[CHEM] Chapter 5 - Energy Changes

1 DARRELL ER (COPYRIGHTED) © TOPIC 5: ENERGY CHANGES DARRELL ER (COPYRIGHTED) ©

2 CHAPTER ANALYSIS THE ABOUT TIME EXAM WEIGHTAGE • Relatively straight forward chapter • Understand big idea: ‘system’ & ‘surroundings’ • Learn how to calculate bond energy • Learn how to sketch energy profile diagram • Light-medium overall weightage • Constitute to 3% of marks for past 5 year papers DARRELL ER (COPYRIGHTED) ©

3 ENERGY FROM CHEMICALS EXOTHERMIC ENDOTHERMIC ENERGY PROFILE DIAGRAM Pay special attention to your understanding of ‘system’ & surrounding. It is the foundation of this chapter. KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

4 ENERGY FROM CHEMICAL REACTIONS ENERGY FROM CHEMICAL REACTION During chemical reactions, chemical bonds within the reactants are broken and new bonds are formed, creating new products. The reactants and products make up the system and when it releases or absorbs heat energy, the surrounding would experience a change in temperature. For an exothermic reaction, the chemical reactions causes the system to release heat energy to the surroundings. The overall energy level of the system would hence decrease. For an endothermic reaction, the chemical reactions causes the system to gain heat energy from the surroundings. The overall energy level of the system would hence increase. DARRELL ER (COPYRIGHTED) ©

5 EXOTHERMIC REACTION EXOTHERMIC REACTION An exothermic reaction occurs where energy is expelled out to the surroundings. Examples of exothermic processes would include condensation and freezing as heat is expelled out to the surroundings to lower the units’ temperature. - Combustion reactions are also exothermic. Burning a hydrocarbon in the presence of oxygen will produce heat and raise the temperature of the surroundings. C2H4 (g) + 3 O2 (g)  2 CO2 (g) + 2 H2O (l) + heat - Neutralisation reactions are also exothermic. Heat is produced when H+ ions undergo bond forming with OH– ions to form water. H+ (aq) + OH– (aq)  H2O (l) + heat DARRELL ER (COPYRIGHTED) © Exothermic reaction Progress of reaction Energy level AE ΔH C2H4 (g) + 3 O2 (g) 2 CO2 (g) + 2 H2O (l)

6 ENDOTHERMIC REACTION ENDOTHERMIC REACTION An endothermic reaction occurs when energy is absorbed from the surroundings. Examples of endothermic process are melting and boiling where heat energy is absorbed to change states. - Dissolving ionic salts in water is endothermic. For example, dissolving sodium chloride in water causes the temperature of the solution to decrease. NaCl (s) + heat  NaCl (aq) - Thermal decomposition is an endothermic process as heat is required for most compounds to be decomposed. PbCO3 (s) + heat  PbO (s) + CO2 (g) - Electrolysis is an endothermic reaction which occurs without absorbing heat energy. Instead, electrolysis occurs by absorbing in electrical energy for a reaction. DARRELL ER (COPYRIGHTED) © Endothermic reaction Energy level Progress of reaction 2H2O (l) 2H2 (g) + O2 (g) AE ΔH

7 ENERGY PROFILE DIAGRAM ENTHALPY CHANGE Enthalpy change is the total amount of energy given out or absorbed during a chemical reaction in the form of heat energy. Enthalpy change (ΔH) would be positive for endothermic reactions and be negative for exothermic reactions. C2H4 (g) + 3 O2 (g)  2CO2 (g) + 2 H2O (l) ΔH = −1411 kJ (Exothermic) This is an exothermic reaction. 2H2O (l)  2H2 (g) + O2 (g) ΔH = + 262 kJ (Endothermic) This is an endothermic reaction. DARRELL ER (COPYRIGHTED) © Energy level diagrams In exothermic reactions, reactants lose energy to the surroundings to form resultant products. Hence, the energy level of the products is lower than the energy level of the initial reactants. In endothermic reactions, reactants gain energy from the surroundings to form resultant products. Thus, the energy level of the products is higher than the energy level of the initial reactants. Endothermic reaction Exothermic reaction Energy level Progress of reaction Progress of reaction Energy level 2H2O (l) 2H2 (g) + O2 (g) AE ΔH AE ΔH C2H4 (g) + 3 O2 (g) 2 CO2 (g) + 2 H2O (l)

8 BOND BREAKING & BOND FORMING ACTIVATION ENERGY COMMON FUELS & HYDROGEN FUEL CELL KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

9 BOND BREAKING & BOND FORMING BOND BREAKING AND BOND FORMATION Bond energy is defined as the amount of energy that is released when the bond is formed or the amount of energy that is required to be absorbed to break the bond. ΔH, = Energy absorbed to break bonds + Energy released to form bonds For an exothermic reaction, Energy absorbed for bond breaking < Energy released from bond forming For an endothermic reaction, Energy absorbed for bond breaking > Energy released from bond forming DARRELL ER (COPYRIGHTED) © Remember: “Your bb (bae) is more important than your bf (best friend)” BB – BF (Bond breaking – Bond forming)

10 ACTIVATION ENERGY ACTIVATION ENERGY Activation energy is defined as the minimum energy required to start a reaction. The symbol, Ea, is used to represent activation energy. For a reaction to occur, it has to possess at least the amount of energy that is equivalent to its activation energy. This is the same level of energy required to be absorbed in order to break the bonds in the reactants. *Recall 2 conditions for reactions from “Rate of Reaction” 1) Sufficient energy (activation energy) 2) Collide in correct orientation DARRELL ER (COPYRIGHTED) ©