Energy and Chemical Change Grade 11 PDF

Energy and Chemical Change Grade 11 Chapter 15 (pg.207-214)  2 Course of a Chemical Reaction Reactants Products 𝑁2 + 3 𝐻2 → 2 𝑁𝐻3  Energy is ALWAYS absorbed from the surroundings at the start of a reaction 3 to...

Energy and Chemical Change Grade 11 Chapter 15 (pg.207-214)  2 Course of a Chemical Reaction Reactants Products 𝑁2 + 3 𝐻2 → 2 𝑁𝐻3  Energy is ALWAYS absorbed from the surroundings at the start of a reaction 3 to break existing bonds. Energy is ALWAYS released to the surroundings at the end of a reaction as new bonds are formed. Definition: An activated complex is the unstable transition state from reactants to products. Definition: Activation energy (𝑬𝑨 ) is the minimum energy needed for a reaction to take place. Definition: A (positive) catalyst is a compound which speeds up the reaction rate by lowering the activation energy without undergoing any chemical change itself.  4 Heat of Reaction Enthalpy (𝑯) is a measure of the internal energy of a chemical system. Only changes in enthalpy (∆𝐻) can be measured. Definition: The heat of reaction (aka change in enthalpy) (∆𝑯) is the energy absorbed or released per mole in a chemical reaction. Mathematically: ∆𝐻 = 𝐻products − 𝐻reactants Practically: 𝐞𝐧𝐞𝐫𝐠𝐲 𝐞𝐧𝐞𝐫𝐠𝐲 ∆𝑯 = 𝐚𝐛𝐬𝐨𝐫𝐛𝐞𝐝 − 𝐫𝐞𝐥𝐞𝐚𝐬𝐞𝐝 (to break existing bonds) (as new bonds form) Unit: 𝑘𝐽. 𝑚𝑜𝑙−1 1. Calculate the heat of reaction for: 𝑁2 + 3 𝐻2 → 2 𝑁𝐻3 5 Bond Energies: 𝑁 ≡ 𝑁 ∶ 942 𝑘𝐽. 𝑚𝑜𝑙 −1 ; 𝐻 − 𝐻 ∶ 435 𝑘𝐽. 𝑚𝑜𝑙 −1 ; 𝑁 − 𝐻 ∶ 390 𝑘𝐽. 𝑚𝑜𝑙 −1 Reactants Products 𝑁≡𝑁 + 3𝐻−𝐻 → 𝐻 − 𝑁 − 𝐻 2 | 𝐻 2. Calculate the heat of reaction for: 𝐶𝐻4 + 𝑂2 → 𝐶𝑂2 + 𝐻2 𝑂 6 Bond Energies: 𝐶 − 𝐻: 413 𝑘𝐽. 𝑚𝑜𝑙−1 ; 𝑂 = 𝑂: 495 𝑘𝐽. 𝑚𝑜𝑙 −1 ; 𝐶 = 𝑂: 799 𝑘𝐽. 𝑚𝑜𝑙 −1 ; 𝑂 − 𝐻: 467 𝑘𝐽. 𝑚𝑜𝑙 −1 Reactants Products  7 Classwork/Homework Do Think Tank 15.1, pg.210, Q 3.1-3.2. Leave 15 lines open for question 3.3 (which will be taught in the next lesson). Do Exercise 15.1, pg.213, Q 5.1-5.2. Just calculate the energy change. Leave 10 lines open for the energy diagrams (next lesson). 8  9 Exothermic & Endothermic Reactions 𝐞𝐧𝐞𝐫𝐠𝐲 𝐞𝐧𝐞𝐫𝐠𝐲 ∆𝑯 = 𝐚𝐛𝐬𝐨𝐫𝐛𝐞𝐝 − 𝐫𝐞𝐥𝐞𝐚𝐬𝐞𝐝 to break existing bonds (as new bonds form) Definition: An exothermic reaction is a reaction which releases energy. 𝐞𝐧𝐞𝐫𝐠𝐲 𝐚𝐛𝐬𝐨𝐫𝐛𝐞𝐝 to break existing bonds < 𝐞𝐧𝐞𝐫𝐠𝐲 𝐫𝐞𝐥𝐞𝐚𝐬𝐞𝐝 (as new bonds form) ∴ ∆𝑯 < 𝟎 (i.e. negative). Definition: An endothermic reaction is a reaction which absorbs energy. 𝐞𝐧𝐞𝐫𝐠𝐲 𝐚𝐛𝐬𝐨𝐫𝐛𝐞𝐝 to break existing bonds > 𝐞𝐧𝐞𝐫𝐠𝐲 𝐫𝐞𝐥𝐞𝐚𝐬𝐞𝐝 (as new bonds form) ∴ ∆𝑯 > 𝟎 (i.e. positive).  10 Potential Energy Diagram (exothermic reactions) Potential Energy (kJ.mol-1) Course of Reaction  11 Potential Energy Diagram (endothermic reactions) Potential Energy (kJ.mol-1) Course of Reaction  12 Classwork/Homework Do Think Tank 15.1, pg.210, Q 3.3. Do Exercise 15.1, pg.213, Q 5.1-5.2. Draw the energy diagrams. DoE, Nov 2022, P2 13 DoE, Nov 2019, P2 14 DoE, Nov 2019 Exemplar, P2 15 DoE, Nov 2018, P2 16 DoE, Nov 2014, P2 17 DoE, Nov 2020 Exemplar, P2 18 DoE, Nov 2017, P2 19