ENTHALPY

OBJECTIVES Exothermic changes cause heat to be released to the surroundings Endothermic changes cause absorption of heat from the surroundings. A potential energy diagram can be used to show the energy pathway for a reaction. The enthalpy change is the energy difference between products and reactants.

Enthalpy Diagrams Exothermic reactions energy course of reaction

Energy Level Diagrams Exothermic reactions energy course of reaction reactants

Energy Level Diagrams Exothermic reactions energy course of reaction reactants products

Energy Level Diagrams Exothermic reactions energy course of reaction reactants products energy given out ∆H is negative

Energy Level Diagrams Endothermic reactions energy course of reaction

Energy Level Diagrams Endothermic reactions energy course of reaction reactants

Energy Level Diagrams Endothermic reactions energy course of reaction reactants products

Energy Level Diagrams Endothermic reactions energy course of reaction energy taken in ∆H is positive reactants products

Summary Table Exothermic reactions Endothermic reactions

Summary Table Exothermic reactions Endothermic reactions Energy is given out to the surroundings Energy is taken in from the surroundings

Summary Table Exothermic reactions Endothermic reactions Energy is given out to the surroundings Energy is taken in from the surroundings ∆H is negative∆H is positive

Summary Table Exothermic reactions Endothermic reactions Energy is given out to the surroundings Energy is taken in from the surroundings ∆H is negative∆H is positive Products have less energy than reactants Products have more energy than reactants

∆H How much energy is given out or taken in? Energy is needed to break chemical bonds Energy is given out when bonds are made ∆H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products

C H H HH O O O O

C HHHHOOOO Energy absorbed to break bonds

C HHHHOOOO

C HHHHOOOO Energy released when new bonds are made

Overall energy change = Energy released on making new bonds - Energy taken in to break old bonds

Energy Level Diagrams Exothermic reactions energy course of reaction methane + oxygen carbon dioxide + water

Reaction mixture Test tube

Heat given out to surroundings EXOTHERMIC

Heat taken in from surroundings ENDOTHERMIC

Review of Exothermic Reactants Ep is higher than Products Ep. Now, we must consider the activation energy (the energy needed so that the reactants bonds will break and reform to make product)

Review of Endothermic Reactants Ep is lower than Products Ep. Need to add more energy to the system for the forward reaction to take place. Still need to consider activation energy

Activated Complex Is the short-lived, unstable structure formed during a successful collision between reactant particles. Old bonds of the reactants are in the process of breaking, and new products are forming Ea is the minimum energy required for the activation complex to form and for a successful reaction to occur.

Fast and slow reactions The smaller the activation energy, the faster the reaction will occur regardless if exothermic or endothermic. If there is a large activation energy needed, that means that more energy (and therefore, time) is being used up for the successful collisions to take place.

Watch the following Flash Review of what is occuring during a chemical reaction for both endothermic and exothermic. KNOW THIS!! chemistry/flash/activa2.swf chemistry/flash/activa2.swf

2.Analyze the activation energy diagram below. What is the Ea for the forward reaction? For the reverse reaction? What is the ΔH for the forward reaction? For the reverse reaction? What is the energy of the activated complex?

Answer The activation energy (Ea) for the forward reaction is shown by (a): Ea (forward) = H (activated complex) - H (reactants) = = 300 kJ mol-1 The activation energy (Ea) for the reverse reaction is shown by (b): Ea (reverse) = H (activated complex) - H (products) = = 100 kJ mol-1 The enthalpy change for the reaction is shown by (c): H = H (products) - H (reactants) = = +200 kJ mol-1 for the forward and reverse reaction.

The Collision Theory 1.Matter consists of moving particles. 2.As the temperature increases the particles move faster and collide more often and with more energy. 3.In chemical reactions bonds must be broken and new ones formed. 4.The energy for this comes from particle collisions. 5.The collisions have a variety of energy, as some are harder than others. 6.A collision energy diagram is a graph of the number of the collisions versus the energy of each collision.

Low Collision energy High 100 % Percent of Collisions With Energy 0 % Collision Energy Diagram SimulationSimulation

Low Collision energy High Collision Energy Diagram Activation Energy Ea- minimum energy required for a successful collision- to break the bonds! 100 % Percent of Collisions With Energy 0 %

Low Collision energy High Collision Energy Diagram Activation Energy Ea- minimum energy required for a successful collision- too break the bonds! 100 % Percent of Collisions With Energy 0 %

Low Collision energy High Collision Energy Diagram Activation Energy Ea- minimum energy required for a successful collision- too break the bonds! 100 % Percent of Collisions With Energy 0 % This area represents the fraction of collisions that do not have the Ea- not successful.

Low Collision energy High Collision Energy Diagram Activation Energy Ea- minimum energy required for a successful collision- too break the bonds! 100 % Percent of Collisions With Energy 0 % This area represents the fraction of collisions with the Ea -successful.

What happens to the number of successful collisions if we add a catalyst, which lowers the activation energy Ea? Watch!

Low Collision energy High Collision Energy Diagram Activation Energy Ea- minimum energy required for a successful collision- too break the bonds! 100 % Percent of Collisions With Energy 0 % This area represents the fraction of collisions with the Ea -successful.

Low Collision energy High Collision Energy Diagram Activation Energy Ea- minimum energy required for a successful collision- too break the bonds! 100 % Percent of Collisions With Energy 0 % Lowering the Ea increases successful collisions!

What happens to the number of successful collisions if we increase the temperature- so that the average collision energy is greater? Watch!

Low Collision energy High Collision Energy Diagram Activation Energy Ea- minimum energy required for a successful collision- too break the bonds! 100 % Percent of Collisions With Energy 0 %

Low Collision energy High Collision Energy Diagram Activation Energy Ea- minimum energy required for a successful collision- too break the bonds! 100 % Percent of Collisions With Energy 0 % Increasing the temperature increases successful collisions- increases rate! Push the graph down and right!

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires:

Collision Theory Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires:

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 1.Favourable Geometry

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 1.Favourable Geometry products

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 1.Favourable Geometry versus Poor Geometry products

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 1.Favourable Geometry versus Poor Geometry products

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 1.Favourable Geometry versus Poor Geometry products

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 1.Favourable Geometry versus Poor Geometry products

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 1.Favourable Geometry versus Poor Geometry no products products

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 1.Favourable Geometry versus Poor Geometry products no products

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 2.Sufficient Energy to break the chemical bonds

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 2.Sufficient Energy to break the chemical bonds

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 2.Sufficient Energy to break the chemical bonds

Collision Theory You need a collision to have a reaction. Collisions provide the energy required to break bonds. Most collisions are not successful A successful collision requires: 2.Sufficient Energy to break the chemical bonds Activation energy is the minimum amount of energy required for a successful collision.

The Collision Theory can be used to explain how the rate of a reaction can be changed. Reaction rates can increase due to 1.More collisions 2.Harder collisions- greater collision energy 3.Lower activation energy or Ea, which allows low energy collisions to be more effective. And that’s it!

The Collision Theory can be used to explain how the rate of a reaction can be changed. 1.Increasing the temperature increases the rate because there are:

The Collision Theory can be used to explain how the rate of a reaction can be changed. 1.Increasing the temperature increases the rate because there are: More frequent collisions

The Collision Theory can be used to explain how the rate of a reaction can be changed. 1.Increasing the temperature increases the rate because there are: More frequent collisions Harder collisions

The Collision Theory can be used to explain how the rate of a reaction can be changed. 2.Increasing the reactant concentration increases the rate because there are:

The Collision Theory can be used to explain how the rate of a reaction can be changed. 2.Increasing the reactant concentration increases the rate because there are: More frequent collisions

The Collision Theory can be used to explain how the rate of a reaction can be changed. 3.Adding a catalyst increases the rate because:

The Collision Theory can be used to explain how the rate of a reaction can be changed. 3.Adding a catalyst increases the rate because Lower activation energy or Ea, which allows low energy collisions to be successful Movie-Movie- The catalyst KI is added to H 2 O 2, food colouring, and dishwashing detergent. The O 2 produced makes foam.

The Collision Theory can be used to explain how the rate of a reaction can be changed. 4.Changing the nature of the reactant for a more reactive chemical changes the rate because

The Collision Theory can be used to explain how the rate of a reaction can be changed. 4.Changing the nature of the reactant for a more reactive chemical changes the rate because Lower activation energy or Ea, which allows low energy collisions to be more effective

The Collision Theory can be used to explain how the rate of a reaction can be changed. 5.Increasing the surface area of a solid reactant increases the rate because:

The Collision Theory can be used to explain how the rate of a reaction can be changed. 5.Increasing the surface area of a solid reactant increases the rate because: More frequent collisions

Explain each Scenario Using the Collision Theory 1.A balloon full of H 2 and O 2 do not react at room temperature. A small spark ignites causes an explosion.

Explain each Scenario Using the Collision Theory 1.A balloon full of H 2 and O 2 do not react at room temperature. Ea is too high for the room temperature collisions A small spark ignites causes an explosion.

Explain each Scenario Using the Collision Theory 1.A balloon full of H 2 and O 2 do not react at room temperature. Ea is too high for the room temperature collisions A small spark ignites causes an explosion. The spark provides the Ea and it explodes because it is exothermic

Explain each Scenario Using the Collision Theory 2.A candle does not burn at room temperature A match causes the candle to burn. The candle continues to burn

Explain each Scenario Using the Collision Theory 2.A candle does not burn at room temperature Ea is too high for the room temperature collisions A match causes the candle to burn. The candle continues to burn

Explain each Scenario Using the Collision Theory 2.A candle does not burn at room temperature Ea is too high for the room temperature collisions A match causes the candle to burn. The match provides the Ea The candle continues to burn