experiment 1 chem 18

8/3/2019 Experiment 1 Chem 18

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Logronio.Turalde.


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1) Determine the effects of some factors onreaction rates

2) Determine the rate law expression using

the method of initial rates3) Evaluate the value of the activation energy

of a reaction


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rate of reaction frequency of collision frequency of collision = (number of

collisions)/(time) effective collision is required


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1. reactants must have sufficient energy toovercome the activation energy

activation energy: minimum energy that isrequired for the reaction to occur, energy barrier

Ea, effective collisions, rate

1. Proper Orientation


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the reactants will form a transition statebefore forming the products

Unstable arrangement of atoms reactants in a short-lived, high-energy,

intermediate state

exact structure cannot be determined

contains partial forming and partial breaking ofbonds


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rate of reaction ease of formation of thetransition state

Ease of formation is related to the activationenergy

Ea, ease of formation


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Some reactions are naturally faster whileothers are naturally slower

Causes:

liquid vs. gas (different phases)

position in the reactivity series

the activation energy


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A B


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Reactants Visible Results

A Slow gentle bubbling

B Violent reaction, fast

bubbling with explodingaction and loud fizzing

sound


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Test Tube A (Mg ribbon + water) has higher

activation energy because it has slowerreaction rate.

The activation energy is dependent on the

nature of reacting substances. The reaction slow when the Ea is higher

because it is harder to overcome.


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Concentration, Rate(Except for Zero-Order Reactions)

more molecules are available for collision


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Determined by the rate law

the order of reaction is experimentallydetermined

it is not always equal to the coefficient in thebalanced equation


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getting the general ratio of two reactions thathave one reactant varying while the other

reactants being constant


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Using linear regression between a set of datawith only one reactant


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Used for rate laws with more than onereactants

Isolating the desired reactant by finding datasets where the other reactants have constantconcentration


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check if the x mark is initiallyclearly seen through the solution. Time the reaction from the

moment the mark is no longervisible

Do not agitate the solution


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I. Constant HCl Concentration

II. Constant Na2S2O3 Concentration


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0.15 M Na2S2O3 (mL) H2O (mL) 3 M HCl (mL)5 0 14 1 13 2 12 3 11 4 1


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0.15 M Na2S2O3(mL) H2O (mL) 3 M HCl (mL)

5 0 2.55 0.5 25

1

1.5

5 1.5 15 2 0.5


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[Na2S2O3] [HCl] ln [Na2S2O3] Time Rate(1/time) ln Rate

0.125 M 0.5 M -2.07944 842 0.11875 /s -2.131

0.1 M 0.5 M -2.30259 1316 0.076 /s -2.577

0.075 M 0.5 M -2.59027 2339 0.04275 /s -3.153

0.05 M 0.5 M -2.99573 5263 0.019 /s -3.963

0.025 M 0.5 M -3.68888 33053 0.00475 /s -5.350


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The rate decreases as the concentration of

Na2S2O3 decreases. Its relationship is exponential: [Na2S2O3]

2rate. So, as the concentration of [Na2S2O3]

increases, rate increases exponentially.


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-6

-5

-4

-3

-2

-1

0

-4 -3 -2 -1 0

lnRate

ln [Na2S2O3]

ln Rate vs. ln [Na2S2O3]


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Theoretically, the order of reaction is

second.


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[Na2S2O3] [HCl] ln [Na2S2O3] Time Rate(1/time) ln Rate

0.1 M 1 M 0 1293 0.07734 /s -2.560

0.1 M 0.8 M -0.2231 1377 0.07262 /s -2.622

0.1 M 0.6 M -0.5108 1424 0.07022 /s -2.656

0.1 M 0.4 M -0.9163 1551 0.06447 /s -2.741

0.1 M 0.2 M -1.6094 1672 0.05981 /s -2.817


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There is only a very little proportional change

in the rates as the concentration of HClchanges.

If ln rate would be rounded off to -3, the veryvery small change can be assumed that thereis no change with the rate at all even of theconcentration of HCl is increased.


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-3

-2

-1

0

-2 -1 0

ln

rate

ln [HCl]

ln rate vs ln [HCl]


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The order of reaction is zeroth-order

reaction.


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Overall order of the reaction is 2

(second order reaction).

Computation: 2+0= 2


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Rate= k [Na2S2O3]2


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T, KE, mobility, frequency of collision,rate

increasing the temperature increases theinitial energy of the reactants, making iteasier to overcome the activation energy

not dependent on H of the reaction the temperature alters the rate constant

T, k


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relates rate with temperature


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* One approximately 10 C lower

than room temperature


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[Na2S2O3 ]= [HCl]= 0.4138 M

Temp(C)

1/T (K) Time Rate(1/Time) k ln k

25.5 3.35 x 10-3 4973 0.02011 0.389 -0.944

54 3.058 x 10-3 1848 0.05411 1.047 0.0455

9.3 3.542 x 10-3 2562 7.9605 x 10-3 0.1540 -1.871


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-2

-1.5

-1

-0.5

0

0.5

0.003 0.0031 0.0032 0.0033 0.0034 0.0035 0.0036

ln

k

1/T

ln k vs 1/T


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SA, Rate due to the high number of molecules exposed

to collision & hence a greater probability forthe reaction to occur


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A B


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Strip of Mg Fast bubbling(evolution of gas)

Pieces of Mg Faster bubbling action(evolution of gas)


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substance that is not consumed in thereaction

alters the activation energy of a reaction


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1. Positive catalyst:

speeds up the reaction (increases Ea)

2. Negative catalyst (inhibitor):

slows down the reaction (decreases Ea)


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a. homogenous catalyst: catalyst in the samephase as the reactants

it provides an alternate pathway with a series of

steps with a lower activation energy each


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a. heterogenous catalyst: catalyst in thedifferent phase as the reactants(usually solid)

acts through adsorption through binding sites in the solid's surface


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H2O2 + Rochelle Salt Light evolution ofgas, gentle bubbling

H2O2 + Rochelle Salt+ CoCl2

Violent evolution ofgas, larger bubbles


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CoCl2 is a catalyst in this reaction. It speeds

up the reaction by having steps with muchlower Ea without being itself beingconsumed.


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The nature of reactants dictates the magnitude of

reaction: lower Ea, faster; higher Ea, slower. The largerthe surface area exposed, the faster the reaction is. Thisis because more reacting molecules will be available forcollision. Rat is directly proportional to theconcentration of reactants raised to the order of

reaction. The presence of a catalyst speeds up a reactionwhile the presence of an inhibitor slows down achemical reaction without itself being used up in theprocess. If one increases the temperature of the

experiment 1 chem 18

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