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IIIIIIIVV I.Intro to Reactions Ch. 7 – Chemical Reactions
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What is a chemical reaction? n It is a process in which substances undergo a change to form completely new substances n During this process chemical bonds break and reform, thus changing the substances
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Endo vs. Exo-thermic n When chemical bonds break and reform, the net bond energy determines if heat is released or absorbed. n If new bonds formed don’t take as much energy to form, then the excess energy is released. You feel it get hot. Thus the term, Exo-thermic!
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How do you know a chemical reaction has taken place? n The new substance is altered so much that it cannot be changed back to the original substance without undergoing another chemical change.
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Other ways to know a chemical reaction… n Heat and/or light is produced.
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Other ways to know a chemical reaction… n Color change.
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Other ways to know a chemical reaction… n Gas is produced (Sometimes you can smell it even if you can’t see it…think burning toast)
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Other ways to know a chemical reaction… n A precipitate is formed. n PRECIPITATE – a solid that forms when two liquids are combined
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B. Law of Conservation of Mass n mass is neither created nor destroyed in a chemical reaction 4 H 2 O 4 H 2 O 4 g32 g 36 g n total mass stays the same n atoms can only rearrange
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C. Chemical Equations A+B C+D REACTANTSPRODUCTS
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2NaOH + CaBr 2 2NaBr + Ca(OH) 2 n REACTANTS – the starting materials of the reaction. n PRODUCTS – the ending materials of the reaction. n (the arrow) – means “yields.” Shows the direction from the reactants to products.
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2NaOH + CaBr 2 2NaBr + Ca(OH) 2 n COEFFICIENTS – numbers placed in front of a compound to indicate how many molecules are present. Can be changed to balance the equation. n SUBSCRIPTS – numbers that indicate how many atoms are present. May not be changed to balance the chemical equation.
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You may also see… n Words or symbols over the arrow. n Δ – heat is required n hv – light is required n elec – electricity is required
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C. Chemical Equations
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IIIIIIIVV D. Chemical Reactions Types of Chemical Reactions
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1. Combustion CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) n the burning of any substance in O 2 to produce heat A + O 2 B
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Na(s)+ O 2 (g) C 3 H 8 (g)+ O 2 (g) 5 3 4 1. Combustion contain oxygen as a reactant hydrocarbons form CO 2 + H 2 O CO 2 (g)+ H 2 O(g) Na 2 O(s) 4 2
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2. Synthesis n the combination of 2 or more substances to form a compound n only one product A + B AB
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2. Synthesis H 2 (g) + Cl 2 (g) 2 HCl(g)
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3. Decomposition n a compound breaks down into 2 or more simpler substances n only one reactant AB A + B
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3. Decomposition 2 H 2 O(l) 2 H 2 (g) + O 2 (g)
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4. Single Replacement n one element replaces another in a compound metal replaces metal (+) nonmetal replaces nonmetal (-) A + BC B + AC
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4. Single Replacement Cu(s) + 2AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + 2Ag(s)
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Fe(s)+ CuSO 4 (aq) Cu(s)+ FeSO 4 (aq) 5. Single Replacement n Products: metal metal (+) nonmetal nonmetal (-) free element must be more active (check activity series) Br 2 (l)+ NaCl(aq) N.R.
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AB + CD AD + CB 5. Double Replacement n ions in two compounds “change partners” n cation of one compound combines with anion of the other
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5. Double Replacement Pb(NO 3 ) 2 (aq) + K 2 CrO 4 (aq) PbCrO 4 (s) + 2KNO 3 (aq)
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Pb(NO 3 ) 2 (aq)+ KI(aq) PbI 2 (s)+ KNO 3 (aq) 5. Double Replacement n Products: switch negative ions one product must be insoluble (check solubility table) NaNO 3 (aq)+ KI(aq) N.R. 2 2
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6. Acid – Base Reactions n This reaction takes place between an Acid, HA and a Base B(OH). n The products are always water and the “left-overs” BA. HA + B(OH) = H 2 O + BA
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