?Enhtalpy change – Hess’ law Essay Sample

Enhtalpy alteration – Hess’ jurisprudence

Vanier college

Part A.

Aim:

The aim of this lab was to find the standard heat formation ( ?H°F ) of MgO, utilizing a calorimeter and finding the heat content of two reactions. Using Hess’ jurisprudence we were able to find the standard heat formation of MgO.

Introduction

Energy exchanged in a chemical reaction can either be in the signifier of heat or visible radiation. If light is involved a freshness is seen, if heat is involved the temperature of the system will alter ( lab manual page 35 ) . The sum of heat exchanged under changeless force per unit area is called the heat content alteration, this can either be endothermal or exothermal. Endothermic if heat is absorbed by the reaction ( positive mark ) , and exothermal if heat is released by the reaction ( negative mark ) . If a reaction is reversed, the mark of ?H is besides reversed. The heat alteration that consequences from the formation of one mole of a compound from its elements in the regular provinces is known as its standard heat of formation ( ?°f ) . The specific heat of the solution is the sum of heat required to raise the temperature of 1 gm of the solution by 1°C. To happen the enthalpy alteration of a reaction utilizing a calorimeter we added the Mg and MgO solids to two HCl solutions in two separate Styrofoam cups, and utilizing a thermometer and taking note of the temperature alteration every 30 seconds for 7.5 proceedingss we so plotted this on a graph.

Generalizing the points from the concluding temperature after the 7.5 proceedingss, through where the heat of the reaction begins chilling, we were able to find the maximal heat that would hold been achieved if their was no heat loss in the reaction. Using this and the initial temperature we found ?T for both of the reactions. We so calculated our multitudes of solutions by weighing exactly the sum of Mg and MgO used, every bit good as the multitudes of the HCl solutions. The specific heat of solution was given which was 4.184 J/g°C. Using the expression q= ( ?T ) ( Specific heat of solution ) ( Mass of solution ) we were able to find the sum of heat released by the reactions. Knowing our restricting reagents this let us cipher the heat content alterations of each reaction in Kj/mol. The enthalpy alteration for reaction 3 was given. Using Hess’ jurisprudence utilizing all three equations we were able to find the heat of formation ( ?°f ) of MgO in Kj/mol.

Experimental

Refer to lab manual pages 38-39, besides refer to attached pre-lab.

Data/Results
Chemical reaction of Mg + 2H ?Mg +H2

Initial temp: 19°C
Extrapolated temp: 60.8°C
Temperature difference ( ?T ) : 41.8°C
Mass of Styrofoam cup: 3.533g
Mass of Styrofoam cup and solution: 64.194g
Mass of HCl solution: 60.661g
Measure of heat released: 10.71680 Kj
Mass of Mg used: 0.616g
Sum of Mg used: Millimeter: Mg = 24.31g/mol ? ( .616g Mg ) ( 1mol/24.31g Mg ) =.0253 mols of Mg used.

This chart is of reaction temperature, over seven and a half proceedingss, of the equation Mg ( s ) + 2H ( Aq ) ? Mg ( Aq ) + H2. We added Mg to our 2H+ and noted down the temperature alteration at every 30 seconds. We so extrapolated the values back from our concluding temperature through the point in where the chilling started. This made it possible for us to find ?T for the reaction.

?T= Our initial temperature was 19°C and the extrapolated temperature was 60.8°C. Text. –Tint. = 41.8°C ( ?T )

Mass of solution = We added the mass of the HCl solution with the mass of Mg used: 60.661g+0.616g =61.277g of solution

Specific heat capacity was given as 4.184 J/g°C because the solution is largely H2O.

Measure of heat released = ?T ? Mass of solution ? Specific heat of solution:
q= ( 61.277g ) ( 4.184J/g°C ) ( 41.8°C ) = 10,716.80J
10,716.80 ? 1000 = 10.71680 Kj = q solution
qrxn= ?qsoln
qrxn= ?10.71680 Kj
?Hrxn = qrxn/mol of solid used
?Hrxn= ?10.71680Kj/.0253mols =?424 Kj/mol

Chemical reaction of MgO + 2H ? Mg + H2O

Initial temp: 19°C
Extrapolated temp: 31.45°C
Temperature difference: 12.45°C
Mass of Styrofoam cup: 3.546g
Mass of Styrofoam cup and solution: 63.990g
Mass of HCl: 60.444g
Measure of heat released: 3.20119Kj
Mass of MgO used: 1.010g
Sum of MgO used: MM MgO: 24.305 + 16.00= 40.305 g/mol ? ( 1.010g MgO ) ( 1 mol/40.305g ) = 0.025 mols of MgO used

This chart has the same fable as the old one but it is for the reaction MgO ( s ) + 2H ( Aq ) ? Mg ( Aq ) + H20 ( L ) . Again, generalizing the values back made it possible to happen maximal temperature reached and determine ?T.

By utilizing the same methods of computations seen above we obtain the heat released for this reaction.

Measure of heat released = ?T ? Mass of solution ? Specific heat of solution:
q= ( 61.45g ) ( 4.184J/g°C ) ( 12.45°C ) = 3,201.19J
3,201.19J? 1000 = 3.20119 Kj = q solution
qrxn= ?qsoln
qrxn= ?3.20119Kj
?Hrxn = qrxn/mol of solid used
?Hrxn= ?3.20119Kj/0.025mols =?128 Kj/mol

Hess’ Law
Using Hess’ jurisprudence we can utilize all three reactions to find the heat of formation of Mg oxide. Equation 1 releases 424 Kj of energy, 2 releases 128 Kj, and 3 releases 285 Kj. Because they are let go ofing this energy during the reaction they are exothermal and hence are negative vaues. We use Hess’ jurisprudence and unite these three reactions ciphering them algebraically, for equation 2 we need to change by reversal the reaction for it to suit in our equation to utilize Hess’ jurisprudence. To execute the reaction Mg ( s ) + ?02 ? MgO we need to utilize Hess’ jurisprudence because if non it would give off an unmanageable fire which would do it impossible to utilize a calorimeter. Some mistakes that may hold occurred in our informations would be ; from the transferring of the Mg or MgO to the Styrofoam cup, we may hold spilled some, from the heat loss to the milieus seeing as they were merely Styrofoam cups that weren’t covered, and besides because of our human physiological reaction that may hold caused little mistake in temperature readings.

1. ) Mg ( s ) + 2H ( aq ) ? Mg ( aq ) +H2 ?H= ?424 Kj
2. ) MgO ( s ) + 2H ( aq ) ? Mg ( aq ) + H20 ?H= ?128 Kj
3. ) H2 ( g ) + ?O2 ( g ) ? H2O ( L ) ?H= ?285.5 Kj

Mg ( s ) + ?O2 ( g ) ? MgO ( s ) ?H= ? ? ?

1 ) Mg ( s ) + 2H ( aq ) ? Mg ( aq ) +H2 ?H= ?424 Kj
Reversed:2 ) Mg ( aq ) +H2O ( L ) ? MgO + 2H ?H= 128 Kj
3 ) H2 ( g ) + ?O2 ( g ) ? H2O ( L ) ?H= ?285.5 Kj

Mg ( s ) + ?O2 ( g ) ? MgO ( s ) ?H= ?581.5 Kj

Decision
After happening the ?H of equations 1 and 2 we were able to use Hess’ jurisprudence to find the standard heat of formation of MgO. The ?H of reaction 1 was ?424 Kj, for 2 ?H= ?128 Kj and for 3 ( given value ) ?H= ?285.5 Kj. After using Hess’ jurisprudence we found the the ?H°F for the reaction Mg ( s ) + ?02 ? MgO was ?582 Kj/mol. This tells us that in the formation of MgO there is 582 Kj of energy released.

Part B.

?H.lattice & gt ; ?H.hydration = endothermal procedure

The procedure of fade outing NH4Cl is endothermal. In order for the crystal solid to disassociate into ions, energy must be absorbed, in this instance the lattice energy was overcome and the solid separated into ions and no solid was left. Knowing that the solid wholly dissolved in the solution we are able to find that the lattice energy is greater so the hydration energy significance that the reaction is endothermal. After the dissociation of the NH4Cl the temperature of the H2O dropped from 16°C to 9°C this is because the ions are chilling down the H2O in an exothermal procedure, known as hydration energy. An illustration of a exothermal procedure is the combustion of a taper, the taper is giving off heat to its milieus. A good illustration of an endothermal procedure is photosynthesis. Plants absorb energy from the Sun leting them to bring forth nutrient and grow.

Part C.

Energy exchanged in a chemical reaction can either be in the signifier of heat or visible radiation. If light is involved a freshness is seen, if heat is involved the temperature of the system will alter ( lab manual page 35 ) . During this experiment we observed that when the clear liquid luminol is assorted with potassium ferricyanide a white bluish freshness of visible radiation is created from the mixture. The energy transmutation in this experiment creates this freshness, it is created from the decay of a high energy intermediate to a lower energy merchandise, the extra energy is seen visually as photons and bring forth the blue freshness. When affair is changed there is ever a transportation of energy. Whether it be in the signifier of heat or force per unit area. For affair to alter provinces the atoms either have to acquire “excited” or “calmed” . The more you heat particles the more aroused they get. This will bring forth a gas if heated high plenty. The more the atoms are cooled the more they slow down and finally if cooled plenty arrive at a solid province.