Unit 7: Redox & Electrochemistry

Whats the point ? Electrical production (batteries, fuel cells) REDOX reactions are important in … Purifying metals (e.g. Al, Na, Li) Producing gases (e.g. Cl 2, O 2, H 2 ) Electroplating metals Protecting metals from corrosion Balancing complex chemical equations Sensors and machines (e.g. pH meter) C 3 H 8 O + CrO 3 + H 2 SO 4 Cr 2 (SO 4 ) 3 + C 3 H 6 O + H 2 O

What is Redox? REDOX stands for REDuction/OXidation Oxidation is often thought of as a combination of a substance with oxygen (rusting, burning) Just like with acid/base definitions the definition of oxidation is expanded Oxidation refers to a loss of electrons Reduction refers to a gain of electrons As a mnemonic remember LEO says GER Loss Electrons = Oxidation Gain Electrons = Reduction Read 12.1 (pg )

Testing concepts Q- What is oxidation? What is reduction? Represent each as a chemical equation. A- oxidation = loss of e – … X X + + e – reduction = gain of e – … X + e – X – Q- Why are 2Na + Cl 2 2NaCl & 2H 2 + O 2 H 2 O considered redox reactions? A- Both involve the transfer of electron density (Na has no charge, the atoms in diatomic molecules have no partial charge. After reaction the atoms have different shares of the electrons because of different EN values) Q- Is it possible to oxidize a material without reducing something else? A- No. A lost e – is taken up by something else.

Testing concepts Q-Define oxidizing agent, reducing agent. A-An oxidizing agent causes oxidation by being reduced itself A reducing agent causes reduction by being oxidized itself Q-PE 1 A-CaCl 2 is an ionic compound with a positive calcium ion and negative chlorine ions Ca + Cl 2 CaCl 2 Ca Ca e –, Cl 2 + 2e – 2Cl –. Thus Ca is losing electrons (oxidation) and Cl is gaining electrons (reduction). These are called half reactions

Oxidation numbers We will see that there is a simple way to keep track of oxidation and reduction This is done via oxidation numbers An oxidation number is the charge an atom would have if electrons in its bonds belonged completely to the more electronegative atom E.g. in HCl, Cl has a higher EN (pg. 255). Thus, oxidation numbers are Cl = -1, H = +1 Notice that oxidation numbers are written as +1 vs. 1+ to distinguish them from charges. Instead of referring to EN chart, a few rules are followed to assign oxidation numbers Refer also to study note

Rules (and rationale ) 1.Any element, when not combined with atoms of a different element, has an oxidation # of zero. (O in O 2 is zero) 2.Any simple monatomic ion (one-atom ion) has an oxidation number equal to its charge (Na + is +1, O 2– is –2) 3.The sum of the oxidation numbers of all of the atoms in a formula must equal the charge written for the formula. (if the oxidation number of O is –2, then in CO 3 2– the oxidation number of C is +4)

4.In compounds, the oxidation # of IA metals is +1, IIA is +2, and aluminum (in IIIA) is +3 5.In ionic compounds, the oxidation # of a nonmetal or polyatomic ion is equal to the charge of its associated ion. (CuCl 2, Cl is –1) 6.F is always –1, O is always –2 (unless combined with F), H is usually +1 rule total Ox.# Rules (and rationale ) or rule PE 2 (450), 12.9, (484), 12.10, 12.11, (484) H N O 3H N O 3 C2H6OC2H6O K 2 Cr 2 O 7 AgI H 2 PO 4 –

More practice PE 2 pg. 450: answers in back of book 12.9, (pg. 484): answers in back of book 12.10: a) Cl=+7, b) Cr=+3, c) Sn=+4, d) Au= : a) Na= +1, H = +1, P= +5, O= -2 b) Ba= +2, Mn= +6, O= -2 c) Na= +1, S= +2.5, O= -2 d) Cl= +3, F= :a) +2, b) +4, c) +3, d) +5, e) -2, f) 0, g) –1, h) -3 I) –1/3 For more lessons, visit