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Chemical Reactions Chemistry
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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
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Chemical Reaction Atoms aren’t created or destroyed (according to the Law of Conservation of Mass) 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
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Symbols in Equations t he arrow (→) separates the reactants from the products (arrow points to products) – Read as: “reacts to form” or “yields” The plus sign = “and” (s) after the formula = solid: Fe (s) (g) after the formula = gas: CO 2(g) (l) after the formula = liquid: H 2 O (l)
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Symbols Used in Equations (aq) after the formula = dissolved in water, an aqueous solution: NaCl (aq) is a salt water solution ↑used after a product indicates a gas has been produced: H 2 ↑ ↓used after a product indicates a solid has been produced: PbI 2 ↓
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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)
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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.
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The Skeleton Equation Uses formulas and symbols to describe a reaction – but doesn’t indicate how many; this means they are NOT balanced All chemical equations are a description of the reaction. The equation provides a “balance sheet” to monitor changes in reactants and products and keep track of atoms.
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Write a Skeleton Equation 1. Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. 1 st - write the basics formulas for reactants and products: Important to write them correctly. Once correctly written, don’t alter them! Fe 2 S 3 (s) + HCl (aq) FeCl 3 (s) + H 2 S (g) Do you have the same number and type of atoms on both sides?....if not, use COEFFICIENTS! Fe 2 S 3 (s) + 6HCl (aq) 2FeCl 3 (s) + 3H 2 S (g)
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Write a Skeleton Equation 1. 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 (s) CO 2 (g) + H 2 O (l) + NaNO 3 (aq) BALANCED? 2HNO 3 (aq) +Na 2 CO 3 (s) CO 2 (g) + H 2 O (l) + 2NaNO 3 (aq)
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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)
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Balanced Chemical Equations Atoms can’t be created or destroyed in an ordinary reaction: – All the atoms we start with we must end up with (meaning: balanced!) A balanced equation has the same number of each element on both sides of the equation.
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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/ balance single atoms last 4) Double-check to make sure it is balanced.
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NEVER Never change a subscript to balance an equation (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.
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Practice Balancing 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
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Net Ionic Equations Many reactions occur in water- that is, in aqueous solution When dissolved in water, many ionic compounds “dissociate”, or separate, into cations and anions Now we are ready to write an ionic equation
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Net Ionic Equations AgNO 3 (aq) + NaCl (aq) AgCl (s) + NaNO 3 (aq) this is the full balanced equation next, write it as an ionic equation by splitting the compounds into their ions: Ag + + NO 3 - + Na + + Cl - AgCl (s) + Na + + NO 3 - Note that the AgCl did not ionize, because it is a “precipitate” (a solid, not aqueous)
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Net Ionic Equations simplify by crossing out ions not directly involved (called spectator ions, same on both sides) Ag + + Cl - AgCl This is called the net ionic equation Let’s talk about precipitates before we do some other examples
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Predicting the Precipitate Insoluble salt = a precipitate Always soluble: salts of Na +, K +, NH 4 +, and NO 3 - Usually soluble: salts of Cl - and SO 4 2-, but AgCl, PbCl 2, PbSO 4 and BaSO 4 are insoluble Usually insoluble: salts of OH -, O 2-, CO 3 2-, and PO 4 3- (unless with Na +, K +, or NH 4 + ) Slightly soluble: Ca(OH) 2 and CaSO 4
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