Kinetics (year 1) Flashcards
Rate of reaction
The change in concentration of a reactant per unit time
Why does the rate of the production of H2 gas (for example) decrease with time in the reaction : Mg + H2SO4 —–> MgSO4 + H2
The concentration of the sulfuric acid decreases with time. There are fewer H+ ions/fewer particles in the same volume so the frequency of collisions decreases. As a result, there are fewer successful collisions in a give time.
Suggest why pressure has little or no effect on the rate of reaction of magnesium and sulfuric acid
Pressure ONLY affects gaseous reactions
Activation energy
The minimum amount of energy needed by the reactant particles to react on collision
Why do most collisions not lead to a reaction
Reactions can only occur when collisoins take place between particles having sufficient energy - activation energy
Which factors increase the rate of a reaction
- Surface area
- Concentration
- Pressure
- Temperature
- Catalyst
How to increase the surface area of a solid
Use powdered (solid) or use small pieces of the solid reactant
Why does using a solid with a greater surface area increase the rate of reaction
The powder has a larger surface area so more particles are exposed to the acid particles. There is a greater surface area for collisions to take place with the acid particles. There are more frequent successful collisions in a given time
How would a graph of volume of product against time differ when you a reactant with a greater surface area
The line for the reaction using a greater surface area will be initially steeper because the rate of reaction is faster.
The final volume will be the same in both experiments because the moles of the acid are unchanged
How does an increase in concentration cause an increase in the rate of reaction
When the concentration increases, there are more particles per unit volume. There is a greater chance of the particles colliding so the frequency of collisions increases. As a result, there are more frequent successful collisions in a given time
How does an increase in pressure cause an increase in the rate of reaction
When pressure is increased, the particles are pushed closer together so there are more reactant particles per unit volume. The frequency of collisions increases therefore there are more frequency successful collisions per unit time.
How to increase the pressure
Reducing the volume of the container
Pumping more reactant gas into the container
How does increasing the temperature cause an increase in the rate of reaction
When temperature increases the particles move faster so they collide more frequently. Further, the particles collide with greater energy, so more of the reactant particles have enough energy greater than or equal to the activation energy to react on collision.
At a higher temperature there are more frequency successful collisions per unit time
How does a catalyst increase the rate of reaction
A catalyst provides an alternative route for the reaction. A greater proportion of reactant particles have enough energy greater than or equal to the activation energy to react on collision, so there will be more frequency successful collisions per unit time
Why must an experiment be repeated 3 times
To improve the reliability of the investigation
To identify anomalous results and discard them
To identify random errors and discard them
What does the area to the right of the activation energy under a Maxwell-Boltzmann curve represent
Number of particles with energy greater than activation energy
What does the area under a Maxwell - Boltzmann curve represent
Number of particles present (total)
Where is the mean energy of the particles found on a Maxwell-Boltzmann curve found
Not at the peak, slightly to the right
Why should a Maxwell-Boltzmann curve start at the origin
There are no molecules with no energy. All the particles are moving so have some energy
Why should a Maxwell-Boltzmann curve never touch the x axis
There is no maximum energy for the molecules
What does a Maxwell-Boltzmann curve look like at a higher temperature
The peak is lower and is more to the right
How does the area under a Maxwell-Boltzmann curve change with temperature
It DOES NOT CHANGE. Total area under each curve does not change
Why does the reaction between methane and oxygen not take place at room temperature
The mixture of methane and oxygen is kinetically stable because the activation energy is very high. There are virtually no particles with energy greater than or equal to the activation energy at room temperature.
Where to find the most probably energy on a Maxwell-Boltzmann curve
At the peak
What is a catalyst
A substance that increases the rate of a reaction but remains chemically unchanged
How would a Maxwell-Boltzmann curve look like when a catalyst is used
Identical, except the area area to the right of Ea is greater with a catalyst. The Ea will be lower and more to the left for a reaction using a catalyst.
What does a homogenous catalyst do
Provide a surface for the reaction to take place
What effect does a catalyst have on the energy distribution of gas molecules
No effect because the energies of molecules depends only on the temperature
What happens to number of particles per unit volume if you double the concentration
Number of particles per unit volume doubles
What happens to a Maxwell-Boltzmann curve if the concentration increases
The shape does not change, the curve will just be higher up as there will be more particles per unit volume
