IB Bussen

Hess Law: Neutralization of NaOH

RESEARCH QUESTION

Is the reaction endothermic or exothermic and can Hess law be used to calculate the enthalpy changes?

INTRODUCTION

In this experiment, we aim to calculate the enthalpy changes in the neutralization of NaOH and HCl, and to find out if the reaction is endothermic or exothermic. Using Hess Law, we will compare and analyze the results.

Independent variables: The type and amount of substance used in the reactions.

Dependent variables: The temperature recorded during the reactions. (Measured by a 30 second interval.)

Controlled variables: The time allowed for the reactions to take place.

Equipment used: Digital thermometer (± 0.1 °c), stopwatch, magnetic stirrer, 100 ml beakers, burette (± 0.04 ml), Digital scale (± 0.1 g), tripod and clamp.

Substances used: Water (H₂O deionised and tap water), Sodium Hydroxide (NaOH), Hydrochloric acid (HCl).

PROCEDURE

The experiment was carried out in three different reactions.

Setup: A 100 ml beaker was put on a magnetic stirrer with a digital thermometer placed on a tripod above it, its measuring device placed in the beaker to allow continuous measurement.

First reaction: 50 cm³ was measured using a burette and put in a beaker. The temperature was recorded. 1g of NaOH in the form of pastilles were also put in the beaker, and the stirrer put in action, while we began to record the temperature every 30 seconds for 240 seconds while the reaction took place, as seen in chart 1.

Second reaction: 25 cm³ of HCl with the concentration 1moldm^-3 was placed in the beaker. The temperature was recorded. 25 cm³ of sodium hydroxide solution with the concentration 1moldm^-3 was added to the beaker. Temperature was recorded every 30 seconds for 240 seconds while the reaction took place, as seen in chart 1.

Third reaction: 25 cm³ of HCl with the concentration 1moldm^-3 was placed in a beaker along with 25 cm³ of deionised water. The temperature was recorded. 1.0g of NaOH was added and the substances were allowed to react. Temperature was recorded every 30 seconds for 240 seconds while the reaction took place, as seen in chart 1.

H2O

REACTION 1: NaOH_(s) à Na⁺_(aq) + OH^-_(aq)

Chart 1: Temperature of substance measured every 30 seconds, during reaction 1

Time (s)	Temperature (°c)
0	20.3
30	21.5
60	22.3
90	22.7
120	23.2
150	23.6
180	23.8
210	23.9
240	24.1

CALCULATION OF ENTHALPY CHANGE (ΔH) IN REACTION 1

Heat evolved in reaction 1:

m*c*ΔT = heat energy

50cm³ = 50g = 0.05kg

ΔT = T_max – T_min

ΔT = 24.1-20.3= 3.8°c = 3.8K

0.05kg*4.18c*3.8K=0.7942 kJ

Amount of substance (NaOH) in moles: 1/40=0.025

Enthalpy change: -0.7942*(1/0.025) = -31.768 kJ mol ^-1

Chart 2. Temperature of substance measured every 30 seconds, during reaction 2.

REACTION 2: Na⁺ + OH^- + H⁺ + Cl^- à Na⁺ + Cl^- H₂O

Time (s)	Temperature (°c)
0	20.2
30	26.0
60	25.9
90	25.8
120	25.7
150	25.6
180	25.5
210	25.4
240	25.3

Graph 2, Temperature of substance measured every 30 seconds, during reaction 2, with values from chart 2.

CALCULATION OF ENTHALPY CHANGE (ΔH) IN REACTION 2

Heat evolved in reaction 2:

m*c*ΔT = heat energy

50cm³ = 50g = 0.05kg

ΔT = T_max – T_min

ΔT = 26-20.2= 5.8°c = 5.8K

0.05kg*4.18c*5.8=1.2122 kJ

Amount of substance (HCl) in moles: 25/1000=0.025

Amount of substance (NaOH) in moles: 25/1000=0.025

Enthalpy change (ΔH): -1.2122 *(1/0.025) = -48.488 kJ mol^-1

	H2O

REACTION 3: NaOH + H⁺+ Cl^- à Na⁺ + Cl^- + H₂O

Chart 2. Temperature of substance measured every 30 seconds, during reaction 2.

Time (s)	Temperature (°c)
0	19.9
30	22.6
60	24.7
90	26.5
120	28.0
150	29.3
180	29.8
210	30.0
240	30.0

Graph 3. Temperature of substance measured every 30 seconds, during reaction 1 with values from Chart 3.

HESS LAW:

ΔH₁ = -31.768 kJ mol^-1

ΔH₂ = -48.488 kJ mol^-1

ΔH₃ = X (-83.6 kJ mol^-1)

ΔH₃ = ΔH₁ + ΔH₂

ΔH₃ = -31.768 + (-48.488) = -80.255 kJ mol^-1

CONCLUSION

As can be seen in the graphs, the temperature of the substances rises, initially. This is evidence that the reactions are exothermic. This is supported by the calculations of enthalpy change where the third reaction showed a change of –83.6 kJ mol^-1. The quickest enthalpy change occurred in reaction 2, where the change in temperature to T_max was almost instant. By disregarding the calculated enthalpy change of the third reaction, we could use Hess law to show how accurate our calculations were. By adding the calculated enthalpy changes in reaction one and two, we got an alternative enthalpy change for the neutralization; -80.255 kJ mol ^-1, which is fairly similar to the initial estimation with a difference of 3.345 kJ mol^-1. Using Hess law is a fairly accurate way of measuring the enthalpy change of the reaction, and is so most definitely if the goal is merely to learn if the reaction is endothermic or exothermic.

EVALUATION

The slight difference in results of the two methods can be explained by several factors. The equipment used to measure the substances each had error margins:

Digital thermometer (± 0.1 °c)

Burette (± 0.04 ml)

Digital scale (± 0.1 g)

Using equipment of higher quality would result in higher consistency in the measurements. To further control the credibility of the results, and for the sake of comparison, average bond enthalpy could have been calculated. Repeating the reactions would also increase the precision of the results. The initial temperature of the substances were different in the reactions, something which might have an affect on the final results, since the reactions occur with greater speed the higher the temperature. By ensuring that each substance were at the same temperature, by the use of e.g. a Bain-marie.