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EQUILIBRIUM

EQUILIBRIUM. REACTION RATES + ENERGY. Chemical energy of a substance = potential + kinetic energy. Kinetic energy = doing energy (mechanical) Potential energy = stored energy. What these energies result from. Attractions between electrons and protons Repulsion between nuclei

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EQUILIBRIUM

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  1. EQUILIBRIUM REACTION RATES + ENERGY

  2. Chemical energy of a substance = potential + kinetic energy. • Kinetic energy = doing energy (mechanical) • Potential energy = stored energy

  3. What these energies result from • Attractions between electrons and protons • Repulsion between nuclei • Repulsion between electrons • Movement of electrons • Vibration and rotation of nuclei

  4. Enthalpy • Is the Chemical energy of a substance is • sometimes also called its HEAT CONTENT • given the sign H

  5. Exothermic • When H (products) < H (reactants) • Energy is released into environment • Feels Hot • Eg burning of petrol reactants Energy released ΔH negative energy products

  6. Endothermic • When H (products) > H (reactants) • Energy is absorbed from environment around the reactants • Feels cooler products Energy absorbed ΔH positive energy reactants

  7. ΔH – Delta H • ΔH = energy (products) – energy (reactants) • The energy released or absorbed in a chemical reaction is called the HEAT OF REACTION • This heat of reaction is the difference in the enthalpy’s of the products and reactants (hence ΔH)

  8. Examples • Exothermic reactions • Burning of fuels including food • Endothermic reactions • Photosynthesis (absorbs energy from sunlight to convert H2O and CO2 into glucose and O2

  9. Thermochemical equations • Show the energy released or absorbed durin a chemical reaction • Energy is measured in Joules (J) or kilo joules (kJ) • The heat of reaction (ΔH ) has the units j mol–1 of kJ mol–1 • The energy is therefore related to the number of moles as given by the equation

  10. Exercises CH4(g) + 2O2(g) CO2(g) + 2H2O(g) ΔH = –890 kJ mol–1 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g) ΔH = 2(–890 kJ) = –1780 mol–1 CO2(g) + 2H20(g) CH4(g) + 2O2(g) ΔH = 890 kJ mol–1(reverse reaction – reverse sign)

  11. Chemical bonds during a chemical reaction • Bonds in the reactants must first be broken • This requires energy to be absorbed • New bonds are created in the products • This requires energy to be released

  12. Activation energy • The energy required to break bonds in the reactants so that the reaction can proceed.

  13. Energy profile Exothermic Energy released as bonds break Activation energy reactants Energy released as bonds form energy ΔH products

  14. Energy profile Endothermic Activation energy products energy ΔH reactants

  15. Factors affecting rate of a reaction • Increasing the surface area of solids • Increasing concentration of reactants or pressure of gases • Increasing temperature • Adding a catalyst

  16. Factors affecting rate of a reaction • Increasing the surface area of solids • Breaking reactant into smaller pices increases the surface area • More particles are present on the surface • The greater number of particles exposed allows for greater number of collisions with other reactant particles

  17. Factors affecting rate of a reaction • Increasing concentration of reactants or pressure of gases • A greater number of particles moving in a given volume of solution will mean a greater chance of collision between particles

  18. Factors affecting rate of a reaction • Increasing temperature • As temperature increases the average speed and kinetic energy of particles increases • This results in a greater chance of collisions occurring

  19. A Catalyst • A substance that will increase the rate of a chemical reaction without being consumed in that reaction - 2 Types • Homogenous • Same state as the reactants eg Atmospheric reactions • Heterogeneous • Different state to the reactants • Easier to separate out at the conclusion of reaction

  20. How catalysts workHaber process • N2(g) + 3H2(g) 2NH3(g)ΔH = –91 kJ mol –1 • Catalysed with powdered Iron • N2 and H2 absorb onto surface of Fe • As they absorb, bonds within them break • NandH molecules now readily form NH3 molecules and move away from Fe

  21. Energy profile diagram change when a catalyst is used Without catalyst ΔH With catalyst energy N2(g) + H2(g) Energy needed to break bonds is LESS ammonia

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