On my chem homework, a diagram called potention energy diagram is given. You need to be able to tell the potential energy of the products and reactants and the activation enrgy, the heat of reaction, and what would happen if a catalyst were added.
Okay, cool!
The diagram is set up so that on the horizontal x-axis is the "reaction coordinate," which is basically just time as the reaction proceeds. The vertical y-axis is energy.
The potential energy of the reactants is where the graph curve starts, touching the y-axis. The potential energy value is the y-value at this location.
Where the graph curve levels off at the end is the products, so the potential energy of the products is the y-value where the curve levels off.
The activation energy is how much energy had to be added into the system to get to the activated complex, located at the peak of the diagram’s curve. Take the y-value at the top of the curve and subtract from the the potential energy of the reactants.
The heat of reaction is the difference in the potential energies of the products and reactants. Take the potential energy of the products, and subtract from that the energy of the reactants.
If a catalyst were added, the potential energy of the products and reactants remain unchanged, but the activation energy is less, so you draw a curve that starts in the same place and ends in the same place as the original diagram, but your curve just won’t go up as high. The hill for a catalyst is less steep.
This answer assumes standard diagram with reaction coordinate along the x-axis, energy on the y-axis.
Reactants are on the left side of the diagram.
Products are on the right side.
The highest point along the reaction pathway is the transition state.
The change in the y value between the reactants and products is the heat of reaction (if products are higher, rxn is endothermic).
The change in the y-value between the reactants and the transition state is the activation energy. The activation energy is always >0.
A catalyst does not affect the energy of either the reactants or products. It only affects the transition state. The activation energy of a catalyzed reaction is lower than the activation energy of an uncatalyzed reaction.
References :
Basic thermodynamics.
Okay, cool!
The diagram is set up so that on the horizontal x-axis is the "reaction coordinate," which is basically just time as the reaction proceeds. The vertical y-axis is energy.
The potential energy of the reactants is where the graph curve starts, touching the y-axis. The potential energy value is the y-value at this location.
Where the graph curve levels off at the end is the products, so the potential energy of the products is the y-value where the curve levels off.
The activation energy is how much energy had to be added into the system to get to the activated complex, located at the peak of the diagram’s curve. Take the y-value at the top of the curve and subtract from the the potential energy of the reactants.
The heat of reaction is the difference in the potential energies of the products and reactants. Take the potential energy of the products, and subtract from that the energy of the reactants.
If a catalyst were added, the potential energy of the products and reactants remain unchanged, but the activation energy is less, so you draw a curve that starts in the same place and ends in the same place as the original diagram, but your curve just won’t go up as high. The hill for a catalyst is less steep.
References :
100%, AP Chem