2. 18.3 Enthalpy Changes in Chemical Reactions

3. Molecules are held together by chemical bonds. The chemical energy in a molecule is a measure of the energy found in the BONDS of that molecule. Molecules are held together by chemical bonds. The chemical energy in a molecule is a measure of the energy found in the BONDS of that molecule. In a chemical reaction, it requires energy to break a bond and energy is released when bonds are formed. In a chemical reaction, it requires energy to break a bond and energy is released when bonds are formed.

4. Two types of Reactions 1. Endothermic: A reaction that ABSORBS energy from the environment, and the energy required to break the bonds is greater than the energy released when the product bonds are formed. 1. Endothermic: A reaction that ABSORBS energy from the environment, and the energy required to break the bonds is greater than the energy released when the product bonds are formed. Usually, you must heat these reactions to proceed, or they get colder as they proceed. (i.e. cooking eggs or instant cold packs) Usually, you must heat these reactions to proceed, or they get colder as they proceed. (i.e. cooking eggs or instant cold packs) The energy term is on the Left Hand Side (LHS) of the equation. The energy term is on the Left Hand Side (LHS) of the equation.

6. In the previous diagram, the products have a greater chemical energy in their bonds than reactants. In the previous diagram, the products have a greater chemical energy in their bonds than reactants. Enthalpy (H): a measure of the total Potential Energy (PE) in a group of molecules. Enthalpy (H): a measure of the total Potential Energy (PE) in a group of molecules. H of Products = PE of products H of Products = PE of products H of Reactants = PE of reactants H of Reactants = PE of reactants

7. ∆H is the change in H (or PE) during the reaction. ∆H = H prod – H react ∆H is the change in H (or PE) during the reaction. ∆H = H prod – H react ∆H is POSITIVE for an endothermic reaction because H products > H reactants ∆H is POSITIVE for an endothermic reaction because H products > H reactants Activation Energy (∆E a ) is the Energy to convert reactants into the activated complex as it requires energy to break those reactant bonds. Activation Energy (∆E a ) is the Energy to convert reactants into the activated complex as it requires energy to break those reactant bonds.

8. Case 2: Exothermic: A reaction that releases energy. The energy required to break the reactant bonds is LESS than the energy released forming the product bonds. Case 2: Exothermic: A reaction that releases energy. The energy required to break the reactant bonds is LESS than the energy released forming the product bonds. Energy exits and heat/energy is released. Energy exits and heat/energy is released. (explosions, etc). The energy is on the Right Hand Side (RHS) of the equation.

10. In the previous diagram, the products have lower energy than reactants and ∆H is negative. In the previous diagram, the products have lower energy than reactants and ∆H is negative. In terms of written reactions, the energy term relates to moles in the balanced equation In terms of written reactions, the energy term relates to moles in the balanced equation Eg 1 : A + 2B + 400.0kJ  2C + D Eg 1 : A + 2B + 400.0kJ  2C + D 6.0 mol B x 400.0kJ = 1200 kJ absorbed 2 mol B 2 mol B Eg 2: 2A + B  C + 2D + 150.0 kJ Eg 2: 2A + B  C + 2D + 150.0 kJ 4.000 mol A x 150.0kJ = 300.0 kJ released 2 mol A 2 mol A

11. 18.3 MC ANS 18.3 MC ANS 1. D 7.D 2. B 8.D 3. B 9.C 4. D 10. B 5. B 11.A 6. C12.D

12. 1) I = activation energy for the forward rx. 1) I = activation energy for the forward rx. B) energy needed to change reactants into the activated complex. B) energy needed to change reactants into the activated complex.

13. 2) 2NH 3(g) + 92kJ  N 2(g) + 3H 2(g) 2) 2NH 3(g) + 92kJ  N 2(g) + 3H 2(g) 3)∆Ea= +200kJ 350 250 ∆H = +100kJ 150Energy of act. Cmplx = 350kJ