redox reactions

oxidation

• Gain of oxygen

  • Ex: C + O2 → CO2

• Also loss of hydrogen

  • Ex: CH4 + O2 → CO2 + H2O

reduction

• Loss of oxygen

  • CuO + H2 → Cu + H2O

• Also gain of hydrogen

  • Ex: N2 + H2 → NH3

redox

• Oxidation is loss of electrons

• Reduction is gain of electrons

• Reactions never take place alone, must occur simultaneously

oxidizing agent

• Oxidizes others causing them to lose electrons

• it is reduced, gains electrons

• Oxidation number of oxidizing agent decreases

reducing agent

• Reduces others cousing them to gain electrons

• it is oxidized, loses electrons

• Oxidation number of oxidizing agent increases

balancing redox reactions

• Write half equation: divide oxidation and reduction into separate equations

• Step 1: balance all atoms except hydrogen & oxygen

• Step 2: balance oxygen by adding water 

• Step 3: balance hydrogen by adding H+

• Step 4: balance charge by adding electrons 

• Step 5: add up & cancel out (electrons must cancel out)

writing equation in neutral medium

• Neutralize H+ by adding OH -

• Combine H and OH -

metals & metal ions

• Metals higher in reactivity can displace less reactive metals

• The more reactive metal acts as a reducing agent

  • The more reactive a metal is the better it is at pushing electrons onto less reactive metal ions

• More reactive atom will have less reactive ion

electrochemical ce

voltaic cells: primary

converts chemical energy from spontaneous redox reactions to electrical energy

voltaic cells: secondary

• (rechargeable cells) that involve redox reactions that can be reversed using electrical energy

voltaic cells: fuel cells

•  convert chemical energy from a fuel , typically hydrogen, and an oxidant such as oxygen into chemical energy through redox reactions, with water and heat as byproducts

electrolytic cells

•  converts electrical energy to chemical energy by bringing about non spontaneous redox reactions

half cells

• Half cells: a strip of metal in solution of it's salt solution 

• Metalls will reach equilibrium with their cations, releasing electrons on surface of metal

• Strip of metal is an electrode

• The position of the equilibrium determines the potential difference between the metal strip and the solution of metal

• When two half cells are connected, a voltaic cell is formed

electrode

• Each half of a voltaic cell is called a half cell or electrodes

• There are two types of electrones: anodes & cathodes

anode

• Where oxidation occurs (more reactive meta, RA)

• It is negative because oxidation is causing the build up of electrons

• Oxidation is driving the current

• Anode loses mass as the solid metal is converted to it's ions

• Electron flows from anode to cathode

cathode

•  Where reduction occurs (less reactive metal, OA)

• Cathode is positive because reduction is using electrons

• Cathode gains masses as the ion is converted to the metal

voltaic cells

• Electrons travel through the wires that connect the two half cells 

• The more reactive metal is oxidized faster, causing electrons to travel to the less reactive metal 

• This shifts the equilibrium in this less reactive half cell causing it to be reduced

without salt bridge

• The anode is negative

• As oxidation occurs, more ions build up in the solution`this cancels out the charge of the anode, preventing the current from flowing

• The cathode is positive

• The solution contains metl ions & anions, as the metal gets reduced the negative ions build up, canceling out the positive charge

• This prevents current from flowing

salt bridge

• Voltage will only be conducted between electrodes when the circuit is complete

• Complete circuit requires: external circuit connected to each electrode, a salt bridge 

• Salt bridge is a glass tube that contains a concentrated solution of a strong electrolyte

• The ions in the electrolyte can flow between electrodes

• This maintains the negative charge of the anode and the positive charge of the cathode 

voltage

• Different half cells make voltaic cells with different voltages

• Direction of electron flow & voltage generated will be determined by the difference in reducing strength of the two metals 

• This can be judged by the position of metals in the reactivity series 

• Greater the gap between metals, greater voltage produced (more reactive metal = higher voltage)