unit 2: redox reactions & gas stoichiometry

S E P T 1 5 , 2 0 1 4

UNIT 2: REDOX REACTIONS & GAS STOICHIOMETRY

Observable phenomena

Sub-microscopic depictions

Chemicalnomenclature

CHEMICAL MODELS

Our basic triangle.

• What you see with your eyes• What you measure with

instruments• Lab work, demos & experiments

• Chemical symbols• Balanced chemical

equations• States of matter (s), (l),

(g), (aq)• Quantitative calculations

• Representational sketches and diagrams (to scale or not)

• Verbal descriptions• Simulations

WHAT DOES A REDOX RXN LOOK LIKE FROM A MACROSCOPIC LEVEL?

• Tin(II)Chloride + Zinc• Carbon and Copper Oxide

R E D OX I S S H O RT F O R “ R E D U C T I O N A N D OX I D AT I O N R E A C T I O N S ”

WHAT IS A REDOX REACTION?

Oxidation-reduction reactions are those in which an electron is transferred from one

reactant to another.

The atom that donates the electron is OXIDIZED.

The atom that accepts the electron is REDUCED.

The electron donor is the REDUCING AGENT.The electron acceptor is the OXIDIZING

AGENT.

Think: reduced in charge.

REDOX RXNS

• Mnemonics for Redox rxns:• “OIL RIG”

• Oxidation Is Loss (electrons)• Reduction Is Gain (electrons

• “LEO GER”• Lose Electrons Oxidation• Gain Electrons Reduction

WHAT DOES A REDOX LOOK LIKE FROM A SUBMICROSCOPIC LEVEL?

• Sub -Microscopic Level: Zinc + Oxygen

• Similar redox reaction: • CuCl2(aq) + Zn(s) Cu(s) + ZnCl2(aq)

• Which reactant is being oxidized? Which is reduced?

HOW DO WE PREDICT IF A S.R. REACTION WILL OCCUR?

• For instance, which of the following reactions will happen?

• Cl2 (g) + 2HBr (aq) 2HCl (aq) + Br2 (l)

• Br2 (g) + 2HCl (aq) 2HBr (aq) + Cl2 (l)

• In other words, which is more “reactive,” chlorine or bromine?

• We need an ACTIVITY SERIES (lab 3) and a way of THINKING about activity (using Coulomb’s Law).

VOLTAIC CELLS

•Voltaic cell - an electrochemical cell that uses a spontaneous chemical rxn to generate an electric current.

•Components of a voltaic cell:•2 half-cells connected by a wire•Salt bridge•cathode and anode

Alessandro Volta - invented the first voltaic cell (1800)

Electrical Potential – Voltaic Cell –Electrode –

Electric Current -Salt Bridge -

HOW DOES A VOLTAIC CELL WORK SPONTANEOUSLY?

• When you connect 2 metals from different positions on the activity series - it creates a electrical potential energy difference b/w the metals.• The greater the activity b/w the two metals,

the greater the electrical potential

• The differing tendency of metals to lose electrons allows an oxidation -reduction to generate electricity!

A battery consists of a number of voltaic cells.

Here’s an example of a single “wet” cell.

Zinc is oxidized.

Copper isreduced.

Sulfate

Cathode

(half-cell)

Copper will be

reduced.

Consonants.

Anode

(half-cell)

Zinc will oxidize

Vowels.

Use oxidation potential series to determine electric potential (“voltage”) of this cell.

Zn Zn2+ + 2e- (3.63 V) Cu2+ Cu + 2e- (-2.53 V)

Difference in electric potential is 1.10 V.

Zinc is oxidized.

Copper isreduced.

Sulfate

BATTERIES - DRY VOLTAIC CELL

•.•Voltaic Pile

A 9 V battery consists of six 1.5 V batteries (like AAA batteries) wired in series.

Capacity refers to how many electrons the series redox reactions will release before no more reactants remain. We will measure this

quantity in MOLES of electrons. It is related to the lifetime of the battery.

Voltage is proportional to the amount of potential energy each electron gains in the reaction. It is related to the power the battery

can provide.

Lead and lead-oxide strips are placed in sulfuric acid. What is oxidized? Which is reduced?

Car Battery

HOW DO CAR BATTERIES WORK?

How Batteries WorkCommon battery chemistries include:Zinc-carbon battery: common in many inexpensive AAA, AA, C and D dry cell batteries. The anode is zinc, the cathode is manganese dioxide, and the electrolyte is ammonium chloride or zinc chloride.Alkaline battery: common in AA, C and D dry cell batteries. The cathode is composed of a manganese dioxide mixture, while the anode is a zinc powder. Lithium-ion battery (rechargeable):, such as cell phones, digital cameras and even electric cars. A variety of substances are used in lithium batteries, such as lithium cobalt oxide cathode and a carbon anode.Lead-acid battery (rechargeable): This is the chemistry used in a typical car battery. The electrodes are usually made of lead dioxide and metallic lead, while the electrolyte is a sulfuric acid solution.

Lithium Ion Batteries

LIFE CYCLE OF BATTERIES

• Recycling Car Batteries• Lithium Ion Rechargable

GAS STOICHIOMETRYCHAPTER 12

GAS STOICHIOMETRY

• Many chemical reactions involve gases as a reactant or a product

• Gas Stoichiometry – the procedure for calculating the volume of gases as products or reactants

• Gases also have a molar volume (L/mol) rather than concentration.

• This is the conversion factor used to convert (litres of gas) to (moles of gas)

• The Ideal Gas Law (PV = nRT) may also be required to:A) find the number of moles of reactantB) Find the V, P, or T of the product

• Molar volume is the same for all gases at the same temperature and pressure (remember, all gases have the same physical properties)

• At STP, molar volume = 22.4 L/mol (101.325 kPa and 0oC)

• This can be used as a conversion factor just like molar mass!

MOLAR VOLUME

At STP, one mole of gas has a volume of 22.4 L, which is approximately the volume of 11 “empty” 2 L pop bottles.

STP = 22.4L/mol

EXAMPLE #1

• If 300g of propane burns in a gas barbecue, what volume of oxygen measured at STP is required for the reaction?

• Remember: 22.4 L/mol for STP

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

m = 300g m = ?44.11g/mol 22.4L/mol

300 g x 1 mol x 5 mol O2 x 22.4 L = 761 L O2(g)

44.11 g 1 mol C3H8 1 mol

**Remember – molar volume is the conversion factor for gases just like molar mass is the conversion factor in stoichiometry

EXAMPLE #2

• Hydrogen gas is produced when sodium metal is added to water. What mass of sodium is necessary to produce 20.0L of hydrogen at STP?

• Remember: 22.4L/mol for STP

2Na(s) + 2H2O (l) 2NaOH(aq) + H2(g)

m = ? V = 20.0L

22.99g/mol 22.4L/mol

20.0L x 1 mol x 2mol x 22.99g = 41.0 g Na(s)

22.4 L 1 mol 1 mol

**Remember – molar volume is the conversion factor for gases just like molar mass is the conversion factor in gravimetric stoichiometry

EXAMPLE #3

• What volume of gaseous ammonia at 450kPa and 80oC can be obtained from the complete reaction of 7.5kg of hydrogen with nitrogen?

N2(g) + 3H2(g) 2NH3(g)

m = 7500g m = ? M = 2.02 g/mol P = 450kPA

T = 353.13K

7500 g x 1 mol x 2 = 2475.2475 mol NH3(g)

2.02 g 3

PV = nRT V = nRT = P

= 16150.10L 1.6 x 104 L of NH3(g)

If the conditions are not STP or SATP, the molar volume cannot be used! You must use the ideal gas law to find the gas values using moles determined from stoichiometry

WHAT DO YOU REMEMBER ABOUT GASES?

GAS VOLUME AND MOLES : AVOGADRO’S LAW

MOLAR VOLUME & GAS DENSITY

IDEAL GAS LAW