Conversion of NaOH to HCL through two different routes with measured enthalpy changes to prove Hess’
Updated: Oct 23, 2019
The aim of this practical was to through two different routes, reach a particular product and measure the enthalpy change and if it would be the same then Hess’s law would be correct. Enthalpy change is the amount of heat evolved or absorbed under constant temperature with the symbol ΔH. (2&3)T his law states that no matter the different steps in a reaction the total enthalpy change for the reaction is the sum of all changes and so enthalpy is a state function. Thus, following the law of conservation of energy, the enthalpy change of a complete enthalpy cycle is zero.
NaCl was achieved through two different routes making an enthalpy/energy cycle, letting NaOH react with the acid HCL and the enthalpy changes of all the routes were measured to prove Hess’s law.
NaOH(s)+2M HCL(aq) ⇒ 2M NaCl(aq)+Dilute ⇒ 1M NaCl(aq)
NaOH(s)+Dilute ⇒ 2M NaCL(aq)+2M HCL(aq) ⇒ 1M NaCl(aq)
(1) Digital thermometer ±0.1 ° C (2) Sparkvue (3) Scale ±0.001g (4) 100 ml Beaker
(5) 50 ml E-flask ±0.6ml
(6) Plastic cup (7) Cupcake case (8) Pipette
(1) 50 ml of HCL was poured into a 50 ml E-flask ±0.6ml with a pipette and then poured into a 100 ml beaker (2) A digital thermometer was put inside the beaker and sparkvue was started for a short while before 4.0 grams of NaOH, measured with a scale with a cupcake case of ±0.001g was put into the beaker and the first enthalpy change was measured with the graph. (3) The beaker was placed into a plastic cup filled with water for a more efficient way of cooling it down before 50 ml of water was poured into the solution and the second enthalpy change was measured. (4) 4.0 grams of NaOH was yet again measured with a scale and poured into HCL and the enthalpy change was measured. (5) The solution was then mixed with 50 ml of HCL and the last enthalpy change was measured reaching the same product being 1M NaCl(aq).
Eye protection is very crucial and must be used during this practical, considering Hydrochloric acid as well as Sodium Hydroxide is used. These two substances should not get into contact with your skin and if they do so, rinse immediately. Environmentally, all residues can be poured out in the sink.
Conclusion and evaluation
Even though Hess’s law is technically proven to be true my values at the end did not match up to prove the law with a difference of 9271 in value. However, I believe that is just because of the heat loss from the system, considering this was not really done in a calorimeter, but in a open system. I think that another reason would also be that on the second enthalpy change calculation, we should have waited longer so that the graph would have just been a straight line and no temperature change would have occured, considering no reaction should occur when a solution is just diluted. If this was done in a calorimeter, less heat would have been lost as well, which explains the failed results of the calculation. The technique with putting the solution in cold tub to speed up the process of cooling it down might have made the solution of the third enthalpy change too low (19.6 degrees celsius), which is why we had such a huge temperature difference in the equation.
(1) BBC.Using Hess’s law (Available online) :
(2) Libretexts. Hess’s law (Available online) :
mics/Thermodynamic_Cycles/Hess%27s_Law (Accessed 2017-02-17)
(3) Khan academy. Hess’s law example (Available online) :
https://www.khanacademy.org/science/chemistry/thermodynamics-chemistry/enthal py-chemistry-sal/v/hess-s-law-example (Accessed 2017-02-17)