Chemistry - Thermochemistry

• Why is hydrogen the best element to burn to measure enthalpy change?

  Give off the most energy per mole
• what is each equation for each enthalpy change?

  H1= H2-H3

  H2=H1-H3

  H3=H1-H2
• what are the steps to work out change in enthalpy?

  1. work out heat transfered - mxcxdeltaT

  2) work out the moles of reactants - moles = mass/mr

  3) work out molar enthalpy change - -heat transfer/ moles
• how do you calculate the bond enthalpy of formation / make bonds

  The bond enthalpy of products - the bond enthalpy of reactants
• How do you calculate the bond enthalpy of breaking bonds

  The bond enthalpy of reactants - the bond enthalpy of products
• How do you measure the temperature change of a system

  In a calorimeter - insulated container
• What is bond enthalpies

  The amount of energy required to break one mole of a covalent bond

  Breaking bonds are positive - absorbed energy

  Making bonds are negative - given energy out
• When calculating energy using Hess’s law for the formation what directions should the arrows point?

  From the elements at the bottom to the compounds at the top
• When calculating the energy of combustion using hess’s law - what direction should the arrows go?

  Down to the carbon dioxide and water symbols
• When using Hess’s law , if teh direction changes on the diagram to original arrow , what must you do

  Swap the symbol on that energy ( positive to negative/ negative to positive)
• What is Hess’s Law?

  The enthapy change fro any chemical reaction is independent of the route providing the conditions remain the same
• How do you calculate energy change?

  M x C x delta T

  Mass of water

  Specific heat capacity

  Temperature change
• What can you use when balancing thermochemistry equations?

  Fractions
• What is the principle of the conversation of energy

  The total energy of the system of reaction chemicals and its surroundings are constant
• what happens is you form a mole of an element from its element?

  nothing , no reaction occurs
• how do you go from celsius to kelvin?

  add 273
• what is 0K

  absoulte zero- lowest possible temperature
• why is anthaply done under standard condtions?

  to compare other data
• what are the standard enthaply conditions ?

  1 ATM of pressure

  298K/ 25 degrees

  1 mol/dm 3
• what is enthaply change?

  the heat energy change per mole
• what is enthaply ?

  the total energy content of a system held at constant pressure
• what is the internal energy of a system?

  the sum of kinetic energy of all particles in a system ( chemical reaction) and potencial energy
• what does the enthalply of combution always make?

  carbon dioxide and water
• what does this symbol mean ?

  gained energy from surroundings 

  endothermic

  
• what does this symbol mean?

  lost in energy to surrondings

  exothermic

  
• what reaction is this?

  standard enthaply reaction

  
• what is standard enthaply reaction ?

  enthalpy change which occurs when equation quantities of materials react under standard conditions,

  
• what type of reaction is this?

  endothermic

  
• what type of reaction does this show?

  exothermic

  
• what is enthalpy change of combution?

  when one mole of a substance is completly burned in excess oxygen under standard conditons

  
• what is enthaply change of formation?

  when 1 mole of a compound is produced from its element in standard conditions