Chapter 5: Kinetics Flashcards
What is the Collision Theory?
For a chemical reaction to happen:
- Particles need to collide
- Have correct orientation
- Have minimum amount of energy (activation energy)
What are the factors that affect the rate of reaction?
- Increasing Temperature
- Increasing Concentration of a Solution
- Increasing the Pressure of a Gas
- Increasing the surface area of a solid
- Using a catalyst
Which factor affects the rate of reaction (energy) and which ones affect the successful collisions?
- Increasing Temperature increases the energy
- Increasing conc, pressure, surface area, and use of catalyst increases successful collisions
What does the rate of a reaction mean?
Changing the concentration of a reactant per unit time
What does the Activation Energy mean?
“The minimum energy which particles need to collide and start a reaction”
What is a reagent?
Chemical you are using in a reaction
What does Intermediates mean?
Means the middle/half way point
What does Enthalpy mean?
- Is heat changed in constant pressure
In an Endothermic reaction, products have ….
In an exothermic reaction products…
- Absorbed energy
- Released energy to the surroundings
What is the Maxwell-Boltzmann distribution?
- Energy of a particle in a gas is linked to its speed
- A plot of energy against number of particles produces a distinctive shape
- This is called Maxwell-Boltzmann distribution curve
What are particles like at the start, between extremes and end?
- Few particles have low energy because collisions cause some particles to slow down
- Most molecules have energies between extremes becuase the distribution is not symmetrical
- Only a few particles have high energy
What is the area under the curve mean in a Maxwell Boltzmann distribution curve?
How many particles there are - if you dont change the number particles
What is the general shape of a Maxwell-Boltzmann distribution curve
- The graph starts at the origin - No particles have no energy
- It never touches the x-axis again - cause all particles have some energy, there is no maximum energy for molecules
What is the most probable energy
Where is the average energy
- The highest point on the curve. This represents the most likely energy of any single molecule
- Is just to the right of the most probable as the graph isnt symmetrical
What happens when there is a temperature increase in a maxwell boltzmann distribution curve
- The peak of the curve is lower and moves to the right
- The number of particles with higher energy increases
- The total area under the curve is the same
- At higher temperatures more particles have energy greater than the activation energy so a higher % of collisions will start a reaction → reaction rate increases
What happens with an increase of concentration on a M-B curve
- Most probable stays the same
- There are more particles so there is a higher peak and the area under the curve will be greater
- This means there are more molecules available to collide - more molecules with energy greater than activation energy
What do catylytic converters do
- Reduce pollution in cars by removing toxic gases from exhaust fumes
- The gases pass over a ceramic honeycomb
- As the car warms up , the ceramic honeycomb reaches its operating temperature and catalyses the reaction between the gases - the polluting gases react with eachother to produce less harmful gases
How do you calculate the rate of the reaction from a graph
- When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction at that moment
What is the trend of a concentration-time graph
- The rate decreases as the reactant is used up
- The initial rate is the rate at the start of the reaction (when t=0) where it is fastest
What is a catalyst
- A substance that speeds up the reaction but is chemically unchanged at the end
- Catalysts provide a different reaction pathway with a lower activation energy
- The Maxwell Boltzman distribution curve is unchanged however more particles have energy greater than the activation energy.
How does increasing temperature affect the rate of reaction
- Increases the speed of the molecules which increases energy and the number of successful collisions
How does increasing concentration and pressure increase the rate of reaction
- At higher concentrations(and pressures) there are more particles per unit volume
- and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions
- However as reaction proceeds reactants are used up and so reaction drops
How do reactions take place on the catalyst
- The gases form weak bonds with the metal atoms of the catalyst (adsorbtion). This holds the gases in the right position for them to react together. The gases then react on the surface
- The products break away from the metal atoms (desorbtion). This frees space so more gases can react
The bonds must be strong enough to hold the gases on the surface but weak enough to release the products easily
Why do some molecules have very low energies
- Because collisions cause some particles to slow down and lose energy
Why is a small increase in temperature has a large effect on the initial rate of a reaction
- A small increase in temperature results in many more/ significantly higher proportion of particles with energy greater than the activation energy
Explain why a decrease in temperature decreases the rate of reaction
- Decrease in the proportion of molecules that have energy greater than the activation energy
- Therefore fewer effective/ successful collisions
What are the 3 different ways to investigate how the rate of reaction changes with temperature
- Time taken for a percipitate to form
- Change in mass
- Gas volume
How is time taken for percipitate formed carried out
You mx colourless sodium thiosulfate and colourless Hydrochloric acid to form a yellow percipitate of sulfur.
- Stand a conical flask on a white tile and put a black mark on
- Put fixed volumes of reactants to the flask and start stopwatch
- When mark is no longer visible stop stopwatch
- Repeat this reaction with different temperatures using a water bath. Volumes of reactants must be same eachtime
How do you work out the change in mass
- When the product is a gas the rate of formation can be measured using a mass balance
- When reaction starts start a timer and read off mass at regular intervals. Wait till mass balance stops decreasing. Gas escapes top.
- Can repeat this using different temperatures. Higher the temp - faster the reaction
How do you use gas volume to investigate how rate of reaction changes with temperature
If a gas is given off, measure rate by collecting it in a gas syringe and recording the amount at regular intervals
- Start a timer when reaction starts and read off volume of gas syringe at regular intervals. Gas volume stops increasing when reaction is finished.
- Gas syringes are accurate (give results to 0.1cm3. Can use this method for flammable/ toxic gases (nothing escapes)
- The reaction can be repeated with different temps
