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Presentation On Exothermic And Endothermic Reactions (Part 2)

Home > PowerPoint Slides > Presentation On Exothermic And Endothermic Reactions (Part 2)
Published in: Chemistry | Science
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This PPT is suitable for the IGCSE and O Level Chemistry students. It only provides theoretical background of the topic. The practice questions, worksheets and topical questions will be discussed during the tutoring sessions

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  1. Exothermic and Endothermic Reactions (Part 2) Chemistry (IGCSE / O-Level)
  2. What will you learn in this lesson? • Enthalpy and Enthalpy Change • Activation Energy (Ea) • Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include: (a) reactants (b) products (c) enthalpy change of the reaction, AH (d) activation energy, Ea • Bond making and bond breaking • Calculate the enthalpy change of a reaction using bond energies
  3. Enthalpy and Enthalpy Change (AH) • The thermal energy (heat) content of the system is called enthalpy (H) • The heat change during the course of the reaction. It can be either exothermic (negative value) or endothermic (positive value) is called enthalpy change (AH) Reactants Heat is given out to the surrounding, enthalpy of the system is negative Calculated per mole of a specific -AH—728kJtmol reactant or product (kJ/mol) Products Regress ofreaetion
  4. Bond Making and Bond Breaking • To make new bonds the chemical bonds between the reactant needs to be broke. • Breaking this bond requires energy; energy must be taken in from the surrounding to pull the atoms apart • Breaking chemical bonds takes-in energy (endothermic process) • Making new chemical bonds gives away (releases) energy (exothermic process) bond:breaking energy Progress reac*ion
  5. Bond Energy The energy required to break a particular type is called bond energy More energy required to break a particular bond, releases more energy when new bonds are formed 0-0 N-N tabTe7Ä: Bond energy / kJ/mol 436 435 464 347 498 803 945 Bond energiesforsome covalent bonds. in hydrogen average Of four bonds in methane in water average of many compounds in oxygen in carbon dioxide in nitrogen
  6. Bond Energy (contd.) B Making bonds — EXOthermic B Energy Breaking bonds — ENDOthermic Energy Memory aidfOrcatculatingtheenthalpy Changeof reaction: MB(OBENDO.
  7. Bond Energy (contd.) can use bond energy values to ind the enthalpy change, LH, for the burning of methane. The equation Is: The left-hand side involves bond breaking and needs energy: two O O 2 498 = 996 kJ / mol total = 2736 kJ / mol The right-rand side invdlves bond making and gives out energy.' two C = O four O—H bonds total enew 803=1606 kJ/m01 4 x 464 = 1856 kJ / mol = 3462 kJ / mol The enthalpy of reaction. AH, is teenerqy dhanqe on going from reactants to produ&. So, for the buming of methane: enthalpy of 'reaction — energy difference LH — (total •lerqy needed) — (total energy qtven out) 2736 3462 AH = -726 kJ/m01 It is useful to remember that combustion reactions are always exothermic AH = (energy needed to break the bond) — (energy given out when bonds from)
  8. Activation Energy (Ea) The minimum energy required to start a chemical reaction — for a reaction to take place the colliding particles must possess at least this amount of energy. Reactions can be thought of as the result of collisions of atoms, particles or ions. So, the energy ot particles must be greater than activation energy For e.g., when fuels are burnt energy is required to ignite them in the form of spark, a match or sunlight Activation energy of burning sodium or potassium is lower than the burning of magnesium with help pf burner + 202(g) Progress of reaction Some bonds must be activation broken before any energy reaction can take place enthalpy of reaction * 2HP(g)
  9. Practice Questions Is bond breaking an endothermic or an exothermic process? Hydrogen peroxide decomposes to produce water and oxygen. The equation is: 2H202(g) — 2H20(q) + 02(g) using the following values, calculate the heat change for the reaction and say whether it is exothermic or endothermic. Bond energies: 0-0 - 144 kJ/mol 0-0 - 498 kJ/mol 0-H - 464 10/'mol Explain why ever,' chemical reaction must have an activation energy, even though for some reactions it may have a low value.

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