Energetics (3.1.4) Flashcards
what can reactions be?
endothermic or exothermic
what is enthalpy change (△H)?
the change in heat energy measured under conditions of constant pressure
what is standard pressure?
100 kPa
what is standard temperature?
the stated temperature, but usually 298 K
what is standard state?
the normal physical form of a substance under standard conditions eg. for carbon it is graphite
what is standard enthalpy change (∆HƟ)?
the change in enthalpy measured under standard conditions with all substances being in their standard states
what is the standard enthalpy of combustion (∆c H Ɵ)?
the enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions with all substances being in their standard state
what is the balanced equation for the combustion of butanethiol (C4H9SH) where the enthalpy change is the standard enthalpy of combustion?
C4H9SH (l) + 7 1/2 O2 (g) -> 4 CO2 (g) + 5 H2O (g) + SO2 (g)
what do you need to remember for balancing equations for standard enthalpy of combustion?
there must only be one mole of the substance reacting with oxygen, so other balancing numbers may involve fractions (balance normally and then divide)
what is the standard enthalpy of formation (∆f H Ɵ )?
the enthalpy change when 1 mole of a substance is made from its elements under standard conditions with all substances being in their standard state
what is the balanced equation for the formation of butanethiol (C4H9SH) where the enthalpy change is the standard enthalpy of formation?
4 C (s) + 5 H2 (g) + S (s) -> C4H9SH (l)
what do you need to remember for balancing equations for standard enthalpy of formation?
the substance being formed should be the only product and there must only be one mole of it formed
how do you calculate the enthalpy change in a reaction?
q = m x c x ∆T
q = heat energy change in that specific process (J)
m = mass of substance that is changing temperature (g)
c= specific heat capacity of substance changing temperature
∆T = change in temperature (degrees celsius or Kelvin)
why isn’t q the same as ∆ H?
q is the enthalpy change for the amount of substance used, while ∆ H is the molar enthalpy change
how do you convert from q to ∆ H?
∆ H = q/ moles (of substance providing energy) x 1000
what do you need to remember when writing the final value for ∆ H?
the sign will be the opposite of whatever the sign for q was
ie. for an exothermic reaction ∆ T and therefore q will be positive, so ∆ H will be negative
what does Hess’ Law state?
the energy change for a process is independent of the route taken - it only depends on the initial and final state
how can you use Hess’ Law to find enthalpy changes for reactions?
- write the chemical equation for which you are trying to find ∆ H
- for each substance in that equation, identify what other information is provided (standard enthalpy of combustion or formation ie. is it being used or formed?) and add the appropriate arrows
- fill in the second line (products of combustion or original elements)
- label the arrow with the appropriate value from the data
- check the balancing (do number of moles match data provided) and multiply up as appropriate
- check whether the arrows are being followed in the same or opposite direction and show addition (same) or subtraction (opposite) as appropriate
- do the sum
how do you calculate ∆ H when given ∆ H f data?
𝛴 ∆ H f (products) - 𝛴 ∆ H f (reactants)
how do you calculate ∆ H when given ∆ H c data?
𝛴 ∆ H c (reactants) - 𝛴 ∆ H c (products)
what is the definition of bond enthalpy?
the energy required to break a specific type of covalent bond (atoms must be gaseous)
what is the definition of mean bond enthalpy?
the average value for a specific type of bond over many different compounds containing that bond
how can mean bond enthalpies be used to calculate ∆ H for reactions in the gaseous phase?
sum of ∆ H for bonds broken - sum of ∆ H for bonds formed
