Energetics 1 Flashcards
standard conditions
298K and 100KPa
enthalpy change
heat energy change measured at constant
pressure
exothermic
when more energy is given out from bonds being formed than is taken in by bonds being broken
endothermic
When the breaking of bonds in a reaction requires more energy than is given out by the formation of new bonds
energy diagrams for endo and exo
exo= negative, products lower than reactants endo= positive, products high than reactants
Enthalpy of formation, ∆Hθf
The enthalpy change that takes place when mole of substance is formed from its elements in their standard state at a temperature of 298K and pressure of 100kPa
Enthalpy of combustion,∆Hθc
The enthalpy change that takes place when mole of substance is completely burned in oxygen at a temperature of 298K and pressure of 100kPa
Enthalpy of neutralisation, ∆Hθc
The enthalpy change that takes place when one mole of water is formed in a neutralization reaction at a temperature of 298K and pressure of 100kPa
Enthalpy of reaction, ∆Hθr
The enthalpy change that takes place when the number of moles of substances in the equation react at a temperature of 298K and pressure of 100kPa
energy transferred equation
mass of water x 4.2 x change in temperature
∆H
heat energy/moles
using a calorimeter
- A measuring cylinder is used to measure out 100cm3 of water.
- The water is placed in a calorimeter (basically a metal can).
- The liquid to be investigated is placed in a spirit burner. The spirit burner is weighed.
- The temperature of the water is noted
- The apparatus is set up as shown below.
- The liquid is ignited and allowed to heat the water.
- After a temperature rise of around 20 oC, the flame is extinguished.
- The exact temperature is noted
- The burner is weighed to find out how much liquid has been burned.
two main reasons for inaccurate values for enthalpy of combustion
Heat loss to the surroundings
Incomplete combustion
Hess’ law
For a given chemical change, the overall energy change will always be the same whether the change takes place in one step or a number of steps
combustion Hess at the bottom
oxidised elements
arrows going downwards
Formation Hess at the bottom
elements
arrows going upwards
bond enthalpy
∆H when one mole of a bond is broken and reactants and products are gaseous, always endothermic
mean bond enthalpy
∆H for the breaking of a particular type of bond in a particular molecule, averaged out over all those bonds
limits of mean BE
bond energies are averages
everything has to be gaseous
bond energy
broken - made
reality of standard enthalpy of combustion
exothermic make temp higher than 298K
effects the states of products
Reasons enthalpy change of a reaction is different to the data book
- Heat loss/gain to/from surroundings
- approximation in specific heat capacity of solution
- neglecting specific heat capacity of calorimeter/apparatus
- reaction may be incomplete/slow
- temp change is very small
- density of solution is taken as the same as water
- conditions not standard
