IIIIIIIVV I.Intro to Reactions Ch. 7 – Chemical Reactions

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

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!

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.

Other ways to know a chemical reaction… n Heat and/or light is produced.

Other ways to know a chemical reaction… n Color change.

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)

Other ways to know a chemical reaction… n A precipitate is formed. n PRECIPITATE – a solid that forms when two liquids are combined

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

C. Chemical Equations A+B  C+D REACTANTSPRODUCTS

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.

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.

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

C. Chemical Equations

IIIIIIIVV D. Chemical Reactions Types of Chemical Reactions

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

Na(s)+ O 2 (g)  C 3 H 8 (g)+ O 2 (g)  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

2. Synthesis n the combination of 2 or more substances to form a compound n only one product A + B  AB

2. Synthesis H 2 (g) + Cl 2 (g)  2 HCl(g)

3. Decomposition n a compound breaks down into 2 or more simpler substances n only one reactant AB  A + B

3. Decomposition 2 H 2 O(l)  2 H 2 (g) + O 2 (g)

4. Single Replacement n one element replaces another in a compound  metal replaces metal (+)  nonmetal replaces nonmetal (-) A + BC  B + AC

4. Single Replacement Cu(s) + 2AgNO 3 (aq)  Cu(NO 3 ) 2 (aq) + 2Ag(s)

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.

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

5. Double Replacement Pb(NO 3 ) 2 (aq) + K 2 CrO 4 (aq)  PbCrO 4 (s) + 2KNO 3 (aq)

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

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