Differential Scanning Calorimetry

What is DSC

measure energy difference between sample and reference during heating or cooling based on heat flux (similar to DTA) or heat compensation; newer version of DTA

What type of data does DSC provide and why is it useful

quantitative; studying chemical reactions involving heat

How does heat flux DSC work

sample and reference in same oven; integrated temp sensors measure the difference in heat flow at the same heating rate

How does power (or heat) compensation DSC work

sample and reference in different ovens; measure the power required to obtain and maintain the same temp

What can DSC be used to study

anything related to heat generation/absorption (similar to DTA); phase changes; adsorption/desorption; decomposition; crystallisation

How is DSC used

identify materials; phase transition temps; heat produce/required for chemical processes; measure heat capacity

What is an Ellingham diagram

indicates change in gibbs FE of reactions at different temp and pressure (consider what gas is involved); reactions occur spontaneously when DG<0; all dashed lines show DG=0

What characterises a first order transition

latent heat involved; phase change; no temp change; larger peak

What characterises a second order transition

no latent heat; only one phase; only heat capacity changes; temp keeps changing during transition

For a 1st order transition the change in derivative of the gibbs free energy is

non zero

For a 2nd order transition the change in derivative of the gibbs free energy is

0; second derivative is non zero

How does heating rate affect DSC

slow reactions shift to higher T when heated at a higher rate; rapid heating can cause events to overlap; slow heating is better for determining event temperature; too slow leads to broad peaks which are less visible

How is melting temp determined for pure and impure samples

pure Tm = onset temp, sharp symmetric peak; impure Tm = peak temp, broad asymmetric peak, additional peak from impurity

How can non reversible reactions affect the DSC plot

on heating dehydration or gas production produces peaks; on cooling won’t see these peaks as the gas doesnt come back

How is heat capacity measured using DSC

measure different in heat capacity between sample and reference; also measure known sample to eliminate systematic errors