Thermal analysis: Differential Scanning Calorimetry

Question 1

What is the gibbs free energy equation?

Answer 1

Delta G = DeltaH - TDeltaS

Question 2

In the gibbs free energy equation what does it mean if the delta H is more than 0?

Answer 2

Postive= Endothermic

Question 3

In the gibbs free energy equation what does it mean if the delta H is less than 0?

Answer 3

Negative= exothermic

Question 4

Define what crystallisation temperature (Tc) is?

Answer 4

Temperature at which a solid form crystallises into an ordered form under application of heat

Question 5

Define what melting temperature (Tm) is?

Answer 5

Temperature at which a solid form transforms into a liquid under the application of heat

Question 6

Define what glass transition temperature (Tg) is?

Answer 6

Temperature amorphous solid turns from solid and brittle into soft and flexible under application of heat

Question 7

Describe what the differential scanning calorimetry (DSC) is for?

Answer 7

1. Measuring when a material undergoes a physical change of state or chemical change
2. Exothermic (liberated) or Endothermic (absorbed) reaction is given off which is measured
3. The inert sample and reference is maintained at T=0 by this instrument

Question 8

How do we interpret the graph on the differential scanning calorimetry?

Answer 8

Look under the notes

Question 9

How do you interpret the DSC and TGA data? (draw out how evaporation, decomposition etc)

Answer 9

Look under the notes

Question 10

How do you find out extra information from the DSC data?

Answer 10

Combining TGA with DSC

Question 11

How do you get the temperature changes associated with a thermal event?

Answer 11

Take the peak value and centre of the curve to get the temperature changes

Question 12

How do you get the heat or enthalpy changes associated with a thermal event?

Answer 12

Look at the area under the curve

Question 13

How do you calculate the DSC %crystallinity?

Answer 13

1. Scan the glass transition temperature and measure heat flow or change in heat capacity
2. Do this for 100% crystalline sample
3. Do this for a sample with some amorphous content

Question 14

How do you calculate DSC %purity and what is it used for?

Answer 14

1. Useful for almost pure substances
2. Based on melting point depression by impurities
3. Need to know the enthalpy of fusion per mole

Question 15

What are the DSC practicalities?

Answer 15

1. Flow of purge gases such as N2 to create an inert atmosphere
2. Sample weighing 1 to 5mg
3. Sample is thinly spreaded across the pan to allow for even heat distribution
4. Pan has to be made of aluminium to allow for suitable conductivity
5. Scan rates are between 1 to 10 degrees per minute