Unit 1 - Reaction Profiles Flashcards
Describe an exothermic reaction
Energy is released to the surroundings.
- The temperature rises.
Describe an endothermic reaction
Energy is taken in from the surroundings.
- The temperature falls.
What does a potential energy diagram show?
The energy pathway for a reaction.
What is enthalpy?
The energy content or potential energy of a substance.
What is enthalpy change?
The energy difference between products and reactants.
- ∆H
What are the units for enthalpy?
Kilojoules (kJ) and Joules (J).
What are the units for molar enthalpy?
Kilojoules per mole (kJ mol⁻¹).
Sketch the reaction profile for an exothermic reaction
Reaction Progress on x axis.
Energy (kJ mol⁻¹) on y axis.
- Reactants have more energy than products.
- ∆H is negative.
- Activated complex is at the highest point of the reaction pathway.
Sketch the reaction profile for an endothermic reaction
Reaction Progress on x axis.
Energy (kJ mol⁻¹) on y axis.
- Products have more energy than reactants.
- ∆H is positive.
- Activated complex is at the highest point of the reaction pathway.
What sign does ∆H have for exothermic reactions?
Negative.
What sign does ∆H have for endothermic reactions?
Positive.
How would you calculate ∆H for a reverse reaction?
It is the same value as for the forward reaction but has the opposite sign (-v or +ve).
i.e. if forward reaction +50 kJ mol⁻¹ then reverse reaction will be -50 kJ mol⁻¹.
What is the activation energy?
The minimum energy required by colliding particles to form an activated complex.
What is the activated complex?
An unstable arrangement of atoms found at the maximum of a potential energy barrier during a reaction.
Sketch a Maxwell-Boltzmann distribution curve for a lower and higher temperature
Reaction Progress on x axis.
Number of particles on y axis.
- 2 curves on graph, one at lower temperature and one at higher temperature.
- Ea is fixed on the y axis.
- The lower temperature curve has only a small proportion of particles with energy greater than Ea.
- The higher temperature curve has a larger proportion of particles with energy greater than Ea. It appears to have moved over to the right compared to the lower temperature graph.
- Both curves have the same area underneath them.
What is temperature?
A measure of the average kinetic energy of the particles in a substance.
Explain how a small rise in temperature leads to a greater rate of reaction
An increase in temperature means a greater number of molecules with energy greater than the activation energy Ea.
- There are more successful collisions.
- Increased rate of reaction.
Describe how use of a catalyst increases rate of reaction
A catalyst provides an alternative pathway with a lower activation energy.
Sketch a potential energy diagram to show the effect of a catalyst on activation energy
Reaction Progress on x axis.
Energy (kJ mol⁻¹) on y axis.
- A potential energy diagram should be drawn showing 2 curves.
- The uncatalysed curve should have a higher Ea than the catalysed curve but both should start and end at the same point.
