Abstract:

Calorimetry is used to find the enthalpy of a MgO solution.

Going through the procedure properly, the enthalpy of the formation for MgO

will be found.

This will be done by first determining the heat of the first

and second reaction. The known value for formation of water will be plugged in

the equation to give the enthalpy of MgO. The goal of this experiment is to us

Hess’s Law to calculate rxn using the equation reaction= products,- reactants .

Introduction:

The main purpose of this

experiment is to find the heat of formation for magnesium. The reaction and

heat between Magnesium and hydrochloric acid is measured and recorded. To find

the heat given off by these reactions the equation used in coffee

cup calorimetry initially takes the initial and final temperature of the water

to find the amount of heat that is being absorbed or released during the

reaction. The formula used to find the heat is . As m is the mass of the solution, s is the specific heat

and is the final temperature- the initial temperature of the

solution. Next, the magnesium oxide and HCl is mixed and the heat in the reaction is recorded again using the

formula for coffee calorimetry. This method used to measure the heat of the

reactions, coffee cup calorimetry is designed to measure the heat lost or

gained, as mentioned before. Although Hess’s Law has the equation reaction =

products- reactants, another rule it has is by adding all the enthalpy changes

for all the steps is the same as the overall enthalpy of a reaction. Coffee

Calorimetry also has a useful formula although Hess’s Law is used, another way

of finding the specific heat is the formula .

Data:

Table 1: Initial and Final Temperatures of Mg

Initial Mass (g)

Minimum temperature (Celsius)

Maximum temperature (Celsius)

Trial 1

0.50

21.5

56.6

=+35.1

Trial 2

0.55

21.7

60.48

=+38.7

Table 2: Initial and Final Temperatures of MgO

Initial Mass (g)

Minimum temperature (Celsius)

Maximum temperature (Celsius)

Trial 1

0.846

21.7

32.4

Trial 2

.927

21.6

33.3

Table 2- Trial 2 Initial and Final Temperatures of Mg and MgO

Results:

rxn Trial 1

(kJ/mol)

rxn Trial 2

(kJ/mol)

rxn Average for 2 trials

(kJ/mol)

4(kJ/mol)

Mg

MgO

–

–

Discussion:

The experiment went smoothly, the procedure was successful

and the results were able to help accomplish the goal of finding the amount of

heat released in the reaction. There may have been a few sources of error but

the most effecting source that may have caused error is measurement. Getting

more than necessary of a solution is the most likely. Although the measurements

read to be correct, transferring solutions from one thing to another may also

affect the amount of solution. Not cleaning the measuring equipment properly may

lead to contamination; that also may have affected the outcome.

For an exothermic reaction the temperature rises due to heat

release to the surroundings of the system and H is negative. For reaction 1 and

2 the reactions are exothermic because the H number is negative, which

indicates the heat is being released from the system. In this lab quantity or q

is different from H°rxn, the definition of q is how much heat is lost or gained

in a system (chemwiki).The definition for H is a thermodynamic quantity equivalent to

the total heat content of a system. It is equal to the internal energy of the

system plus the product of pressure and volume.(google def of enthalpy) H = E + PV. H and q are only similar when it

comes to constant pressure.

Further Analysis

A.) The

first step is to obtain the mass of the alloy, then dissolve the alloy in the

HCl solution. Then, one would need to take out the non reacted metal and weigh

it. This finds the mass of Mg by taking the whole mass of HCl and adding the

mass of the alloy.

B.) The

percent would be calculated by multiplying the answer by 100. If the sample is

30% then it would be 5g multiplied .3, which equals 1.5grams of Mg, then it would

be multiplied by 24 mol of Mg to get .0625.