energetics

• Enthalpy Change

  A measure of HEAT ENERGY given out or taken in when a CHEMICAL or PHYSICAL CHANGE happens at CONSTANT PRESSURE
• Standard Condition

  Temperature 298K and pressure 100kPa
• Bond Breaking

  Is an endothermic process energy put in to break bonds
• Bond Making

  Is an exothermic process (energy is released in bond formation)
• Bond Enthalpy


  The enthalpy change for breaking 1 mole of a particular covalent bond in the gas phase to form gaseous atoms
• Mean Bond Enthalpy

  The average enthalpy change for breaking one mole of a particular covalent bond in a range of different compounds in the gas phase.
• Why are calculated enthalpy changes and experimental enthalpy changes slightly different?

  Calculated enthalpy changes use mean bond enthalpy values, which are slightly different to actual bond enthalpy values
• What is the equation linking bond enthalpy and enthalpy of reaction

  ∆Hreaction = ∑(bonds broken) - ∑(bonds made)  
• Exothermic reactions

  •  More energy is RELEASED in bond FORMATION than is TAKEN IN by bond BREAKING   

  The surroundings get HOTTER

  ∆H is NEGATIVE
• Examples of Exothermic reactions

  Combustion

  Respiration

  Neutralisation
• Endothermic reactions

  More energy is TAKEN IN to BREAK bonds than is RELEASED in BOND FORMATION

  The surroundings get COLDER

  ∆H is POSITIVE
• Examples of Endothermic reactions

  Photosynthesis

  Thermal Decomposition
• Standard Enthalpy of Combustion

  ∆Hc

  The Enthalpy change when ONE MOLE of substance is completely burned in oxygen, with all reactants and products in their standard state under standard conditions
• Standard Enthalpy of Formation

  ∆Hf

  The enthalpy change when ONE MOLE of substance is formed from its elements, with all reactants and products in their standard state under standard conditions
• Standard Enthalpy of Reaction

  ∆Hr

  The enthalpy change of a reaction, (per mole of limiting reactant), with all reactants and products in their standard state under standard conditions
• Standard Enthalpy of Neutralisation

  ∆Hneutralisation

  The enthalpy change when ONE MOLE of WATER  is formed from the reaction between an acid and base, with all reactants and products in their standard state under standard conditions
• In calorimetry calculations for Enthalpy change ∆H, what must we do to the final answer if the reactions is exothermic? (gets hotter)

  Add a negative sign (eg ∆Hc = - 56 kJmol-1)
• What are the biggest sources of error in Enthalpy of Combustion experiments

  The values calculated during experiments are vastly different from data book values. Reasons include:

  1)    Heat loss

  2)    Incomplete combustion

  Evaporation of the fuel
• What is the major source of error in Enthalpy changes in solution (Enthalpy of reaction or enthalpy of neutralisation). How can this be reduced?

  Heat loss

  (using a polystyrene cup and lid also help to reduce the heat loss)
• Finding a more accurate ∆T for enthalpy change of solution without changing the practical equipment

  1. Record the temperature of the solution every minute for three minutes

  2. At the fourth minute, add the second reactant (the solid or the second solution) and stir the mixture thoroughly

  3. Record the temperature of the mixture at the fifth minute, and every minute for a further ten minutes

  4. Plot a graph of temperature(y) against time(x)

  5. Draw two lines of best fit (before and after the addition time) and extrapolate both lines beyond the fourth minute

  6. Draw a vertical line at minute four. The theoretical temperature rise at the fourth minute (more accurate ∆T) is the difference between the two extrapolated lines
• State Hess’s Law

  The overall enthalpy change for a reaction is independent of the route taken.
• Why is the Standard Enthalpy of formation of any element Zero? Even a diatomic molecule

  By definition
• Why is there not value for the Standard Enthalpy of combustion of oxygen

  Because you can’t combust oxygen
• Draw the equipment needed to measure Enthalpy of combustion.

  Give the equations used, the units and meaning for each symbol in the equations

  q(J)= mc∆T

   

  q is heat energy transferred

  Where:-

  m(g) = mass of water in can

  c (JK-1g-1) = specific heat capacity of water

  ∆T = change in temperature of the water (with T1 and T2 BOTH in either oC or K)

  ∆H=q/n

  q (kJ) = heat energy transferred

  n(moles) = moles of fuel combusted

   

  Note should be NEGATIVE for combustion (add -,ve sign manually)