Calorimetry

<< Chemistry Practical

 

Calorimetry is a technique to determine the amount of heat evolved/ taken in in a reaction by measuring the temperature rise/ drop.

 

Calorimetry

• Calorimetry can be used to determine the enthalpy change of a reaction by measuring the temperature change involved in a reaction (usually exothermic).
• Examples: neutralisation, displacement of metals.
• Calculation of enthalpy change using temperature data is not required in the O-level syllabus.

 

 

Thermometric Titration

 

Calorimetry can be also used to determine the end-point in an acid-base titration by:

• mixing different volumes of acid and alkali in a foam cup and measuring the highest temperature reached (Method 1)
• recording the temperature of the solution as the acid is progressively added to the alkali (or vice versa). (Method 2)
• End-point: the point at which the maximum temperature rise is obtained (must be inferred from graph).

 

Recording Data

 

Method 1

• mixing different volumes of acid and alkali in a foam cup and measuring the highest temperature reached

Expt No.	vol. of acid/ cm 3	vol. of alkali/ cm 3	initial temp/ ºC	highest temp reached/ ºC	temp rise/ ºC
1	10.00	40
2	15.00	35
3	20.00	30
4	25.00	25
5	30.00	20
6	35.00	15
7	40.00	10

 

Method 2

• recording the temperature of the solution as the acid is progressively added to the alkali (or vice versa).

total vol. of titrant added/ cm 3	total temp rise/ ºC
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00

 

NOTE:

• all data must be recorded to 1 d.p. and ±0.5 ºC(precision of thermometer).

 

Graph Plotting

• Label axes with units.
• Choose suitable scale for graph to cover at least half of graph paper. Avoid using odd scales e.g. 1 square represents 3 units.
• For this experiment (and many others in O levels), the graph will pass through origin (0,0)
• Draw two lines of best fit (equal number of points above and below the line; perpendicular distance of the points above and below the line roughly cancels out.
• Read point of intersection for titre value.

 

Discussion

• Heat is released when an acid reacts with an alkali
• Max. temp rise increases as the amount of acid and alkali reacted increases
• (Method 1) Max. temp rise is the highest the acid and alkali are added in stoichiometric ratios (amount of acid added is just enough to react with all alkali) → largest amount of acid and alkali reacted
• (Method 2) Max. temp rise decreases after ___ cm³ of (acid/alkali) was added as all the (alkali/acid) has been reacted. No more reaction takes place (no more heat is released) and heat is lost to the excess acid/ alkali added (surroundings).

 

Experimental Errors + Precautions

• [Error] heat loss to surroundings leading to smaller temperature rise measured.
• [Precaution] use 2 foam cups/ use a lid/ turn off fans.