potential energy curve

On the y axis why does it say -436 kJ/mol and -104 kcal/mol? What does this mean and why is the part where I drew a dot the stability point (where attractive and repulsive forces cancel), shouldn't the stability point be the part that's touching the x axis as this would show 0 energy?

I'm so confused can someone please help me

(edited 10 years ago)

Reply 1

tazmaniac97

OP

12

also does U stand for potential energy in this formula

F = - \frac{dU}{dr}

I'm confused because my lecturer wrote that U(r) stands for potential energy :confused:

(edited 10 years ago)

Reply 2

teachercol

9

U stands for potential energy

U(r) indicates that it is a function of r

KJ and Kcal are alternative units for the same thing.

Reply 3

teachercol

9

And youre confusing force/separation with energy separation.

Min energy = zero force

Reply 4

tazmaniac97

OP

12

Original post by teachercol

U stands for potential energy

U(r) indicates that it is a function of r

KJ and Kcal are alternative units for the same thing.

Original post by teachercol

And youre confusing force/separation with energy separation.

Min energy = zero force

Why is the energy negative?

Reply 5

teachercol

9

Potential energy is negative because of how we define zero energy.

Zero PE is when the two particles are infinitely separated and there is no force

You have to put energy in to separate particles (aka break bonds) so PE is negative.

Reply 6

tazmaniac97

OP

12

Original post by teachercol

Potential energy is negative because of how we define zero energy.

Zero PE is when the two particles are infinitely separated and there is no force

You have to put energy in to separate particles (aka break bonds) so PE is negative.

I still don't get it, the part where the dot is (in the diagram) is the stability point and it's where (according to my lecturer) is where the attractive and repulsive forces cancel out, but if they cancel out then shouldn't the energy be 0?

(edited 10 years ago)

Reply 7

Stonebridge

13

Original post by tazmaniac97

I still don't get it, the part where the dot is (in the diagram) is the stability point and it's where (according to my lecturer) is where the attractive and repulsive forces cancel out, but if they cancel out then shouldn't the energy be 0?

Why should it? Potential energy is something gained or lost as a result of a force moving its point of action.
If I applied a constant force to a spring to stretch it, it would expand until the force it was pulling back with equalled the force I was pulling it out with. At that point the forces are balanced. At that point there is potential energy in the spring as a result of the force having done work. The fact that the forces are balanced (me vs the spring) doesn't mean the potential energy is zero. You are confusing force and energy.

In the potential vs distance graph, the gradient (actually negative potential gradient) is equal to the force. To the right of the dip the gradient is positive (attraction) and to the left it's negative (repulsion). At the stable point at the bottom of the dip the gradient is zero and force is zero.

Reply 8

tazmaniac97

OP

12

Original post by Stonebridge

Why should it? Potential energy is something gained or lost as a result of a force moving its point of action.
If I applied a constant force to a spring to stretch it, it would expand until the force it was pulling back with equalled the force I was pulling it out with. At that point the forces are balanced. At that point there is potential energy in the spring as a result of the force having done work. The fact that the forces are balanced (me vs the spring) doesn't mean the potential energy is zero. You are confusing force and energy.

In the potential vs distance graph, the gradient (actually negative potential gradient) is equal to the force. To the right of the dip the gradient is positive (attraction) and to the left it's negative (repulsion). At the stable point at the bottom of the dip the gradient is zero and force is zero.