Calorimetry

What is it?

Measuring enthalpy
changes experimentally

Definition

Transferring heat to known mass
of another substance (usually H2O)
and measuring temp rise

Enthalpy of Combustion Reactions
(of an alcohol)

apparatus

copper can used as calorimeter

  • copper good conductor of heat
  • low specific heat capacity
  • does not absorb too much heat

source of
errors

temperature measurement
(use sensors)

heat loss to air
(side shields)

copper absorbing
some heat
(thinner can)

following measurements
taken:

mass of water (g)

temperature rise
of water (K)

loss in mass of fuel (g)
(mass of fuel burned)

method

  1. weigh known mass water
    in metal can + note temp
  1. weigh spirit burner
    containing ethanol
  1. place under metal can +
    light the wick
  1. stir water + note final
    temp of water
  1. extinguish spirit burner
    and reweigh

diagram

Enthalpy of Neutralisation
of an Acid + Base

method

  1. 50cm3 of 2.0 moldm-3 solution
    HCl in insulated polystyrene cup

note its temperature

  1. Add 60cm3 of 2.0 moldm-3
    NaOH solution (excess to ensure
    complete reaction of acid)
  1. Stir continuously with
    thermometer & note
    MAXIMUM temperature

apparatus

Polystyrene cups used as calorimeters

  • excellent insulators
  • low specific heat capacity.

source of
errors

heat loss to surroundings

  • put lid on polystyrene cup
  • place polystyrene in beaker

diagram

Enthalpy Changes of
Solution

when ionic solids dissolve
in water, ions in lattice become spread through solution

  • energy must be put in to PULL LATTICE APART
  • energy is released as ATTRACTIONS FORM between ions + water molecules
    (hydration) (+ w/ other solvents = solvation)

Hydration energy
D: energy released when 1 mole of
ions become attached to water molecules
in solution (exothermic)

Lattice energy
D: energy required to break 1 mole
of ionic lattice into constituent ions
(endothermic)

Heat of solution
D: sum of lattice + hydration energies
ΔHsoln = ΔHlattice + ΔHhydration

Enthalpy change of Solution
D: enthalpy change that occurs when 1 mole
of solute dissolved in water to form an
infinitely dilute solution

water good solvent for ionic

  • water molecules polar
  • strongly attracted to charged ions

less polar solvents poorer at
dissolving ionic solids

  • energy released solvation of ions
    not compensate for energy put in
    to break up lattice

diagram