Blog

Describing Simple Calorimetry Experiments

Describing Simple Calorimetry Experiments

  • All these involve measuring a temperature change during the reaction.
  • Specific heat is the amount of heat needed to raise the temperature of 1g of a substance by 10 For water, the value is 4.18 J g-1 0C-1 (joules per gram per degree Celsius).

 

Heat Given Out = Mass x Specific Heat x Temperature Rise

 

For Neutralisation, Displacement Reactions and Dissolving

 

Mass of Weighing Bottle + Mg (g) 10.810
Mass of Weighing Bottle Afterwards (empty) (g) 10.687
Mass of Mg used (g) 0.123
Initial Temp. (0C) 17.4
Final Temp. (0C) 27.5
Temperature Rise (0C) 10.1

 

They all follow the same method. This example involves measuring the heat evolved (or energy) when magnesium reacts with dilute sulphuric acid.

  1. Pour an excess of sulphuric acid into a polystyrene cup and measure the temperature of the acid.
  2. Pour some magnesium powder into a weighing bottle and weight it.
  3. Pour the powder into the acid and record the highest temperature.
  4. Weigh the empty weighing bottle.

 

Let’s say the total mass of the solution and Mg is 50g

Heat evolved when 0.123g of Mg reacts = 50 x 4.18 x 10.1J = 2111J = 2.111kJ

To find out the heat evolved when 1 mole of Mg reacts (Mg = 24g):

(2.111/0.123g) x 24 = 412 kJ

 

The temperature rose, meaning the reaction is exothermic so:

 

Mg(s) + H2SO4(aq) -> MgSO4(aq) + H2(g)                   ∆H = -412 kJ mol-1

 

This is actually smaller than the accepted value, which is around -417 kJ mol-1. A reason for this could be that heat was lost too quickly. Using a mercury thermometer may give better results.

 

Combustion

  1. Put 100cm3 of water into a conical flask and record the temperature.
  2. Fill the spirit burner with alcohol (let’s say ethanol) and weight.
  3. Light the spirit burner and record the temperature of water until there is say, a 400C increase.
  4. Reweigh the spirit burner.

 

Volume of water (cm3) 100
Mass of water being heated (g) 100
Mass of burner before (g) 37.355
Mass of burner after (g) 36.575
Mass of ethanol burnt (g) 0.780
Original temp. of water (0C) 21.5
Final temp. of water (0C) 62.8
Water temperature increase (0C) 41.3

 

Heat gained = 100 x 4.18 x 41.3 = 17260 J = 17.26 kJ

 

Ethanol is C2H5OH

One mole of ethanol = 46g

 

Amount of heat produced from 1 mole of ethanol = (17.26/0.780) x 46 = 1020 kJ

 

KEY POINTS FOR THIS UNIT

  • 1 mole is the Avogadro constant number of particles
  • The molar volume is 24 dm3 or 24 000 cm3
  • Number of moles = mass / RFM
  • 1 dm3 = 1000 cm3
  • Mol dm-3 = mol per 1000 cm3
  • G dm-3 = grams per 1000 cm3
  • Energy/Heat = mass x specific heat x temperature rise