1.5 kinetics Flashcards
Collision theory
Collision theory says that a reaction won’t take place between two particles unless:
* they collide with correct orientation
* they collide with a kinetic (movement) energy greater than the activation energy
activation energy
The activation energy is the minimum amount of kinetic energy particles need to react. This much energy is needed to break the bonds within reactant particles to start the reaction.
Reactions with low activation energies often happen pretty easily. But reactions with high activation energies don’t. You need to give the particles extra energy by heating them.
You can show the activation energy of a reaction on an enthalpy profile diagram
Maxwell-Boltzmann distribution
If you plot a graph of the numbers of molecules in a gas with different kinetic energies you get a Maxwell-Boltzmann distribution.
The Maxwell-Boltzmann distribution is a theoretical model that has been developed to explain scientific observations.
area under a Maxwell-Boltzmann distribution curve
The area under a Maxwell-Boltzmann distribution curve is equal to the total number of molecules.
The effect of temperature on reaction rate
If you increase the temperature of a gas, the molecules will on average have more kinetic energy and will move faster. This results in more frequent collisions and therefore a greater proportion of molecules will have at least the activation energy and be able to react.
The effect of temperature on reaction rate on a Maxwell-Boltzmann distribution curve
This changes the shape of the Maxwell-Boltzmann distribution curve - it pushes the peak lower and to the right. The total number of molecules is
still the same, which means the area under each curve must be the same.
The effect of concentration (of solutions) / pressure (of gases) on reaction rate
If you increase the concentration of reactants in a solution, the particles will on average be closer together. This leads to more frequent successful collisions and a greater rate of reaction.
If the reaction involves gases, increasing the pressure of the gases works in just the same way. Raising the pressure pushes all of the gas particles closer together, increasing the number in a given volume. This leads to more frequent successful collisions and a greater rate of reaction.
What is a catalyst?
A catalyst increases the rate of a reaction by providing an alternative reaction pathway
with a lower activation energy. The catalyst is chemically unchanged at the end of the reaction.
Catalysts save heaps of money in industrial processes.
catalyst Example - Haber-Bosch process
The Haber-Bosch process uses an iron catalyst to increase the rate of forming ammonia from nitrogen and hydrogen
Conditions are around 400-500 ℃ and 20 MPa.
Maxwell-Boltzmann distribution curve with a catalyst
With a catalyst present, the molecules still have the same amount of energy, so the Maxwell-Boltzmann distribution curve is unchanged. But because the
catalyst lowers the activation energy, more of the molecules have energies above this threshold and are able to react,
Calculating reaction rates
Rate of reaction is the change in the amount of a reactant or product over time. The units of reaction rate will be ‘change you’re measuring / unit of time’
(e.g. g s-1 or cm3 s-1).
Ways of measuring the rate of a reaction
- Time taken for a precipitate to form - You can use this method when the product’s a precipitate that clouds a solution.
- Change in mass - When the product is a gas, its rate of formation can be measured using a mass balance.
- Gas volume - If a gas is given off during a reaction, you can measure reaction rate by
collecting it in a gas syringe and recording how much you’ve got at regular
time intervals.
Ways of measuring the rate of a reaction - Time taken for a precipitate to form
When you mix colourless sodium thiosulfate solution and colourless
hydrochloric acid solution, a yellow precipitate of sulfur is formed:
Na2S2O3 (aq) + 2HCI (aq) => 2NaCI (aq) + SO2(g) + S (s)
You can stand a conical flask on top of a white tile with a black mark on it. Then you add fixed volumes of the reactant solutions to the flask and start a stopwatch. Look through the solutions to observe the mark
on the tile. As the precipitate forms, the mark will become harder to see clearly. Stop the timer when the mark is no longer visible. The reading on the timer is recorded as the time taken for the precipitate to form.
You can repeat this reaction for solutions at different temperatures to investigate how temperature affects reaction rate. Use a water bath to gently heat both solutions to the desired temperature before mixing them. The volumes and concentrations of the solutions must be kept the same each
time. The results should show that the higher the temperature, the less time it takes for the mark to disappear and the faster the rate of the reaction gets.
Ways of measuring the rate of a reaction - Change in mass
When the reaction starts, start a stop clock or timer, then read off the mass at
regular time intervals. You’ll know the reaction is finished when the reading on the mass balance stops decreasing.
This method is very accurate and easy to use but does release gas into the room, which could be dangerous if the gas is toxic or flammable. So it’s best to carry out the experiment in a fume cupboard.
You can repeat this reaction for acids at different temperatures to investigate how temperature affects reaction rate. All other experimental variables must be kept the same. The results should show that the higher the temperature, the faster the mass decreases and the faster the reaction rate gets.
Ways of measuring the rate of a reaction - Change in Gas volume
start a stop clock or timer when the reaction starts, then read off the volume of gas in the gas syringe at regular time intervals. You know that the reaction has finished when the gas volume stops increasing.
This method is accurate because gas syringes usually give volumes the nearest 0.1 cm3.
Because no gas escapes, you can use this method for reactions that produce toxic or flammable gases (although you should still do reactions like these in a fume cupboard to be safe). Vigorous reactions can blow the plunger out of the syringe, so you should do a rough calculation of how much gas you expect the reaction to produce before you begin. Then you can use an appropriate size of gas syringe.
The reaction can be repeated with the acid at different temperatures to investigate the effect on reaction rate. The results should show that the higher the temperature, the faster the gas is produced, and therefore the faster the rate of reaction.
