TOPIC 6 & 16: KINETICSlorenowicz.weebly.com/.../6/1/6/4616010/sch3u7-kinetics1.pdf6, DECOMPOSES AS...

TOPIC 6 & 16: KINETICS

RATES OF CHEMICAL REACTIONS

Monday, May 14, 2012

WHAT ARE RATES?

IF YOU LEAVE MILK OUT ON THE COUNTER, HOW LONG DOES IT TAKE TO GO BAD?

HOW QUICKLY DOES AN AIRBAG NEED TO DEPLOY IN ORDER TO SAVE A LIFE?

HOW LONG DOES NUCLEAR WASTE TAKE TO DECAY?

HOW LONG DOES IT TAKE FOR DIAMONDS TO FORM FROM GRAPHITE?


WHAT IS KINETICS?

THE RATE AT WHICH CHEMICAL REACTIONS OCCUR

THE CHANGE IN CONCENTRATION OF REACTANTS OVER TIME

THE CHANGE IN CONCENTRATION OF PRODUCTS OVER TIME


REACTION RATES

THE RATE OF A CHEMICAL REACTION CAN BE DETERMINED BY MONITORING THE CHANGE IN CONCENTRATION OF EITHER REACTANTS DISAPPEARING OR BY THE PRODUCTS APPEARING AS A FUNCTION OF TIME. !


Example 1: Reaction Rates

In this reaction, the concentration of butyl chloride, C4H9Cl, was measured at various times, t.

C4H9Cl(aq) + H2O(l) ⎯⎯→ C4H9OH(aq) + HCl(aq)

[C4H9Cl] M

Rate =Δ[C4H9Cl]

Δt


Calculating Reaction Rates

The average rate of the reaction over each interval is the change in concentration divided by the change in time:

C4H9Cl(aq) + H2O(l) ⎯⎯→ C4H9OH(aq) + HCl (aq)

Average Rate, M/s

7.


Reaction Rate Determination

Note that the average rate decreases as the reaction proceeds.

This is because as the reaction goes forward, there are fewer collisions between the reacting molecules.



Reaction Rates

A plot of concentration vs. time for this reaction yields a curve like this.

The slope of a line tangent to the curve at any point is the instantaneous rate at that time.

Secants yield the average rate



Reaction Rates

The reaction slows down with time because the concentration of the reactants decreases.

So there are less molecules to collide and react



HOW DO WE MEASURE RATES?

CHANGE IN MASS

CHANGE IN CONCENTRATION

CHANGE IN VOLUME

CHANGE IN PRESSURE

CHANGE IN COLOUR

CHANGE IN CONDUCTIVITY

CHANGE IN LIGHT ABSORPTION


Reaction Rates and Stoichiometry

In this reaction, the ratio of C4H9Cl to C4H9OH is 1:1.

Thus, the rate of disappearance of C4H9Cl is the same as the rate of appearance of C4H9OH.


Rate = -Δ[C4H9Cl]Δt

= Δ[C4H9OH]Δt


Reaction Rates & Stoichiometry

Suppose that the mole ratio is not 1:1?

Example H2(g) + I2(g) ⎯⎯→ 2 HI(g)

2 moles of HI are produced for each mole of H2 used.

The rate at which H2 disappears is only half of the rate at which HI is generated


TRY IT!

WHAT IS ALWAYS REPRESENTED BY THE SLOPE OF THE TANGENT TO A GRAPH OF

CONCENTRATION VS. TIME FOR A CHEMICAL REACTION?

A. THE ACTIVATION ENERGYB. THE ENTHALPY CHANGE

C. THE INITIAL RATE OF REACTIOND. THE AVERAGE RATE OF REACTION

E. THE INSTANTANEOUS RATE OF REACTION


TRY IT!

A REACTION TAKES PLACE IN AQUEOUS SOLUTION IN WHICH COLOURLESS SOLUTES REACT TO FORM A

BLUE-COLOURED SOLUTE AND A SOLID. WHICH OF THE FOLLOWING PIECES OF EQUIPMENT WOULD MOST LIKELY BE USED TO MEASURE THE RATE?

A. A GAS SYRINGEB. A BALANCE

C. A THERMOMETERD. A SPECTROPHOTOMETER

E. A PRESSURE SENSOR


TRY IT!

MOLTEN CRYOLITE, NA3ALF6, DECOMPOSES AS

SHOWN: NA3ALF6 --> 3NA(L) + AL(L) + 3F2(G)IF THE AVERAGE RATE OF FORMATION OF F2 (G) IS

0.75 MOL/S, WHAT IS THE AVERAGE RATE OF

NA3ALF6 IN MOL/S?

A. 2.25B. 0.75C. 0.42D. 0.25E. 0.013


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