Energetics - 5th Flashcards
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
what are the two formulas needed to work out enthalpy change after doing calorimetry?
- Q = mc deltaT
- enthalpy change = -Q/moles
what is c in Q=mc deltaT?
- constant value
- specific heat capacity of water
- always has the value 4.18 J/g/degrees celcius
describe the simplest type of calorimetry.
- the simplest type of calorimetry involves combusting a fuel to heat up the water
- to measure deltaH (enthalpy change) for the combustion of the liquid fuel methanol, you would combust some methanol using a burner under a can of water
method for simplest type of calorimetry.
- weigh out 50g of water and place in a can above the burner
- measure the initial mass of the burner and the initial temp of the water
- ignite the burner, stir water for 30s, then extinguish the burner
- measure the final mass of the burner and the final (maximum) temperature of the water
what are the two significant sources of error in this experiment?
- heat loss - most of the heat energy dissipates into the air instead of the water
- incomplete combustion - this releases less heat energy than expected
what is solution calorimetry?
- sometimes the reaction you are measuring happens dissolved in some water
- in this case, the reaction still releases heat energy and the water still absorbs it- but it just happens that the reaction and the water are in the same place
describe the method of solution calorimetry.
- e.g. to measure enthalpy change for the displacement reaction between Mg(s) and CuSO4(aq)
- pipette 25cm3 of CuSO4(aq) into a polystyrene cup
- measure the initial temp of sol
- add 1g of Mg(s) and stir continuously
- measure the max temp of the solution
- use equations to calculate
what happens if you do a calorimetry experiment for endothermic reactions?
- reaction absorbs heat energy from water, so the water cools down
- this results in a negative temp change for the water
- the minus sign gets passed through the equations and comes out at the end as a plus, because a minus times a minus is a plus
