Topic 16 - Further Kinetics Flashcards
Equations to measure the rate of reaction?
Rate = change in conc of product/time
This is the concentration of product increases
Rate = -change in conc of reactant/time
This is where conc of reactant is decreasing giving positive value for the rate
Describe how to measure the rate of reaction by measuring the volume of a gas involved
You could either collection over water into measuring cylinder of using a gas syringe
Gas syringe gives greater degree of precision but if a large volume of gas is being measured then the difference in the degree of measurement uncertainty becomes small so either instrument is precise
You time how much at each interval and use ideal gas equation for moles and then molar ratio
Describe measuring rate of reaction by measuring the change in mass of a reaction mixture
Applicable when gas is involved - reaction flask and contents placed on digital balance and the decrease in mass is measured as reaction proceeds. Most precise when gas given off has relatively high density e.g. CO2 but with a low density like Hydrogen, mass changes are so small the measurement uncertainties become significant
Method of measuring the rate of reaction by monitoring a colour change (colorimetry)
Can be done with the naked eye but under certain circumstances greater precision can be obtained using a colorimeter - detect more subtle changes and provides a quantitative measurement. It measures absorbance (light absorbed b solution) the more concentrated the colour, the higher the absorbance is e.g. Reaction between propanone and iodine the brown colour fades so the absorbance decreases.
Methods of measuring the rate of reaction by analysis using titration
Use a pipette to remove an aliquot from a reaction mixture at regular intervals. Reaction in aliquot is quenched using an ice bath to stop the reaction - this is then titrated to determine the concentration of reactant or product species. If aliquot involves an acid it can be titrated against standard sodium hydroxide to investigate reaction between iodine and propanone
Measuring rate of reaction by measuring the electrical conductivity of the pH
Electrical conductivity - if number of ions in solution changes during a reaction it might be possible to follow the reaction by measuring changes in electrical conductivity using an conductivity meter
pH - if reaction produces/uses H+ ions, pH will change so measure pH at regular intervals and calculate concentration of H+ ions
What is a rate equation?
If a relationship where the rate is directly proportional to the concentration of a reactant A then an be represented by..
Rate = k[A]
Where k is the proportionality constant - this is a first order rate equation
Rate constant only change value with change in temperature e.g. Second order equal k[A]2 or zero order with respect to A is just k
Units of rate constants
Zero = mol dm-3 s-1 First = s-1 Second = dm3 mol-1 s-1 Third = dm6 mol-2 s-1
What is the overall order?
Reaction is the sum of all individual orders
Describe reaction mechanisms with regards to rate determining step
Orders of reaction suggest a possible mechanism for a relation. Mechanism cannot be inferred from stoichiometric equation since mathematical relationship between rate of reaction and concentration of reactants can be determined through experiments
Consider: A + B + C –> D + E
Rate = k[A][B]
Could mean C in such a large excess that changes in its concentration were not negligible and therefore no measurable effect on rate of reaction or if changes to [C] don’t have effect on overall rate of reaction then different explanation must be sought.
Must be step involving reaction between A and B that has effect on RoR and another step involving C that has no effect on rate of reaction. Assume A and B takes place before C has a change to react and reaction between A and B is significantly slower - step 1 A + B –> Z is slower and rate determining step before step 2 C + Z —> D + E
Rate determining is the. Slowest step and determines overall rate of reaction
What is the continuous method to determine the rate equation
One reaction mixture is made up and samples are withdrawn at regular time intervals - stopped by quenching. Concentration is then determined by experimental technique like titration - draw a conc-time graph and find out the half life for the reaction. If half life is a constant value then reaction is first order with respect to reactant
If half life doubles as reaction proceeds then it is second order
If graph straight one with negative gradient then rate of reaction is constant no matter what conc of reaction is so zero order with respect to the reactant
How to calculate the rate from concentration-time graph and volume-time graph
Draw a tangent to the curve at the given time and calculating the gradient of the tangent
Draw a triangle from the tangent and measure the X and y axis
Rate = -change in conc of reactant / time
Rate of reaction is called instantaneous reaction rate - exactly same method
Describe the initial-rate method for determining the rate equation
Several reaction mixtures are made up and the initial rate is measured - times can be calculate the mathematical relationship between RoR and con of the reactant
E.g. A + B –> C you can see patterns in the [A] and [B] and the initial rate of formation of C e.g. If [A] doubles and initial rate doubles then first order with respect to [A]
Determining order from a rate-concentration graph
Initial rate of reaction is proportional to the reciprocal of the time measured so rate = 1/t
Zero order = straight line
First Order = positive through origin
Second order = positive curve
Describe reaction mechanisms and determining order/mechanism order
Talk about elementary
Use NO2 + CO —> NO + CO2 rate = k[NO2]2
If reaction is elementary then single collision between two reactant particles
But if not elementary then not possible to deduce the rate equation from stoichiometric equation e.g. Decomposition of dinitrogen pentoxide into nitrogen dioxide and oxygen is first order with respect dinitrogen pentoxide not second
Takes place via series of interconnected elementary reactions that are called the mechanism of a reaction.
The NO2 + CO only rate = k[NO2]2 so isn’t elementary. Two possible rate determining steps are NO2 + NO2 –> N2O4 or NO2+NO2 –> 2NO + O2
Using second of two possible rate determining steps the following mechanism following mechanism consists of NO2 + NO2 –> 2NO + O2
CO + O2 –> CO2 + O and then NO + O -> NO2
But further investigation leads to NO2 + NO2 –> NO3 + NO and then NO3 + CO -> NO2 + CO2
