enthalpy changes Flashcards
chemical reaction
is a change in energy
enthalpy change in reaction
heat change in a reaction at constant pressure kjmol-1
endothermic
absorb energy from surroundings
enthalpy of products greater than enthalpy of reactants
eg thermal decomposition
exothermic
release energy to surroundings
enthalpy of products smaller than enthalpy of reactants
eg combustion of ethane
standard enthalpy change of reaction
enthalpy change of a reaction according to two molar quantities in the equation under standard conditions
standard enthalpy change of formation
enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
standard enthalpy change of neutralisation
enthalpy change when an acid and an alkali react to form 1 mole of water under standard conditions
enthalpy change of combustion
enthalpy change when 1 mole of substance completely burns in oxygen to make co2 and h2o under standard conditions
bonds broken
energy absorbed
bonds broken in reaction is an endothermic reaction so delta h is positive
more energy needed to break bond than energy given out when bonds formed
bonds made
energy released and products are produced
energy released so products are being produced
more energy released when bonds are formed than what was needed to break the bonds so an exothermic reaction
enthalpy change
total energy 2 break bonds - the total energy released from forming bonds
calorimetry
enthalpy change of combustion
full burned to raise the temp of water
weigh the fuel before and after burning to work out the mass of fuel burned
energy from fuel transferred into water and some is lost to surroundings
calorimetry method help
lid placed on top to prevent heat loss
windshield placed by flame to prevent draught
Polystyrene cup to prevent heat loss
add acid first measure the temp and add the alkali stir and measure the temp change
hess law
total enthalpy change of a reaction is independent of the route taken
formation cycle
arrows point upwards
