Topic 6 Flashcards
Collision theory
Reactant particles must collide with the correct orientation
Reactant particles must also collide with sufficient energy to overcome the energy barrier for the reaction, which is called the activation energy (Ea).
Activation energy definition
The activation energy (Ea) of a reaction is the minimum amount of kinetic energy that colliding particles must have for a chemical reaction to occur.
Rate of reaction definition
Change in concentration of a product/change in time
Transition state definition
The transition state is the highest energy state on a reaction coordinate;
It indicates a point at which new bonds are being formed at the same time as old bonds are being broken.
Where on a graph is the activation energy
Difference between transition state and reactant concentration
What is the area under graph of Maxwell Boltzmann equal to?
Total number of particles in the sample
How does increasing temperature change Maxwell Boltzmann curve?
The particles gain kinetic energy and the curve flattens out.
The area under the curve is the same because the number of particles is constant
The most probable value of kinetic energy has increased
Maxwell Boltzmann axes?
Y = number of particles
X = kinetic energy
The instantaneous rate of reaction?
The instantaneous rate of reaction is the rate of reaction at any particular point in time.
Units for rate of reaction
mol dm^-3 s^-1
Describe how a time v concentration of a product graph looks like
At the start of the reaction, the rate is fastest; this is shown by the gradient being the steepest. At this point in time, the concentration of reactants is at its highest, which means that there will be a high frequency of collisions between reactant particles.
As the reaction proceeds, the gradient becomes less steep as the concentration of reactants continues to decrease and there are fewer collisions between reactant particles.
Once all the reactants have been consumed in the reaction, there is no more product formed and the line becomes horizontal.
How to measure rate of reaction
VOLUME OF GAS PRODUCED:
Use a gas syringe to measure the volume of a gas collected
AT REGULAR TIME INTERVALS
CHANGE IN MASS:
The rate of production of a gas can also be measured using a mass balance, as shown in Figure 3. In this experiment, the change in mass per unit time is measured as the carbon dioxide escapes from the conical flask
MEASURE THE CHANGE IN ION CONCENTRATION:
For a solution to conduct electricity, it must contain free-moving ions. Where a reaction involves ions, we can determine the rate of reaction by using a conductivity meter to measure the change in ion concentration of an aqueous solution. Reactions that involve aqueous ions include acid–base reactions (which involve changes in hydrogen ion (H+) concentration). The increase or decrease in hydrogen ion concentration can be measured using a pH probe
FORMATION OF A PRECIPITATE
The reaction between hydrochloric acid and sodium thiosulfate is an example of a clock reaction in which the time taken for a reaction to reach a certain point is measured. In this reaction, it is the formation of a precipitate of yellow sulfur which obscures a cross drawn under the reaction mixture (Figure 6). By varying either the concentration or temperature of the acid or sodium thiosulfate solution, the effect of the change can be measured by recording the time taken for the cross to be obscured.
Na2S2O3 (aq) + 2HCl (aq) → 2NaCl (aq) + SO2 (g) + H2O (l) + S (s)
Measuring the change in concentration by titration
The final method we will look at is quenching (or stopping) a reaction. This involves removing a sample of the reaction mixture and quenching the reaction by placing it in cold water (or ice). We can then determine the concentration of either reactant or product by titrating with a standard solution
An example of a reaction that can be analysed by redox titration is the one between iodine and propanone. These substances react in the presence of a sulfuric acid catalyst according to the following equation:
CH3COCH3 (aq) + I2 (aq) → CH3COCH2I (aq) + HI (aq)
Samples of the reaction mixture are removed at regular time intervals and quenched with sodium hydrogencarbonate solution. This removes the acid catalyst, quenching the reaction. The amount of iodine remaining can then be determined by titration with sodium thiosulfate solution, with a starch indicator.
How does temperature affect the rate of reaction
Firstly, the frequency of collisions between reactant particles increases because particles have more kinetic energy at higher temperatures.
Secondly, and more importantly, at a higher temperature, a greater proportion of reactant particles will collide with energy equal to or greater than the activation energy for the reaction.
Describe effect of temperature on MX boltzmann curve
If the temperature of a sample of gas is increased, a number of changes to the shape of the Maxwell–Boltzmann distribution take place
The peak of the distribution curve shifts to the right, which means there is an increase in the most likely value for the kinetic energy of the particles. In other words, at higher temperatures, the average kinetic energy of the particles increases.
The curve flattens, becoming broader so that the total area under it remains constant. This is logical as there is the same number of particles in the sample.
There is an increase in the area under the curve to the right of the activation energy value, Ea. At higher temperatures, a greater proportion of particles will have energy equal to or greater than the activation energy, Ea.
How does increasing the concentration affect rate of reaction
Increasing the concentration of a solution increases the rate of reaction. Figure 3 shows the reaction of zinc with both diluted and concentrated solutions of hydrochloric acid.
The rate of reaction is greater in the concentrated solution because of the increased frequency of collisions between the hydrogen ions and the zinc.
An increase of collisions results in a greater probability of successful collisions between the reactant particles and consequently an increase in the rate of reaction.
How does increasing pressure affect rate of reaction
Increasing the pressure for a reaction that involves gases means that there are more gas particles in a given volume, effectively increasing the concentration. This results in an increase in the frequency of collisions between reactant particles and an increase in the rate of reaction
How does surface area affect rate of reaction
Breaking up a solid into smaller pieces increases the surface area per unit volume and causes an increase in the rate of reaction. Figure 5 shows the increase in surface area as a solid is broken up into a powder. The rate of reaction would get progressively faster as the surface area per unit volume increases.
How does a catalyst affect rate of reaction
Catalysts lower the activation energy of a reaction - they provide an alternative reaction pathway that has a lower activation energy.
It is important to note that a catalyst remains chemically unchanged at the end of the reaction and in some cases the catalyst can be re-used, which provides a further economic benefit
What changes the value of ‘k’ (rate constant)
Temperature
Catalyst
How to calculate overall order of reaction?
Add up exponents in rate equation
1st order reaction units
s-1
2nd order reaction units
mol-1 dm3 s-1
3rd order reaction units
mol-2 dm6 s-1
0 order rate units
mol dm3 s-1
Half lives
The half-life (t1/2) is the time it takes for the concentration of a reactant to decrease by half.
What does factor A in the arrhenius equation relate to?
Constant - collision orientation of particles
Using the following graphical data, calculate the value of the Arrhenius constant. Explain the steps
Find the y-intercept which is Ln(A)
Ln(A) = whatever Y intercept is
e^y intercept = final answer
EASY!
What is the x and y axis label for the arrhenius constant graph
X axis = 1/t
Y axis = Ln(k)
Y = mx + c form of arrhenius equation
Ln(k) = -Ea/rt + lnA
Y = mx + c form of arrhenius equation
Ln(k) = -Ea/rt + lnA
Gradient is -Ea/r
Activation energy UNITS
J mol^-1
OR
KJ mol^-1
