Energetics Flashcards
What is enthalpy change ΔH?
the heat change at a constant pressure
What is the ΔH in an exothermic reaction?
negative
an exothermic reaction gives out heat
What is the ΔH for an endothermic reaction?
positive
and endothermic reaction takes in heat from the surroundings
What is Standard enthalpy change?
reactions done under standard conditions
What are the standard conditions for enthalpy change?
temperature - 298K
pressure - 100kPa
What is the standard enthalpy change of combustion ΔcH?
The enpalthy change when 1mol of a substance is completely burned in oxygen
under standard conditions and states
ALWAYS exothermic
What is the standard enthalpy of formation ΔfH?
The enthalpy change when 1 mol of a compound is formed FROM its elements
under standard conditions and states
endo OR exothermic
What is the ΔfH for an ELEMENT?
Zero
What is Hess’s law?
the enthalpy change for a reaction is independent of the route taken
What is the equation for enthalpy of combustion?
Σ∆cH (reactants) - Σ∆cH (products)
What is the equation for enthalpy change of formation?
Σ∆fH (products) - Σ∆fH (reactants)
What is Calorimetry?
used to find the heat released in a reaction
How can enthalpy of combustion be found experimentally?
- burning a known mass of a substance
- use the energy released to increase the temperature of a known mass of water
What is the equation for heat energy?
q= mc∆T
specific heat capacity = 4.18
What is the calculation for ∆H using heat energy?
∆H = q/n
q must be divided by 1000
What happens in a combustion reaction?
a fuel is used to heat up a known amount of water
Why will the value of ∆H be less than a data book value?
heat loss to surroundings
incomplete combustion
What is bond dissociation energy?
Enthalpy change to break the bond in 1 mol of gaseous molecules to form gaseous atoms
What are bond enthalpies?
bonds that only appear in one molecule and values would be accurate
eg Cl-Cl
What is a mean bond enthalpy?
enthalpy required to break 1 mol of covalent bonds averaged over a range of compounds
Which bonds are mean enthalpy bonds?
bonds that appear in many molecules
bind enthalpy is different in in diff molecules
values are not 100% accurate
What is the equation for enthalpy change of a reaction?
∆H = Σ bonds broken - Σ bonds formed
