Introduction:
Thermochemistry is the branch of chemistry that studies the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase may change, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system’s energy exchange with its surroundings. In combination with entropy determinations, it is used to predict whether a reaction is spontaneous or non-spontaneous, favourable or unfavourable. Chemical reactions can be divided into two based on the energy changes that occur during the reaction which are endothermic and exothermic reactions. Endothermic reaction is a chemical reaction that absorbs heat energy from the surrounding. Hence, it has a positive value for enthalpy change of reaction. In contrary, exothermic reaction is a chemical reaction that releases heat energy to the surroundings, resulting in a negative value for enthalpy change of reaction.
Enthalpy change of a reaction is the heat energy change in a chemical reaction, for the number of moles of reactants shown in the chemical equation. Standard enthalpy of reaction, ΔrH⊖ is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions. Enthalpy change of reaction has an interrelationship with Hess’ Law which states that the total enthalpy change during the complete course of a reaction is same whether the reaction is made in one step or in several steps. In other words, if a chemical change takes place by several different routes, the overall enthalpy change is the same, regardless of the route by which the chemical change occurs provided the initial and final condition are the same. The purpose of this experiment are to measure and compare the amount of heat change involved in three separate but related reactions and to study the experimental verification of Hess’ Law.
Experiment 10
Enthalpy change of reaction.
Objectives: To measure and compare the amount of heat change involved in three separate but related reactions, which are: Reaction 1: NaOH(s) + water → Na+(aq) +OH-(aq)
Reaction 2: NaOH(s) + HCl(aq) → NaCl(aq) +H2O(l)
Reaction 3: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Materials: Distilled water, sodium hydroxide and hydrochloric acid. Apparatus: Styrofoam cup, thermometer and measuring cylinder.
Procedure:
Part A: Dissolving sodium hydroxide in water
1. 100 ml of distilled water was added into a nested Styrofoam cup.
2. The distilled water is stirred carefully with a thermometer until a constant temperature is reached. This temperature was recorded.
3. The bottle containing sodium hydroxide solids are weighed. The mass is recorded down to 2 decimal points.
4. All the sodium hydroxide solids are poured into the cups of water.
5. The solution is stirred gently with the thermometer until the sodium hydroxide solid is completely dissolved. The highest temperature reached is recorded.
6. The empty bottle is weighed to calculate the mass of sodium hydroxide solid completely dissolved. The mass is recorded down to 2 decimal points.
Part B: The reaction of sodium hydroxide solid with hydrochloric acid 1. Steps 1-6 are repeated, the 100 ml of water is replaced with 100ml of 0.25M hydrochloric acid.
Part C: The reaction of sodium hydroxide solution with hydrochloric acid 1.100mL of 0.05 M hydrochloric acid was measured using a measuring cylinder and was added into a nested Styrofoam cup.
2.100 mL of 0.05 M sodium hydroxide solution was measured and added into another measuring cylinder.
3.The solutions were stirred carefully and separately with a thermometer until a constant temperature was reached. The temperatures were recorded.
4.The 100mL of 0.5 M sodium hydroxide solution was poured into the Styrofoam cup containing the 100mL of 0.5 M hydrochloric acid.
5.The cup was covered and the solution was stirred gently with the thermometer. The highest temperature reached was recorded.
Results:
Part A: Dissolving sodium hydroxide solid in water
Initial temperature: 26 c
Final temperature: 29 c
Temperature change: 3 c
Mass of bottle containing NAOH solids: 12.31 g
Mass of empty bottles: 11.12 g
Mass of NaOH solids added into water: 1.19g
Part B: The reaction of sodium hydroxide solid with hydrochloric acid Initial temperature: 26 c
Final temperature: 32 c
Temperature change: 6 c
Mass of bottle containing NAOH solids: 14.40g
Mass of empty bottle: 13.19J
Mass of NAOH solids added into 0.25M HCL: 1.21g
Part C: The reaction of sodium hydroxide solution with hydrochloric acid Initial Temperature of 0.5M NaOH = 25°C
Initial Temperature of 0.5M HCl = 25°C
Average Temperature = 25°C
Final Temperature = 29°C
Temperature change = 4°C
Calculation:
Part A
Part B
Part C
OUESTIONS:
1. Add algebraically the chemical equations of reactions 1 and 3. Is the resulting chemical equation the same as that of reaction 2?
2.Add the enthalpies of reaction 1 and 3 as obtained from this experiment. Is this sum about the same as enthalpy of reaction 2 obtained from this experiment? What law explains this phenomenon?
Discussion:
In part A, the reaction involved is the enthalpy change of solution as the heat released when 1 mole of sodium hydroxide solids is dissolved in a large amount of water, so that no further heat released occurs if more water is added to the solution. Sodium hydroxide solids dissolve in water to produce or dissociate into sodium and hydroxide ions as below: NaOH (s) + water → Na+ (aq) + OH- (aq); ΔrH⊖ = –
In part C, enthalpy change of neutralization takes place as heat energy is released when 1 mole of hydrogen ions from hydrochloric acid reacts with 1 mole of hydroxide ions from sodium hydroxide solution to produce 1 mole of water molecules. This reaction can be expressed by the thermochemistry equation below: NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l); ΔrH⊖ = –
In part B, enthalpy change of solution is involved although the reactants are similar to that in part C but solids of sodium hydroxide are used instead. In this case, the hydrochloric acid acts as a solvent to dissolve sodium hydroxide solids and then reacts with it when it is in aqueous state to produce a salt, sodium chloride and water. NaOH (s) + HCl (aq) → NaCl (aq) + H2O (l); ΔrH⊖ = – Since thermochemistry has an interrelationship with Hess’s Law, this explains why the enthalpy change of reaction in part B is higher than that in part C. Hess’ Law states that a reaction consists of a number of steps. The overall heat change is equal to the sum of the heat energy changes for all the individual steps, not dependent on the route taken. In this experiment, the enthalpy change of reaction in part B is the sum of the heat change in parts A and C. In other words, the total heat energy released in part B is the heat energy evolved during sodium hydroxide solids dissolve in the hydrochloric acid and the heat released from its reaction with acid (neutralization) right after it exists in aqueous state. Therefore, part B has a higher enthalpy change of reaction than part C which only involves the neutralization between sodium hydroxide solution and hydrochloric acid. By comparison, in part B, the experimental total heat released is slightly lesser than the theoretical total heat released due to some of the heat are lost to the surroundings and absorbed by the styroform cup as well as the thermometer. All the reactions in parts A, B and C are exothermic due to the following observation and calculations: 1.All the enthalpy changes of reaction have negative value. 2.During the experiment,
the styroform cups become slightly hot. 3.The final temperatures of all reactions are higher than the initial tempereatures. This means that the rate of formation of chemical bonds is higher than the rate of breaking of chemical bonds. Hence, more heat energy is released to the surroundings. The precaution that should be taken is styroform cups must be used because they are poor heat conductors to reduce the heat loss to the surrounding. The thermometer must be placed in the solution for a few minutes before taking the reading to ensure that the solution has reached a uniform temperature. The reaction mixture in the styroform cup must be stirred slowly and continuously so that the temperature of the solution is uniform. The thermometer should not be taken out of the reaction mixture when taking the reading. The styroform cup is covered with a plastic cover to minimise the heat loss to the surrounding. The thermometer reading must be observed throughout the experiment so that the highest temperature reached can be recorded.
Conclusion:
The enthalpy change of reaction in parts A, B and C are -50.208 kJ mol-1, -83.68 kJ mol-1 and -58.576 kJ mol-1 respectively. All reactions are exothermic as the enthalpy change of reaction shows a negative value for each of them. Since the reaction of sodium hydroxide and hydrochloric acid to form sodium chloride and water is carried out in a series of steps plus no external work is done, the principle of Hess’ Law is obeyed as the enthalpy change of reaction is equal to the sum of the enthalpy changes for the individual steps.
References:
Internet:
http://en.wikipedia.org/wiki/Exothermic_reaction
http://www.s-cool.co.uk/a-level/chemistry/chemical-energetics/revise-it/enthalpy-changes
http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Enthalpy/Heat_of_Reaction Chapter
Yew Lee, Loh. 2004.Chemical energetics. STPM Physical Chemistry Volume 2, ed N.Sivaneson. 349 & 350. Selangor ; Penerbitan Pelangi SDN. BHD.
You Sie, Lim. 2007.Thermochemstry. In Pre-U Text STPM Physical Chemistry, ed. Kim Hong, Yip, 202–201,290– 293. 374 &375. Rawang : Pearson Malaysia SDN. BHD.
