate. Heating removes the water of hydration 25 Hydrated crystal is blue Anhydrous crystal is white 26 The anhydrous compound is pure. The hydrated crystal contains 50% water so it is more expensive (you are paying for water in the bottle!!!)

16 Balanced Chemical Equations According to the Law of Conservation of Mass: atoms aren’t created or destroyed in a chemical reaction, they are just rearranged. All the atoms we start with in the reactants we must end up with in the products (meaning: balanced!) A balanced equation has the same number of each element on both sides of the equation.

17 Rules for balancing: 1)Assemble the correct formulas for all the reactants and products, using “+” and “ → ” 2)Count the number of atoms of each type appearing on both sides 3)Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST! (hint: I prefer to save O until the very last) 4)Double-Check to make sure it is balanced.

18 Never change a subscript to balance an equation (You can only change coefficients) If you change the subscript (formula) you are describing a different chemical. H 2 O is a different compound than H 2 O 2 Never put a coefficient in the middle of a formula; they must go only in the front 2 NaCl is okay, but Na 2 Cl is not.

19 Practice Balancing Examples _AgNO 3 + _Cu → _Cu(NO 3 ) 2 + _Ag _Mg + _N 2 → _Mg 3 N 2 _P + _O 2 → _P 4 O 10 _Na + _H 2 O → _H 2 + _NaOH _CH 4 + _O 2 → _CO 2 + _H 2 O

20 Practice Balancing Examples 2AgNO 3 + _Cu → _Cu(NO 3 ) 2 + 2Ag 3Mg + _N 2 → _Mg 3 N 2 4P + 5O 2 → _P 4 O 10 2Na + 2H 2 O → _H 2 + 2NaOH _CH 4 + 2O 2 _CO 2 + 2H 2 O

21 Types of Reactions There are probably millions of reactions. We can’t remember them all, but luckily they will fall into several categories. We will learn: a) the 5 major types. We will be able to: b) predict the products. For some, we will be able to: c) predict whether or not they will happen at all. How? We recognize them by their reactants

22 Synthesis Reaction AB + ABA The general equation is A + B  AB also called Combination Reactions 2 substances combine to make one compound

23 #1 Synthesis Reactions also called Combination Reactions 2 substances combine to make one compound the general equation is : A + B → AB Ca + O 2 → CaO element + element SO 2 + O 2 → SO 3 compound + element CO 2 + H 2 O → H 2 CO 3 compound + compound We can predict the products, especially if the reactants are two elements. Mg + N 2 → _______ Mg 3 N 2

24 Synthesis Reaction HCl (g) + NH 3(g)  NH 4 Cl (s)

25 Complete and balance: Ca + Cl 2 → Fe + O 2 → (assume iron (II) oxide is the product) Al + O 2 → Remember that the first step is to write the correct formulas – you can still change the subscripts at this point, but not later while balancing! Then balance by changing the coefficients only

26 The general equation is : AB → A + B A reaction where a more complex molecule breaks down to form two or more simpler products Decomposition Reaction AB + ABA

27 2NH 4 NO 3(s)  4H 2 O (g) + 2N 2(g) + O 2(g) + energy Decomposition Reaction Timothy McVeigh bombing, 1995 Regular building demolition with ammonium nitrate explosives

28 #2 - Decomposition Reactions one reactant breaks apart into two or more elements or compounds. the general equation is : AB → A + B H 2 O H 2 + O 2 CaCO 3 CaO + CO 2 CuSO 4 5H 2 O CuSO 4 + 5H 2 O 2NaHCO 3(s) Na 2 CO 3(s) + H 2 O (l) + CO 2(g) Note that energy (heat, sunlight, electricity, etc.) is usually required

29 #2 - Decomposition Reactions We can predict the products if it is a binary compound (which means it is made up of only two elements) It breaks apart into the elements: H 2 O HgO mercury (II) oxide cinnabar mercury

30 Single Displacement Reaction A + BCB + BAC A A reaction where an element displaces another element in a compound, producing a new compound and an element  A metal will replace a cation (metal or H)  A non-metal will replace an anion (non-metal) A + BC  AC + B

31 Zn (s) + 2 HCl (aq) → ZnCl 2 (aq) + H 2 (g) zinc metal and hydrochloric acid react to form zinc chloride and hydrogen gas in this single- displacement reaction.

32 #3 - Single Displacement Reactions One element replaces another the reaction follows the form of: compound + element → compound + element Reactants must be an element and a compound. Products will be a different element and a different compound. Na + KCl → K + NaCl (cations switched) F 2 + LiCl → LiF + Cl 2 (anions switched)

33 #3 Single Displacement Reactions Metals will replace other metals (and they can also replace hydrogen) Zn (s) + 2 HCl (aq) → ZnCl 2 (aq) + H 2 (g) Cu (s) + 2AgNO 3 → 2Ag (s) + Cu(NO 3 ) 2 Think of water as: HOH Metals replace the first H, and then combines with the hydroxide (OH). 2Na (s) + 2H 2 O (l) → 2 NaOH (aq) + H 2(g)

34 #3 Single Displacement Reactions We can even tell whether or not a single displacement reaction will happen: More ‘active’ element replaces less active The Activity Series of Metals lists metals (and hydrogen) in order of activity. Elements higher on the list replaces those lower on the list.

35 Single Displacement Reaction Hg(NO 3 ) 2(aq) + 2Ag (s)  Hg (s) + 2AgNO 3(aq) Will this reaction occur? Both of these reactions do not occur. H 2(g) + ZnCl 2(aq)  2HCl (aq) + Zn (s) What about this reaction?

36 The “Activity Series” of Halogens Fluorine Chlorine Bromine Iodine Halogens can replace other halogens in compounds, provided they are above the halogen they are trying to replace in the periodic table. 2NaCl (s) + F 2(g)  2NaF (s) + Cl 2(g) MgCl 2(s) + Br 2(g)  ??? No Reaction! ??? Higher Activity Lower Activity

37 #3 Single Replacement Reactions Practice: Fe + CuSO 4 → Pb + KCl → Al + HCl →

38 Double Displacement Reaction AD + BC  AC + BD + BCB + AC A ADA BDB Two compounds switch parts to make two new compounds the general equation is : AB + CD → AD + CB

39 Double Displacement Reaction sodium chloride and silver fluoride react to form sodium fluoride and silver chloride in this double displacement reaction

40 #4 - Double Replacement Reactions Two compounds switch parts to make two new compounds the reaction is: compound + compound → compound + compound NaOH + FeCl 3 → The positive ions change place. NaOH + FeCl 3 →→  Fe +3 OH - + Na +1 Cl - 1 = NaOH + FeCl 3 →→  Fe(OH) 3 + NaCl

41 Double Displacement Reaction Pb(NO 3 ) 2(aq) + 2KI (aq)  PbI 2(s) + 2KNO 3(aq)

42 #4 - Double Replacement Reactions Have certain “driving forces”, or reasons Will only happen if one of the products: a) doesn’t dissolve in water and forms a solid (a “precipitate”), or b) is a gas that bubbles out, or c) is a molecular compound (which will usually be water).

43 Complete and balance: assume all of the following reactions actually take place: CaCl 2 + NaOH → CuCl 2 + K 2 S → KOH + Fe(NO 3 ) 3 → (NH 4 ) 2 SO 4 + BaF 2 → NaOH (aq) + HCl (aq) → H 2 O (l) + NaCl (aq)

44 How to recognize which type? Look at the reactants: E + E OR C + C =Synthesis C =Decomposition E + C =Single displacement C + C =Double displacement

45 Practice Examples: H 2 + O 2 → → H 2 O → → Zn + H 2 SO 4 →  → HgO →  → KBr + Cl 2 →  → AgNO 3 + NaCl → → Mg(OH) 2 + H 2 SO 3 →→

46 Combustion Reaction fuelO2O2 + A reaction of a fuel with oxygen, releasing energy in the form of heat and/or light X y O z + energy

47 Combustion Reaction

48 Element + O 2  “oxide” + energy 2Mg (s) + O 2(g)  2MgO (s) + energy P 4(s) + 5O 2(g)  P 4 O 10(g) + energy

49 #5 – Combustion Reactions Combustion is a fast reaction of a substance with oxygen to make compounds called oxides. the general equation is : fuel + oxygen → oxides + energy the three things that must be present for combustion to happen are: fuel oxygen spark / heat

50 Combustion Reaction Examples: What is the main purpose for which fuels are burned around the world? The following equations show what happens when different carbon-based fuels are burned. C (s) + O 2(g) → CO 2(g) + energy CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O + energy ethanol CH 3 CH 2 OH (l) + O 2(g) → CO 2(g) + 3H 2 O (l) C 6 H 12 O 6(s) + 6O 2(g) → 6CO 2(g) + 6H 2 O (g) + energy

51 Combustion of other Fuels Some fuels do not contain carbon. The products are oxides of each element in the fuels. Mg (s) + O 2(g) → MgO (s) Cu (s) + O 2(g) → CuO (s) H 2(g) + O 2(g) → H 2 O (l) P 4(s) + 5O 2(g) → P 4 O 10(g) S (s) + O 2(g) → SO 2(g) As you see, many combustion reactions may also be classified as synthesis.

52 SUMMARY: An equation... Describes a reaction Must be balanced in order to follow the Law of Conservation of Mass Can only be balanced by changing the coefficients. Has special symbols to indicate the physical state, if a catalyst or energy is required, etc.

53 Reactions Come in 5 major types. We can tell what type they are by looking at the reactants. Single Replacement happens based on the Activity Series Double Replacement happens if one product is: 1) a precipitate (an insoluble solid), 2) water (a molecular compound), or 3) a gas.


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