Enthalpy Change (Theory) Flashcards
What is the equation for heat change?
q = mcΔT q: Heat change of water (J) m: Mass of water (g) c: Specific Heat Capacity of water (J/K/g) ΔT: Temperature change of water (K)
K is Kelvin (Celcius + 273)
How do you convert from heat change to enthalpy change?
> Find moles of substance ( mass of substance / Mr )
> H = ( q / 1000 ) * moles
What is enthalpy change of formation?
When 1 mole of elements in their standard states (e.g. H₂) under standard conditions form products.
What is enthalpy change of combustion?
When 1 mole of a substance is completely burned in oxygen under standard conditions to form:
[CO₂ + H₂O]
SC?
Temperature: 298 K
Pressure: 100 kPa (or 100,000 Pa)
Symbol: θ
What is Hess’ Law?
OWN WORDS:
Regardless of the reaction route taken, the enthalpy change from start to finish will be the same.
STANDARD DEFINITION:
The enthalpy change of a reaction is independent of the route taken
What is the enthalpy change of a single/diatomic element?
Zero
No effect on total value of enthalpy change unless stated otherwise
What can prevent experimental values in a combustion reaction from being true values?
> Incomplete combustion (not all oxygen reacted)
> Non-standard conditions were used
What is bond enthalpy?
The change in energy that occurs when breaking (endo) / making (exo) bonds (BEndo-MExo)
If the pressure is constant, what is the value of heat change?
q = ΔcH (enthalpy change of combustion)
What does an increase in water temperature mean?
> Exothermic reaction
> Negative enthalpy change (ΔcH = -x or less than zero)
What type of reaction is in breaking bonds?
> Energy goes in to break bonds
> Endothermic reaction
What type of reaction is in making bonds?
> Energy comes out to make bonds
> Exothermic reaction
How do you calculate enthalpy change? (ΔHr)
ΔHr = endothermic (breaking) - exothermic (making)
What does a decrease in water temperature mean?
> Endothermic reaction
> Positive enthalpy change (ΔcH = -x or less than zero)
