chm lab exam 1

for determining energy of activation

-3 different temperature trials -concentrations of H₂O₂ and KI constant

two solutions:

-Fe(NO₃)₃ and NH₄SCN

observations:

-as the concentration of Fe3+ and SCN- increased, the color got darker

Quantitative experiment

-calibration curve: measuring absorbance of the standard solutions -measuring equilibrium constant Keq

Qualitative experiment

-compare the effects of the concentration of both Fe and SCN -keep concentration of SCN- constant when measuring Fe³+ -keep conc. of Fe3+ constant when measuring SCN- (total volume the same!)-5ml

henderson hasselbach equation process

-concentration of [ln-] and [Hln] are proportional to the absorbance at each buffer's pH

units of reaction rates:

-concentration/time Molarity/seconds

dilution effect:

-equilibrium is established at a lower vapor pressure because when solute is added, the number of solvent molecules on the surface is decreased and fewer solvent molecules vaporize, which means fewer need to condense so equilibrium is established at lower VP.

The water must be brought to a steady, but not to vigorous boil. Why?

-if boiled too vigorously, the volume of water will change too much (evaporate) and the calculated molalities will be incorrect

determining 3 things in this experiment:

-kinetic order of H₂O₂ -kinetic order of KI -energy of activation

the addition of a non-volatile solute ______ the vapor pressure of the solution

-lowers

to determine the kinetic order of KI

-room temperature -concentration of H₂O₂ constant -change concentration of KI

to determine the kinetic order of H₂O₂

-room temperature -concentration of KI constant -change concentration of H₂O₂

an airtight system is crucial: why?

-so that the pressure can be measured accurately!

if the solution is boiled too vigorously so that the volume of water is decreased, what will happen to the concentration of the salt solution and the measured boiling point?

-the salt will become more concentrated because water evaporates and salt is left behind -the boiling point will rise because there is a higher salt to water ratio and the more salt, the higher the boiling point.

the purpose of the experiment kinetics of the decomposition of hydrogen peroxide:

-to determine the orders with respect to H₂O₂ and KI in a series of student designed experiments -to determine the rate constant k

non-volatile solutes change:

-vapor pressure -boiling point -freezing point -osmotic pressure

components that must be present in a rate limiting step:

...

trends in concentration of the complex ion as a function of the changes in concentration of each reactant and the change in temperature:

...

two ways to determine the pKa

1. find the derivative of the S curve of (plot of absorbance vs pH) 2.the henderson hasselbach equation

the Fe³+ solutions contain ______

1.0 M HNO₃

the calibration curve is made by using solutions with atleast _____

250 fold excess of Fe³+ so that all the SCN- is converted into the complex ion

the complex ion has an absorption maximum of

450-470nm

the visible range of the electromagnetic spectrum

450-470nm

investigate the reaction between:

Fe³+ and thiocyanate anion (SCN-)

Bromothymol blue: Hln=, ln-=

Hln=yellow ln-=blue

bromocresol purple: Hln=, In-=

Hln=yellow ln-=purple

important oxidizing agent:

H₂O₂ hydrogen peroxide

catalyst used in this experiment:

KI

KI increases the rate of the decomposition of hydrogen peroxide without undergoing a net change in its own composition because:

KI has a lower activation energy and it is broken down and regenergated

for weak acids Ka:

Ka<1 HA is incompletely ionized -both the acid and its conjugate base (A-) will be present at equilibrium

for strong acids Ka:

Ka> 1 acid is completely ionized in water to generate H₃O+ and A-. [H₃O+] us equal to the initial concentration of HA added

two solutions for experiment:

K₂HPO₄, KH₂PO₄

solutions have a _____ freezing point than pure solvents

Lower

complex ion:

[Fe(SCN)]²+

the function of change in those 4 properties is

a number of dissolved particles

net absorbance=

absorbance-absorbance of the blank solution

a plot of absorbance vs pH resembles

an S curve

conjugate acid base pair

an acid (HA) and a base (A-) That differ only in the presence or absence of a proton

the magnitude of Ka (equilibrium constant) indicates the tendency of

an acid to ionize in water

why?

because a higher temperature is needed to raise the solutions vapor pressure to 1 atm (at normal bp, it is less than 1atm)

two acid base indicators:

bromothymol blue and bromocresol purple

conjugate base is also called

buffer

decomposition reaction can be sped up by

catalysts

the change in vp, bp, fp, op=

colligative properties

Fe³+ and SCN- are _______(color)

colorless

ln-

conjugate base

colligative means:

depending on the collection

immediately after the salt was added to the water, the temperature should have _____. Why?

dropped; the salt takes energy from the water so that it can dissolve. water gives off some of its energy so that the ions can be broken. the heat of the solution lowers since the water cools down from the transfer of heat/energy

the equilibrium condition is

dynamic (changing the concentration of one species or another variable will cause concentrations of other species to change until equilibrium can be reestablished)

increase in temp:

endothermic (heat absorbing) -increases the rate constant -reactant+heat=products

decrease in temp:

exothermic -reactant=products+heat

Le chateliers principle:

exothermic reaction =decrease in temperature (of water) -color darker as temp was colder (so as heat was added to the products) more complex ion was formed

therefore:

fewer gas molecules need to condense in order to balance then, and equilibrium is established at a lower vapor pressure

boiling point of a solution is _____ than the boiling point of a pure solvent

higher

reaction orders tell you:

how a reaction takes place (which molecules collide with which other molecules as bonds break, new bonds form and reactants are converted into products)

Le chateliers principle is used to predict:

how an equilibrium system will change when it is disturbed

the rate law expresses

how the rate depends on the concentrations of reactants

rates _____ as temperature _____

increase as temp. increases

catalysts:

increase the rate of a reaction without undergoing a net change in their own structures.

as molality increases, boiling point

increases

the slope in the reaction rate is expressed as:

kPa/s

at normal boiling point of a pure liquid, the vapor pressure of the solution (with dissolved solutes) will be:

less than 1 atm.

to find the activation energy: plot:

ln k vs 1/T (slope = -Ea/R) so Ea=-Slope(R) CONVERT ! TO J

acid

molecule that dissociates in water to yield H₃O+

base

molecule that dissociates in water to yield OH-

condensing

molecules entering the liquid

vaporizing

molecules leaving the liquid

when a reaction reaches equilibrium, the concentrations of all species involved in the reaction

no longer change with time

if [HA]>[A-], pH:

pH<pKa because log of a decimal is negative

First derivative of S curve procedure

pH=largest first derivative (pH where the absorbance is changing fastest)

if [HA]=[A-], pH:

pH=pKa because log 1=0

if [HA]<[A-], pH:

pH>pKa because the log of any number greater than 1 is positive

the transition from one color to the other occurs over a narrow range of pH=

pKa+- 1

the slope of a plot of ______ will measure____

plot of pressure vs. time, measures the rate of the reaction

the end of the lag phase:

pressure will begin the increase with time

base is a

proton acceptor

acid is a

proton donor (H+)

reaction rates depend on:

reactant concentrations

the complex ion has a _______ color

red rust

how can the kinetics of this reaction be monitored?

since O₂ is generated as a product, it can be monitored in a closed vessel where change in pressure is measured as a function of time.

use the _____ graduated cylinder that will contain the entire volume

smallest (100mL for 49.5mL)

a large value of Ka=

strong acid

non-volatile solutes

substances with no measurable vapor pressure

when Delta H rxn = 0

temperature change will have no effect on the concentrations because it has equal effects on the forward and reverse rates

the equilibrium constant is called

the acid dissociation constant, Ka

chemical kinetics:

the area of chemistry concerned with the speeds, or rates of reactions

reaction rate

the change in concentration of reactants or products per unit of time

boiling point elevation:

the difference between the boiling point of the solution and the boiling point of the pure solvent

the concentrations of the reactants and products at equilibrium are governed by

the equilibrium constant Keq

if additional reactant is added

the forward reaction occurs at a larger rate -system reacts to consume the excess reactant (so there will be more products-shifts towards products)

why is there a short initial lag phase?

the initial O₂ will dissolve in the aqueous solution until it reaches the saturation limit. the end of the short lag phase will be obvious

the number of significant figures is based on

the least precise measuring device

energy of activation:

the minimum energy required to bring about the reaction

when solute is added:

the number of solvent molecules on the surface is decreased and fewer solvent molecules vaporize in order to maintain equilibrium

what kinds of solutions have a higher boiling point?

the one with the biggest molality

the pH of an aqueous solution containing a weak acid and its conjugate base is governed by

the pKa of the acid and the log of [A-]/[HA]

to find the exponents(or orders) plot:

the rate (M/s) vs the concentration (of what finding) or log (M/s) vs log (concentration) -the slope=the exponent (rounded to nearest whole number)

the centerpiece of any kinetic study is :

the rate law

Equilibrium occurs in a chemical reaction when

the rates of the forward and reverse reactions are equal

if additional product is added

the reverse reaction occurs at a larger rate -system reacts to consume excess product (so there will be more reactants-shfts towards reactants)

the concentration of the complex ion is

the same as that of the initial concentration of SCN-

For Hln: at pH<pKa:

the solution containing the indicator will be the color of the acid, Hln

at pH>pKa

the solution will be the color of the conjugate base (ln-)

normal boiling point of a liquid

the temperature at which the vapor pressure equals 1 atm

pH=pKa when

there are equal amounts of the two forms of the dye present

neither H+ or NO₃- are involved in thyocyanate equilibrium so

they do not have an effect on the reaction

why?

to prevent precipitation of Fe(OH)₃

equilibrium:

two opposing processes occurring simultaneously (vaporizing and condensing) at equal rates

the concentration of _____ does not appear in the definition of the ionization constant under dilute conditions

water

Hln

weak acid

indicators are solutions of:

weak acids (Hln) and their conjugate bases (ln-) that exibit different colors depending on the pH of the solution.

solute

what is being dissolved

solvent

what is dissolving the substance (water)

le chateliers principle:

when a chemical system at equilibrium is distrurbed by a change in temperature, concentration, or pressure, it re-attains equilibrium by undergoing a net reaction that reduces the effect of the disturbance