Flashcards in Module 3: Chapter 10 (Reaction rates and equilibrium) Deck (24)
What is the rate of chemical reaction?
How fast a reactant is used up OR how fast a product is formed
On a progress curve why is it the steepest at the start?
The rate is the fastest at the start of a reaction as each reactant is at its highest concentration
What are the factors that affect the rate of a chemical reaction?
- Concentration (or pressure if reactants are gases)
- Use of a catalyst
- Surface area of solid reactants
Describe Collision Theory?
For a reaction to occur reactant particles must come together and collide with:
- the correct orientation
- energy greater than the activation energy for the reaction
How does increasing the concentration affect the rate?
Low concentration - less particles in a given volume so less frequent collisions and therefore a slower rate of reaction
Higher concentration - more particles in a given volume so more frequent collisions and therefore a faster rate of reaction
How can the progress of a reaction be monitored?
By monitoring the removal of a reactant or monitoring the formation of a product, for example:
- monitoring volume of gas produced at regular time intervals
- monitoring the loss of mass using a balance
What is a catalyst?
1. A catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent change itself
· not used up itself
· may react with the reactant to form an intermediate or may provide a surface for the reaction to happen on
· at the end of the reaction it is always regenerated
2. A catalyst provides an alternative reaction pathway with a lower activation enthalpy
What is a homogenous catalyst?
A homogenous catalyst has the same physical state as the reactants
E.g. ozone depletion
Cl· radicals from CFC's catalyse the breakdown
2O3(g) <=Cl·(g)=> 3O2(g)
What is a heterogenous catalyst?
A heterogenous catalyst has a different physical state from the reactants
Describe the process of how catalysts work?
1. The reactant molecules form weak bonds with the catalyst surface - this is called adsorption
2. Bonds with the reactant molecules break
3. New bonds form
4. Product molecules leave the catalyst surface - this is known as desorption
How are catalysts used in industry?
- Catalysts increase the rate of many industrial chemical reactions by providing an alternative reaction pathway with a lower activation enthalpy
- This lowers the temperature needed for the reaction
- Lower temperatures means less electricity from combustion of fossil fuels used
- Less combustions leads to a reduction of CO2 emissions and other atmospheric pollutants
- Examples: Rhodium, Platinum and Palladium
What does a Boltzmann distribution curve show?
Shows the spread of molecular energies in a sample of gas molecules
Draw and label and label a Boltzmann distribution curve
- No molecules have zero energy, so the curve starts at the origin
- The area under the curve equals the total number of molecules
- There is no maximum energy for a molecule so the curve never meets the x-axis
- A line of Ea and shaded area shows the proportion of molecules with an energy greater than the activation energy
Describe the line on the diagram is it was at a higher temperature
The peak is lowered and shifted to the right
A greater of proportion of molecules have an energy greater than the activation energy
What happens at a higher temperature?
- more molecules have an energy greater than/equal to the activation energy
- a greater proportion of collisions will lead to a reaction, increasing the reaction rate
- collisions will be more frequent as the molecules are moving faster, which also increases the reaction rate
What happens to in the presence of a catalyst?
- A greater proportion of molecules exceeds the new lower activation energy
- more molecules collide with an energy greater than the activation energy
- more effective collisions, therefore faster rate of reaction
What happens in an equilibrium system?
- the rate of the forwards reaction is equal to the rate of the reverse reaction
- the concentrations of the reactants and products remain constant
- system must remained closed
What is Le Chatelier's principle?
When a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effects of that
· the position of equilibrium indicates how far the reaction has progressed when equilibrium is established
Describe what happens to the equilibrium for changing concentrations
A + B ⇌ C + D
· An increase in conc of A or B, or a decrease in conc in C or D causes the equilibrium position to move to the right (e.g. Add an acid, concentration of H+ ions increases so equilibrium position moves to the right/left)
· An increase in conc of C or D, or a decrease in conc of A or B the equilibrium concentration moves to the left
Describe what happens to the equilibrium for changing pressure
*only affects gases into gases*
2NO2(g) ⇌ N2O4(g)
· an increase in pressure equilibrium position move to the side with fewer gaseous molecules (here it would move to the right, colourless colour)
· a decrease in pressure equilibrium position moves to side with more gaseous molecules
Describe what happens to the equilibrium for changing temperature
- a decrease in temperature, equilibrium favours the exothermic process
- an increase in temperature, equilibrium will favour the endothermic process
What happens to equilibrium position when a catalyst is added?
- doesn't change the position of equilibrium
- it speeds up the rates of the forwards and reverse reaction equally
- it will increase the rate at which equilibrium is established
How is the equilibrium constant calculated?
[C]^c [D]^ [products]
Kc = ---------------- = -----------------
[A]^a [B]^b [reactants]