Energetics - 5th
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what is the enthalpy change of a reaction?
• the heat energy it releases or absorbs per mole
what is the symbol of enthalpy change?
• DELTA H • has a sign, value and units
what does this tell us: N2 + 3H2 -> 2NH3 DELTA H = -92 kJ/mol
• this tells is that the reaction releases 92 kJ of heat energy every time 1 mole of N2 and 3 moles of H2 react to make 2 moles of NH3
what sign tells us whether the reaction absorbs or releases energy?
+ = POSITIVE = ABSORBED = ENDOTHERMIC - = NEGATIVE = RELEASED = EXOTHERMIC
what tells us how much heat energy is absorbed or released?
• the value tells us how much heat energy is absorbed or released • the units are always kJ / mol so that different reactions can be compared easily
what is the meaning of this reaction and enthalpy change: BaCO3 + heat DELTA H = +245 kJ/mol
• absorbs a lot of heat energy
what is the meaning of this reaction and enthalpy change: NaCl + H2O DELTA H = +35 kJ/mol
• absorbs a small amount of heat energy
what is the meaning of this reaction and enthalpy change: CH4 + O2 DELTA H = -890 kJ/mol
• releases a lot of heat energy
what is the meaning of this reaction and enthalpy change: C8H18 + O2 DELTA H = -5330 kJ/mol
• releases a huge amount of heat energy
what does how much heat energy a reaction absorbs or releases depend on?
• the balance between breaking bonds (which absorbs energy) and making bonds (which releases energy)
how are enthalpy changes represented diagramically?
• on an energy level diagram
what happens to heat energy and enthalpy change when the reaction is endothermic?
• heat energy is absorbed • enthalpy change is positive • products end up with more energy than the reactants had
what happens to heat energy and enthalpy change when the reaction is exothermic?
• heat energy is released • enthalpy change is negative • products end up with less energy than the reactants had
what contributes towards a reaction’s enthalpy change?
• breaking bonds and making bonds
is bond breaking exothermic or endothermic and does bond breaking absorb or release heat energy?
• absorbed • endothermic
is bond making exothermic or endothermic and does bond making absorb or release heat energy?
• released • exothermic
how can the enthalpy change for a reaction be calculated?
• bonds broken - bonds made
what is a reaction profile diagram?
• like an energy level diagram • but it shows the path the reaction actually takes from reactants to products
in an exothermic reaction, which is greater, energy absorbed or energy released?
• energy released > energy absorbed
in an endothermic reaction, which is greater, energy absorbed or energy released?
• energy released < energy absorbed
what is activation energy?
• the energy needed to break the bonds
what is calorimetry?
• an experimental method used to measure an enthalpy change
describe the experiment of calorimetry.
• a reaction is carried out near some water
what happens in an exothermic reaction when calorimetry is done?
• reaction releases heat energy • heat energy is absorbed by the nearby water • the temperature of the water increases