Measuring An Enthalpy Change Practical

A thermochemical equation is the chemical equation for a reaction in which

the enthalpy of reaction for the molar amounts stated is written directly after the equation.

In a thermochemical equation it is important to note state symbols (solid, liquid or gas) because the enthalpy change, ?H, depends on the phase of the substances

The following are two important rules for manipulating thermochemical equations:

When a chemical equation is reversed, the value of ?H is reversed in sign.

When a thermochemical equation is multiplied by any factor, the value of ?H for the new equation is obtained by multiplying the ?H in the original equation by that same factor.

How can ΔH be discovered if the enthalpy changes for reactions are too difficult or impossible to determine by experiment? e.g. the reaction is too slow or too fast or by-products are formed.

Hess’s Law states that the overall reaction enthalpy is the sum of the reaction enthalpies of each step of the reaction.

If ΔH for each step is known, then these can be combined to give ΔH for the overall reaction.

using enthalpy of combustion data

The simplest way to apply Hess’s Law is to remember that the enthalpy change of any reaction is equal to the total formation energy of the products minus that of the reactants

Σ is the mathematical symbol meaning “the sum of”, and m and n are the coefficients of the substances in the chemical equation.

Method

Collecting data for the determination of H1

Collecting data for the determination of H2

Hess's Law

In a thermochemical equation, it is important to note state symbols (solid, liquid or gas) because the enthalpy change, ?H depends on the phase of the substances.  In some cases, it is very difficult to determine the enthalpy changes for reactions through experiment like in situations where the reaction is too slow or too fast or by-products are formed. In such situation, Hess's law is applied. When anhydrous copper (II) sulfate is added to water then there will be a reaction which involves the physical change as the anhydrous copper (II) sulfate (white in colour) changes to hydrated copper (II) sulfate (blue in colour). And during that reaction there will be change in temperature. There will increase in temperature as this reaction takes place therefore the reaction is an exothermic reaction. When water is added to the anhydrous copper (II) sulfate, the water molecule will hence chemically combine with the anhydrous copper (II) sulfate to form a hydrated copper (II) sulfate having water molecules chemically combined as seen below;

CuSO₄ (s) + 5H₂O (l)            CuSO₄.5H₂O (s)

To obtain the enthalpy change for the reaction when anhydrous copper (ii) sulfate is reacted with water to get hydrated copper (ii) sulfate, this enthalpy cannot be measured directly.

Enthalpy change is the amount of energy which is absorbed or evolved during chemical reaction at a constant pressure. An easier way to obtain the change in enthalpy for a chemical reaction is to take change in the temperature as a result of the same reaction.

ΔH = m c_p ΔT…………………………………………………………………………………… 1

ΔH = the enthalpy of the reaction

ΔT = change in temperature

C_p = the heat capacity of the substance which changes temperature

m = the mass of the sample which changes the temperature

In a thermochemical equation, it is important to note state symbols (solid, liquid or gas) because the enthalpy change, ?H, depends on the phase of the substances.  In some cases, it is very difficult to determine the enthalpy changes for reactions through experiment like in situations where the reaction is too slow or too fast or by-products are formed (Cemic, 2015). In such situation, Hess's law is applied.  The Law states that the enthalpy change for a chemical reaction is independent of the route taken. This implies that the change in enthalpy for the overall reaction will be identical irrespective of how many steps are covered (Dahm, 2014). This law can be illustrated as below.

Collecting data for the determination of H1

The enthalpy change moving from point A to B can be obtained by adding the values of the values of the enthalpy changes for the reaction A to X, X to Y and Y to B (Sato, 2014)..

Hr=  +  +        

Hess's Law using enthalpy of formation data can be illustrated as below

DH[reaction] = sum DH_f^o[products] - sum DH_f^o[reactants]

Hess's Law using enthalpy using bond enthalpy term data can be illustrated as below

DH[reaction] = sum E[reactants] - sum E[products]

Hess's Law using enthalpy of combustion data

DH[reaction] = sum DH_com^o[reactants] - sum DH_com^o[products]

For this experiment, it basically involves the reversible reaction where heat is evolved after the fourth minute (after adding anhydrous copper (II) Sulphate). That illustrates that the reaction is exothermic- heat is evolved to the environment.

Collecting data for the determination of H1

A suitable table of results is constructed to allow recording of the temperatures at an interval of 15 minutes. Anhydrous copper (ii) sulfate was weighed between 3.9 g and 4.10 g in a dry stoppered weighing bottle. The stock of the solid is kept in a closed container during weighing.  The skin contact with chemical was avoided and precious mass was recorded. A measuring cylinder was used to place 25cm³of deionized water into a polystyrene cup and its temperature was recorded every minute as the liquid was stirred continuously (Letcher, 2017).

 At the fourth minute powdered anhydrous copper (ii) sulfate was added rapidly to the water in the polystyrene cup as stirring continuous but at this time the temperature was not recorded. The solution was stirred and its temperature was recorded in the polystyrene cup.  At the fifth minutes and every minute up to the fifteenth minute, the solution was stirred and its temperature was recorded in the polystyrene cup (Lvov, 2012). A graph of temperature (on the y-axis) against time was plotted. Two separate best fit was drawn one which joins the points before addition while the other which joins the points after addition, both lines were extrapolated to the fourth minute and the temperature change in the experiment was determined.

Collecting data for the determination of H2

A suitable table of results was constructed to allow recording of temperatures at an interval of 15 minutes. Anhydrous copper (ii) sulfate was weighed between 6.20 g and 6.30 g in a dry stoppered weighing bottle.  A measuring cylinder was used to place 24cm³of deionized water into a polystyrene cup and its temperature was recorded every minute as the liquid was stirred continuously. The total amount of water would be approximately the same as in experiment 1 since hydrated crystal contains water.

Collecting data for the determination of H2

At the fourth minute powdered copper (ii) sulfate was added rapidly to the water in the polystyrene cup as stirring continuous but at this time the temperature was not recorded.  At the fifth minutes and every minute up to the fifteenth minute, the solution was stirred and its temperature was recorded in the polystyrene cup (Tsao, 2012).  A graph of temperature (on the y-axis) against time was plotted. Two separate best fit was drawn one which joins the points before addition while the other which joins the points after addition, both lines were extrapolated to the fourth minute and the temperature change in the experiment was determined.  

Equipment and materials used

Balance

Stand and clamp

Beaker (250 or 400 cm³)

Measuring cylinder × 2 (25 cm³)

Weighing bottle × 2

Thermometer (0.10 C divisions)

Polystyrene cup (calorimeter)

Stopwatch

Stirrer

Hydrated copper (ii) sulfate small crystals – harmful, dangerous to the environment

Anhydrous copper (ii) sulfate powder – harmful, dangerous to the environment

Lab coat and eye protection.

As seen from the result above for both from the first minute to the fourth minute there was no change in temperature since there was no reaction taking place. But after the fourth minutes, from the fifth to the fifteenth minute there was a change in temperature hence change in the enthalpy which is obtained from the equation 1 above. For the experiment 1, there was a great change in temperature hence a great change in enthalpy while in experiment 11 there was a slight change in temperature hence a relatively small change in the enthalpy (Dahm, 2014).  . The values of change of enthalpy obtained from calculation from both the experiment had very small disparity from the theoretical values as seen above.

The disparity of the results is less than 2 % shows that the variation between the practical value and the theoretical value is very small thus it is highly acceptable. (Lvov, 2012).  Sources of these errors may be incorrect measuring of the final and initial temperature, incorrect reading of the masses which are used in the reaction (Cemic, 2015). There was increase in temperature after addition of copper (II) sulphate showing change in enthalpy. In some cases the reading may be taken when the anhydrous copper ( II) sulphate was not fully reacted with water to form hydrated copper (II) sulphate .

Conclusion

 At the end of the experiment, it was probable to obtain the enthalpy change for the reaction when anhydrous copper (ii) sulfate is reacted with water to get hydrated copper (ii) sulfate. Therefore the aim of the project was fully met at the end of this experiment. The enthalpy change obtained from calculation was almost the same as the theoretical value as seen in the above calculation giving the negligible percentage error. The negligible percentage error indicates that the value from calculation was very correct therefore the objective and aims of the assignment was fully met. For a future experiment, the above errors which occurred can be eliminated by ensuring that the temperature is taken when the reaction is fully complete.

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