1. Lecture B

2. Day 4 Activities: Exothermic or endothermic? Objectives –Be able to identify exothermic and endothermic reactions based on temperature change.

3. Exothermic Reactions Heat is released from the chemical chemicals and into the surroundings. When you feel this, it is warm. Ex- out Examples: burning wood, heating pack, Combustion of Natural Gas, Neutralization of HCl with NaOH Most spontaneous chemical reactions are this type.

4. Endothermic Reactions Heat is absorbed by the chemicals and stored. When you feel this, it feels cold. En = enter Most endothermic reactions are non- spontaneous and require energy input. Example: Photosynthesis, instant ice pack, breaking up of water into hydrogen and oxygen.

5. Day 5 Activities: –Introduction: Enthalpy notation –Worksheet: Enthalpy intro Objectives: –Write chemical reactions with proper enthalpy notation. –Use notation to determine endothermic or exothermic

6. Enthalpy Enthalpy in chemistry is often talked about as stored ‘heat content’ of a chemical. So we use the symbol ‘H’ when talking about it. Energy is stored in the bonds of chemical compounds. When bonds break, energy is released. When bonds form, energy is absorbed.

7. Enthalpy So, in the process of a chemical reaction: –1) Energy is released when reactants break bonds –2) Energy is absorbed when products form bonds. The difference between energy released and energy absorbed is what determines whether it is an exothermic or endothermic reaction. If absorbed > released, then it will feel _____. That’s endothermic. If released < absorbed, then it will feel _____. That’s exothermic.

8. Enthalpy  H (delta H) means change in enthalpy. The enthalpy of a reaction is calculated by comparing the enthalpy of the products to the enthalpy of the reactants.  H reaction = H products – H reactants Therefore, if H products > H reactants, the reaction is endothermic and  H = (+) If H products < H reactants, the reaction is exothermic and  H = (-)

9. H 2 O(s)  H 2 O(l) Graphing Enthalpy Change CH 4 + 2O 2  CO 2 + 2H 2 O

10. Writing Enthalpy as Part of a Chemical Equation Once we know what the enthalpy change of a particular reaction is, it is good to include it as part of the equation. We can do this in one of two ways. –1) Include it within the reaction itself –2) Separate it from the equation as a ΔH notation.

11. Writing Enthalpy as Part of a Chemical Equation v.1 If we include the enthalpy change within the reaction –Exothermic has heat in the products –Endothermic has heat in the reactants CH 4 + 2O 2  CO 2 + H 2 O + 890 kJ 2H 2 O + 286 kJ  2H 2 + O 2

12. Writing Enthalpy as Part of a Chemical Equation v.2 The more common way of including enthalpy is to write it in ΔH notation. This separates the energy from that matter. In this format: –Exothermic gives ΔH = (-) –Endothermic gives ΔH = (+) CH 4 + 2O 2  CO 2 + H 2 O ΔH = -890 kJ 2H 2 O  2H 2 + O 2 ΔH = +286 kJ

13. Calculating Enthalpy Using a balanced equation, enthalpy can be calculated using stoichiometric ratios, just like we can calculate any other component in a reaction.

14. Law of Conservation of Energy Energy can not be destroyed or created but it is changed from one form to another. When you start a car, electrical energy is converted into mechanical energy. Kinetic Energy- Energy in Motion Potential Energy- Energy at Rest