Chapter 6: Chemical Kinetics Flashcards
What is the rate of a chemical reaction?
The speed at which reactants are used up or products formed
The change in concentration of reactants or products per unit time
How fast or slow the reaction is.
This can be thought of in terms of how quickly the reactants are used up or how quickly the products are formed
What are two ways in which the rate of reaction can be measured?
- measurement of the rate at which COâ‚‚ is produced
- measurement of the rate at which the mass decreases
What are possible problems with experiments using a bung on a flask?
The gas is likely to escape and the pieces (e.g. calcium carbonate pieces) variating in size
How can the average rate or the gradient be measured?
If a reaction is strongly exothermic what will happen to the temperature of the mixture and the rate?
Reaction mixture becomes hotter during the experiment
Cause rates to be higher
What does the cotton wool do in this experiment?
It allows the gas to escape but stops the mass being lost as a result of splashes
What is the rate of reaction?
the change in concentration of reactants or products per unit time
What colour is iodine?
Brown
How does the colour change occur?
Iodine is brown and all the other species are colourless
the reaction mixture fades from brown to colourless as iodine is used up in the reaction and the decreased in the absorption of light can be meausred using the colorimeter
What can a colorimeter be used to measure?
the amount of light absorbed at a particular wavelength - the darker the colour of the sample, the more light is absorbed
What is stated in the collision theory? What are the two conditions that must be fulfilled for a reaction?
For a reaction to occur particles must collide
- A collision must involve more than a certain minimum amount of energy
- Molecules must collide with the correct orientations
Do all collisions result in a reaction?
No because they must also have a minimum amount of energy and collide in the correct orientations
What is activation energy?
The minimum amount of energy that colliding particles must possess to result in a reaction
What is called when a collision result in a reaction?
Successful or effective collision
What is the highest point along the curve called?
Transition state
What is this graph?
a potential energy profile, showing the activation energy for an exothermic reaction
What is happening here in the reaction between two particles, A and BC?
- as the two particles approach, repulsion between the atoms (internuclear and beween electons) causes an increase in the potential energy
- The B-C bond begins to break and the A-B bond begins to form
- At the transition state (activated complex) along the curve, all three atoms are joined together by partial bonds (A- - B - - C)
- As the A-B bond continues to form, the potnetial energy falls (bond-making releases energy)
What are the factors affecting reaction rate?
- Concentration of reactants
- Pressure for (reactions involving gases)
- Surface area of solid reactants
- Temperature
- Catalysis
What is the effect of concentration on the reaction rate?
With more particles in a certain volume, the particles collide more often (the collision frequency is higher) and therefore there is a greater chance of a successful collision (i.e. one that results in a reaction) occurring in a certain time (need to have an idea of time in answer)
What is the effect of pressure on the reaction rate?
The effect of increasing the pressure is essentially the same as that of increasing the concentration of gaseous reactants. As the pressure is increased, the collision frequency increases
Only reactions involving gases are significantly affected by changing the pressure
What is the effect of surface area of solid reactants on reaction rate?
Reactions generally only occur at the surface of a solid.
Making a solid more finely divided increases the surface area and therefore the number of particles exposed at the surface → greater chance of a particle of the other reactant colliding with a particle on the surface and reaction occuring
What is the relationship between temperature and the energy of particles in a gas?
For an ideal gas: The average kinetic energy of the particles in a gas is proportional to its temperature in Kelvin
What is the effect of temperature on rate of reaction?
As the temperature increases, the rate of reaction increases exponentially
As temperature increases, the molecules have more energy and therefore move faster. This means that the collision frequency increases.
Not only do they collide more often, but they also collide harder, that is, with more energy, so that there is a greater chance of a successful collision
At the same temperature, do lighter or heavier particles travel faster?
light particles travel faster
What does the Maxwell-Boltzmann distribution look like?
What are 4 features of the Maxwell-Boltzmann distribution?
- it is not symmetrical
- no molecules have 0 kinetic energy
- at higher energy the line does not reach the energy axis
- the area under the curve represents the total number of particles and will not change as the temperature changes
What is the main reason that the rate of reaction increases with temperature?
there is an increase in the number of particles with energy greater than or equal to the activation energy
How does the temperature affect the Maxwell-Boltzmann distrbution?
At higher temperatures
* the curve is flatter
* the maximum has moved to the right
* there are fewer particles with lower energy and more particles with higher energy
with more particles having energy greater or equal to the activation energy at the higher temperature, a greater proportion of collisions will be successfull, and therefore the rate of reaction will increase
Why is the area underneath the curve still the same?
because the number of particles (amount of substance) does not change if the temperature is increased
What is a catalyst?
A catalyst is a substance that increases the rate of a chemical reaction without itself being used up in the reaction
It acts by allowing the reaction to process by an alternative pathway of lower activation energy
Why is the catalyst often written abov e the reaction arrow?
because it does not appear in the chemical equation because it does not change in the reaction
Why are catalysts important in many industrial processes?
Without the catalyst, the reaction occurs very slowly, but it is very rapid once the catalyst has been added
i.e. manganese (IV) oxide in the decomposition of hydrogen peroxide solution
What is the effect of a catalyst on the Maxwell-Boltzmann distrubtion?
