Gen Chem 3 Equilibrium

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General Chemistry

April 2012

TTP Presentation

DRAFT


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1

Objectives of Project

How I prepared for MCAT Chemistry:

*REVIEW all of the Gen Chem

sections in your practice MCATs. Ask

questions and

Practice the simple math thats required for

the MCAT (It is 5th grade level math)


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Framework

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Equilibrium

- Kinetics

- Le Chatliers Principle

- Chemical Equilibrium

- Solutions and Solubility


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Equilibrium

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Thermodynamics- Hesss Law

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Equilibrium

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1. Write the rate laws for the forward and reverse reactions

2. Show when the reaction is in equilibrium


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Equilibrium

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Equilibrium- Le Chatliers Principle

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Question: If you added more N2(g) which side would the reaction go towards?


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Equilibrium- Reactions

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Consider the following reaction:

NO2(g) + CO(g) NO(g) + CO2(g)

The reaction has two elementary steps:

1) NO2(g) + NO2(g) NO3(g) + NO(g) slow step

2) NO3(g) + CO(g) NO2(g) + CO2(g) fast step

Notice that if we add these two equations together, we arrive at the original equation.

Elementary steps must add to give the complex reaction. Since the first step is the

slow step, the rate law for the overall reaction is given by this step and is:

Rate = k1[NO2]^2


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Equilibrium- Reactions

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This reaction has two elementary steps:

1) NO(g) + Br2(g) NOBr2(g) fast step2) NOBR2(g) + NO(g) 2NOBr(g) slow step

The rate law of for this equation is:

Rate = K2[NOBr2][NO]

However, the concentration of NOBR2 depends upon the first step. If we assume that thefirst step reaches equilibrium very quickly, the concentration of NOBr2 can be written in

terms of equilibrium constant Keq for step 1, [NOBr2] = Keq[NO][Br2].

[NOBr] = k1/k-1 [NO][Br2].

The resulting rate law is:

Rate = k2k1/k-1 [NO]^2][Br]


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Equilibrium- Catalysis

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Equilibrium- What it means

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We use the reaction quotient Q to predict the direction in which a reaction will proceed.

Since reactions always move toward equilibrium, Q will always change toward K.

- If Q=K then we are at equilibrium- If Q>K then we have more products than we should and we are

moving in the reverse reaction direction

- If Q


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Equilibrium- Practice

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Equilibrium- Solubility

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S


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E ilib i S l bilit


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E ilib i S l bilit U it f C t ti


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Equilibrium- Solubility: Units of Concentration

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Equilibrium Solubility: Units of Concentration


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Equilibrium Solubility: Units of Concentration


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Equilibrium Solubility


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Equilibrium- Solubility Practice


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Equilibrium Solubility Practice

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Equilibrium- Solubility Practice


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Equilibrium Solubility Practice

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Equilibrium Solubility

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q y

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Equilibrium- How solutes will affect solvents Continued


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q

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q

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When a nonvolatile solute (a solute with no vapor pressure) is added to a liquid, some

of those solute molecules will reach the surface of the solution, and reduce the amount of

surface area available for the liquid molecules.

- An example of a nonvolatile solute are salts!!!

Since the solute molecules dont break free of the solution but do take up surface area,

the number of molecules breaking free from the liquid is decreased while the surface area

of the solution and the volume of open space above the solution remain the same. From

the ideal gas law, PV=nRT, we know that a decrease in n at constant volume and

temperature is proportional to a decrease in P. The vapor pressure of the solution

Pvis given by Raoults Law; and is proportional to the mole fraction, a , of liquid aand vapor pressure of the pure liquid Pa!!!

Pv= aPa

So when we add a nonvolatile solute the VP decreases!!

- If 97% of the solution is solvent, then the vapor pressure will be 97% of thepure solvent



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Graph 1 shows only the partial pressure of the solvent as its mole fraction increases. As

predicted by Raoults law, the relationship is linear.

Graph 2 shows the vapor pressure of an ideal solution and the individual partial pressures

of each solvent. Notice that the partial pressures add at every point to equal the total

pressure

Graph 3 & 4 represent non-ideal solutions and the deviations that occur. The

straight lines are Raoults law predictions and the curved lines are the actual pressures.

Notice that the partial pressures still add at every point to equal the total pressure. Notice

also that a positive heat of solution (endothermic) leads to an increase in vapor pressure,

and a negative heat of solution, to a decrease in vapor pressure:

- Also note that the pure components and their pressures are representedby the end points of the straight lines

- And that for negative heats of solutions the vapor pressure of the mixturecant be greater than the pressure pure of the substance with the higher

vapor pressure

- And that for positive heats of solutions the vapor pressure cant be lowerthan the pressure of the pure substance with the lowest vapor pressure



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Equilibrium- PracticeSolubility


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Thermodynamics- Specific Heat and Phase Changes


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Thermodynamics- Specific Heat and Phase Changes


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Final Practice


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Gen Chem 3 Equilibrium

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