Chemistry- Energetics

• All matter possesses energy in one form or another
• Forms of energy


  • Light
  • Sound
  • Heat
  • Kinetic (movement)
  • Potential (stored)
  • Electrical
• Law of Conservation of Energy


  Energy can neither be created nor destroyed, it can only change from one form to another
• Kinetic energy of particles

  Particles within a chemical are in a constant state of movement, the higher the temperature the more the particles will vibrate
• Bond energy


  Energy stored within the bonds between atoms, molecules or ions
• Types of bonding


  • Intramolecular (within a molecule)
  • Intermolecular (between molecules)
• Intermolecular attraction is much weaker than intramolecular attraction
• Exothermic reaction


  Reaction which gives out heat
• Endothermic reaction


  Reaction which takes in heat from the surroundings

  
• In an exothermic reaction


  The temperature rises
• In an endothermic reaction


  The temperature falls
• Enthalpy


  The heat energy a substance has, given the symbol H

  
• Enthalpy change (ΔH)


  If ΔH is negative, the reaction is exothermic. If ΔH is positive, the reaction is endothermic

  
• ΔH is given under standard temperature (0°C) and pressure (1 atmosphere)
• In an exothermic reaction


  The products have lower energy than the reactants, the difference in energy is given out as heat
• In an endothermic reaction

  The products have higher energy than the reactants, the difference in energy is taken in as heat

  
• Bond breaking requires energy, bond forming liberates energy
• Intramolecular forces are stronger than intermolecular forces, so more energy is required to break them and more energy is liberated when they form
• Endothermic reactions have a positive ΔH value, exothermic reactions have a negative ΔH value
• In an exothermic reaction, heat is released to the surroundings
• In an endothermic reaction, heat is absorbed from the surroundings

  
• Activation energy


  The energy that needs to be absorbed to break the bonds of the reactants
• Catalyst


  Lowers the activation energy, providing an alternative reaction pathway that requires less energy
• In an endothermic reaction

  The energy of the reactants is higher than the energy of the products
• Heat of reaction (ΔH reaction)


  The heat change when the number of moles of reactants shown in the equation react together

  
• Heat of combustion (ΔH combustion)


  The heat change which takes place when one mole of substance is completely burned in oxygen

  
• The heat of combustion of a substance is always negative
• Experiment - Estimation of the Heat of Combustion of Ethanol

  1. Set up apparatus

  2. Weigh tin can and water

  3. Record initial water temperature

  4. Light wick and heat water

  5. Extinguish flame and record final water temperature

  6. Weigh lamp and ethanol before and after burning

  
• Q = mc∆T, where Q is heat energy, m is mass, c is specific heat, and ∆T is change in temperature
• Experiment procedure

  1. Weigh can with 0g of water

  2. Record mass of water placed in can

  3. Fill spirit bottle approximately half full of ethanol and weigh

  4. Clamp tin can above spirit bottle, insert thermometer and record initial temperature of water

  5. Light wick of lamp and arrange can so flame touches bottom to heat water

  6. Stir water frequently and when temperature increases by 25oC, put out flame and re-weigh bottle and contents

  7. Record final temperature of water
• Results


  • Initial temperature of water = 21.3oC
  • Final temperature of water = 50.1oC
  • Mass of water in tin can = 100g
  • Mass of lamp + ethanol before burning = 136.82g
  • Mass of lamp + ethanol after burning = 136.20g
• Q

  Heat energy (Joules, J)
• m

  Mass of a substance (kg)
• c

  Specific heat (units J/kgK)
• ΔT

  Change in temperature (K)
• Energy evolved = mass of water X specific heat capacity of water X change in temperature
• Specific heat capacity of water as 4.2J/gK
• Combustion reaction


  C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l)
• Heat gained by the water = Heat given out by the fuel
• The exact heat of combustion of ethanol found from a data book is -1367kJ/mol