Activation Energy. Notes #22

Transcription

1 Activation Energy Notes #22

2 Energy (notes) All molecules store chemical-potential energy in the bonds between atoms Some molecules store more chemical energy than others Propane (the gas used in outdoor grills) contains high-energy bonds. When the reaction occurs, some of this energy is released as heat and light. The remaining energy is stored in the lower-energy bonds in water and carbon dioxide

3 Graphing Energy (notes) The energy of the reactants and products can be graphed, allowing us to compare how much energy each has. In the graph on the left the amount of chemical-potential energy (PE) is represented on the Y-axis. Time (progress of the reaction) is on the X-axis. In this particular graph it is showing that the reactants (A + B) have more energy than the products (C + D).

4 Favorable Reactions (notes) In general reactions in which the reactants have more energy than the products are favorable and will happen on their own When you put magnesium metal in water it automatically reacts, creating magnesium oxide (MgO).

5 Unfavorable Reactions (not notes) There are also unfavorable reactions, and these usually require energy being added to force them to go

6 Why Doesn t Everything Burn? (not notes) So here is the question: if favorable reactions happen automatically why don t they all go off immediately? For example, paper burning is a favorable reaction yet all the paper in this room isn t on fire just because there is oxygen

7 Activation Energy (notes) All reactions have an activation energy that is required to get them started This energy is usually higher than the energy of either the reactants or products Back to our graph: you can see how even though the reaction A + B C + D is favorable, there is a large hump in the middle. That is the activation energy.

8 Why doesn t everything burn, again? (not notes) So for a fire to start you have to have a spark: something that provides the energy to begin the reaction This is what striking a match does: the end of the match is a chemical compound that wants to react with oxygen, but can t until it gets the necessary energy from the friction of the match head against the box

9 Up a Hill (not notes) One way to think of it is like rolling a ball up and down a hill. The ball wants to roll downhill on its own (going from higher to lower potential energy), but it can t if there is a bump in the way. Activation energy is like that bump, keeping the ball from being able to go downhill unless something pushes it over.

10 Simulation (not notes) In your simulation today, the sides of the simulation are the same as the graph The height of the sides and ruler represent the amount of potential energy How fast the molecules are bouncing around represents their kinetic energy The teal line is the graph! The left side is the reactants and the right is the products, with the ruler in the middle representing the activation energy

11 Simulation (not notes) Important: in a reaction, molecules do NOT move physically from one side to another. Moving from one side to another is only a REPRESENTATION of changing the reactants to products When molecules move from the left side to right side in this simulation, it is representing a high energy reactant (such as propane) turning into a low energy product (such as carbon dioxide).

12 Activation Energy A molecular look (not notes) 7 th Grade Review: Molecules are always in motion Increasing energy (heat) is the molecules moving faster In order for a reaction to happen two molecules need to run into each other with enough speed Most of the time molecules don t have enough energy (speed) to start a reaction But if you add enough energy (the activation energy) then they will have enough to react when they hit each other

13 Collision Theory (not notes) With enough energy a reaction happens But just a bit short and they will bounce off each other