Chapter 4- Energetics Flashcards
What is enthalpy change?
The heat energy change of a reaction measured under conditions of constant pressure
What is standard enthalpy of formation?
The enthalpy change on forming 1 mole of a substance from its elements, all substances in their standard states under standard conditions.
Units are kJmol-1
What is standard enthalpy of combustion?
The enthalpy change on burning 1 mole of a substance in excess oxygen to form the combustion products, all substances in their standard states under standard conditions.
Units are kJmol-1
What are standard conditions?
100 kPa
A stated temperature (usually 298K)
What is calorimetry?
The process of measuring the amount of energy absorbed or released during a chemical reaction
What are some sources of error for measuring enthalpy of combustion with a spirit burner?
-Heat loss to surroundings: assumption made that all heat lost by alcohol is gained by the solution
-Incomplete combustion: evidenced by black sooty deposit on the beaker’s bottom
-Evaporation of alcohol: alcohol in spirit burner is volatile and evaporates from the wick between mass readings
What is the equation for heat change?
q = mcΔT
m: mass of substance changing temp. (solution) (g)
c: specific heat capacity (4.18 for water)
ΔT: temp. change (degrees celsius)
What is Hess’s Law?
The enthalpy change for a reaction is independent of the route taken
How do you draw the Hess’s cycle when given enthalpy of formation data?
List elements (balanced) at bottom with arrows going up to products and reactants
How do you draw the Hess’s cycle when given enthalpy of combustion data?
List combustion products (not oxygen) at bottom with arrows going down from products and reactants
How do you draw the Hess’s cycle when given bond enthalpy data?
List GASEOUS ATOMS at bottom with arrows going from reactants to products. Cannot be diatomic molecules.
Energy to break bonds is positive, energy released from new bonds is negative.
Need to account for changes in state.
What is mean bond enthalpy?
The enthalpy change when one mole of a specified covalent bond is broken to produce the atoms, all substances must be gaseous.
Value in the data book is an average taken across a range of compounds.
Why do calculated enthalpy changes or bond enthalpies differ from the actual values?
Bond enthalpy data is average data and so not specific for individual compounds meaning calculations based on this data are approximations.
What is an exothermic reaction?
A reaction that transfers energy to the surroundings. The energy released when forming new bonds is greater than the energy required to break existing bonds.
What is an endothermic reaction?
A reaction that takes in energy from the surroundings. The energy required to break existing bonds is greater than the energy released when forming new bonds.
