Chapter 9 Rates of reaction Flashcards
The rate of a reaction
is the speed at which a chemical reaction takes place and has units mol dm-3 s-1 The rate of a reaction can be calculated by:
Collision theory
states that for a chemical reaction to take place the particles need to collide with each other in the correct orientation and with enough energy
-The minimum energy that colliding particles must have for a collision to be successful and a reaction to take place is called the activation energy
(a) shows an ineffective collision due to the particles not having enough energy whereas (b) shows an effective collision where the particles have the correct orientation and enough energy for a chemical reaction to take place
Increase in reaction rate
- The collision frequency is the number of collisions per unit time
- When more collision per unit time take place, the number of particles with energy greater than the Ea increases
- This causes an increase in rate of reaction
- A catalyst is a substance that increases the rate of reaction without taking part in the chemical reaction by providing the particles an alternative mechanism with a lower activation energy
Rate of Reaction: Concentration
- The more concentrated a solution is, the greater the number of particles in a given volume of solvent
- An increase in concentration causes in an increased collision frequency and therefore an increased rate of reaction
Rate of Reaction: Pressure
- An increase in pressure in reactions that involve gases has the same effect as an increased concentration of solutions
- When the pressure is increased, the molecules have less space in which they can move
- This means that the number of effective collisions increases due to an increased collision frequency
- An increase in pressure therefore increases the rate of reaction
Rate of Reaction: Experimental Calculations
- During a reaction, the reactants are used up and changed into the products
- This means that as the reaction proceeds, the concentration of the reactants is decreasing and the concentration of the products is increasing
- Because of this, the rate of the reaction is not the same throughout the reaction but changes
- The rate of reaction during the reaction can be calculated from a concentration-time graph.
When taking the measurements, the temperature should be kept the
the same at all times as a change in temperature will change the rate of reaction
Calculating the rate at the start of a reaction
At the start of the reaction, the concentration-time curve looks almost linear
-The rate at this point can therefore be found by treating the curve as a linear line and by using:
create more accuracy when Calculating the rate as the reaction proceeds
- The smaller the time intervals, the more accurate the reaction rate value is
- Even more accurate is to find the rate of reaction at different concentrations of reactant or product at particular time points
- This can be done by drawing tangents at several points on the graph
Activation Energy
For a reaction to take place, the reactant particles need to overcome a minimum amount of energy
exothermic reactions
the reactants are higher in energy than the products
endothermic reactions
the reactants are lower in energy than the products
-Therefore, the Ea in endothermic reactions in endothermic reactions is relatively larger than in exothermic reaction
the reaction will not take place
Even though particles collide with each other in the same orientation, if they don’t possess a minimum energy that corresponds to the Ea of that reaction
-Therefore, for a collision to be effective the reactant particles must collide in the correct orientation AND possess a minimum energy equal to the Ea of that reaction
Boltzmann distribution curve
is a graph that shows the distribution of energies at a certain temperature
- In a sample of a substance, a few particles will have very low energy, a few particles will have very high energy, and many particles will have energy in between
- The graph shows that only a small proportion of molecules in the sample have enough energy for an effective collision and for a chemical reaction to take place
Boltzmann distribution curve: Changes in temperature
- When the temperature of a reaction mixture is increased, the particles gain more kinetic energy
- This causes the particles to move around faster resulting in more frequent collisions
- Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction
- With higher temperatures, the Boltzmann distribution curve flattens and the peak shifts to the right
Therefore, an increase in temperature causes an increased rate of reaction due to
- There being more effective collisions as the particles have more kinetic energy, making them move around faster
- A greater proportion of the molecules having kinetic energy greater than the activation energy
A catalyst
is a substance that increases the rate of a reaction by providing the reactants with a different reaction pathway which is lower in activation energy than the uncatalyzed reaction
Boltzmann distribution curve: catalyst
provide the reactants another pathway which has a lower activation energy
- By lowering Ea, a greater proportion of molecules in the reaction mixture have sufficient energy for an effective collision
- As a result of this, the rate of the catalysed reaction is increased compared to the uncatalyzed reaction
