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
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