Chemistry - 3.1.5: Kinetics

Why does the rate of reaction drop as a reaction proceeds?

The reactants are used up and their concentration falls, fewer collisions so lower rate of reaction

How does increasing the pressure of a gas reaction increase the rate of reaction?

More molecules in a given volume so more frequent collisions

How does increasing the surface area of solid reactants increase the rate of reaction?

The greater the total surface area, the more of its particles are able to collide with molecules, so there are more frequent collisions

How does using a catalyst increase the rate of reaction?

They provide a different route for the reaction with a lower activation energy, reducing the activation energy of the reaction so there are more successful collisions

Activation energy and its symbol

The minimum amount of energy that a particle needs in order to react (Ea)

Where is the transition state and what occurs here?

- It is the species that exists at the top of the curve of an enthalpy diagram
- Some bonds are in the process of being made, some bonds are in the process of being broken

Where is activation energy measured from?

From the reactants to the top of the curve

Maxwell-Boltzmann distribution

The distribution of energies (and so, speeds) of the molecules in a gas or liquid

3 things the Maxwell-Boltzmann distribution tells us

1. No molecules have 0 energy (curve starts from origin)
2. Most molecules have intermediate energies (region around peak of curve)
3. There is no upper limit (a few have very high energies so curve should not cross x-axis after origin)

On the Maxwell-Boltzmann distribution, what is the most probable energy (Emp)?

The peak of the curve

On the Maxwell-Boltzmann distribution, what is the average energy?

Slightly to the right of the peak of the curve

If you draw the Ea on a Maxwell-Boltzmann distribution, what does the area to its left and right mean?

- To its left: number of molecules that have energy less than the activation energy
- To its right: number of molecules that have energy more than the activation energy

Does the Maxwell-Boltzmann distribution curve lean to the right or the left?

The left

What happens to the Maxwell-Boltzmann distribution if the temperature increases?

- The peak of the curve is lower, moves to the right, number of molecules with high energy increases (peak energy increases, more molecules have higher energy so lower peak)
- More molecules have Ea - more collisions are successful and result in a reaction

What happens to the Maxwell-Boltzmann distribution if the temperature deceases?

- The peak of the curve is higher, moves to the left, number of molecules with high energy decreases (peak energy decreases, fewer molecules have higher energy so higher peak - lower Emp)
- Fewer molecules have Ea

What is the symbol for most probable energy?

Emp

What does the area under the curve of the Maxwell-Boltzmann distribution represent?

The number of molecules

Is the area under the curve of the Maxwell-Boltzmann distribution affected by changes in temperature?

No - the number of molecules is not affected by temperature

Catalyst

A substance that increases the rate of a chemical reaction but is chemically unchanged at the end (i.e. without being changed in chemical composition or amount)

Why are catalysts important in industry?

Usually cheaper to use instead of increasing temperature/pressure to speed up a reaction as catalysts are not used up

Do catalysts affect the enthalpy change of a reaction?

No

Do catalysts affect the position of equilibrium for a reversible reaction?

No

How do catalytic converters work? (4)

1. Honeycomb shape - larger surface area for reactions to take place
2. Adsorption: polluting gases first form weak bonds with metal atoms of catalyst, holding gases in correct position to react
3. Gases react
4. Desorption: Products break away from the metal atoms, freeing up room on the catalyst surface for more gases to react

Why does a small increase in temperature lead to a large increase in the rate of reaction?

Many more molecules have energy greater than or equal to the activation energy

2 things to remember when drawing a shift in as given Maxwell-Boltzmann distribution

1. Starts at origin
2. Must only cross original graph once

Why can you not carry out the practical at low temperatures?

The reaction would take too long

Kinetics

The study of factors that affect rates of chemical reactions

What is needed to happen in order for a reaction to occur?

The particles must collide with enough energy to break bonds (the activation energy) and between the parts of the molecule that are going to react together (correct orientation)

How does increasing the temperature increase the rate of reaction?

Molecules have more kinetic energy, so move faster and collide more frequently + more molecules have energy greater than/equal to the activation energy, so there are more successful collisions

How does increasing the concentration of a solution increase the rate of reaction?

More molecules in a given volume, so there are more frequent collisions

Do most collisions between molecules lead to a reaction?

No - they either do not have enough energy or are in the wrong orientation

Describe how to use sodium thiosulfate solution, hydrochloric acid and a cross to measure the rate of reaction. (7)

1. Using measuring cylinders (or pipette for more accuracy), measure 50cm3 of 0.05 mol of sodium thiosulphate and put in a boiling tube. In another- measure 5cm3 of HCl and place it in a different boiling tube.
2. Measure the initial temperature of the 2 solutions using a thermometer
3. Add the sodium thiosulphate to conical flask first and then the HCl. The conical flask should be placed over a white tile with a black cross
4. Start the stopwatch and measure the time it takes for the cross to disappear due to the cloudy suffer being produced.
5. Measure the final temperature, then calculate the mean temperature
6. Repeat with 4 more different temperatures (25, 35, 45 ,55 max) using water baths - add hot water and cold water to adjust temperature, use thermometer to measure if high enough then leave boiling tubes for 10 minutes in water bath so it reaches the desired temperature
7. Plot a graph of 1/t against temperature

Describe an alternate method (not disappearing cross) to measure the rate of reaction

1. Measure the amount of gas produced / the amount the mass of reactants falls by
2. Control temperature with water bath, use same volume etc