Thrmochemistry Lab Report

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Introduction:

A transfer of energy in chemistry is usually measured by heat. This heat change often indicates a chemical reaction taking place. The enthalpy of reaction can be calculated from measuring this heat transfer. By using the enthalpy of reaction, other information concerning the reaction reactant and products can be derived. For example, this information can be used to determine whether the reaction is endothermic or exothermic. The main objective of this lab is exactly that, to calculate the molar enthalpy change of the reaction.

Chemical Principle:

 Mg (s) + H2SO4 (aq)            MgSO4 (aq) + H2 (g) +    H rxn[pic 4][pic 5]

10mL H2SO4 x (1mol/1000mL) =1.0 x 10-2 H2SO4    Limiting reactant

As mentioned, Thermodynamics plays an important role in today’s lab. The first law of Thermodynamics states that energy can be converted from one form to another, but cannot be created or destroyed. In today’s lab, heat transfer occurs with no temperature change. This is also called an isothermal heat transfer. A calorimeter can be used to measure these changes in heat along with volume. These volume changes are measured from the calorimeter compartment, in this case, consisting of ice and water. Another concept from chemistry and thermodynamics being utilized here is Hess’s Law. This states that the change in enthalpy is the same whether a reaction takes place in one step, or a series of steps, when reactants are converted to products. Based on this law, the lab today demonstrated taking the indirect route with the same outcome as would have happened had the more direct approach been used.

Procedure:

1. Test the calorimeter apparatus set-up

1. Ensure there are no leaks

1. Run the initial measurements

1. Measure 10mL of 1M H2SO4 solution into a clean test tube
2. Measure .2-.25 g of magnesium ribbon
3. Chill the acid sample in a beaker of crushed ice
4. Fill the calorimeter beaker with crushed ice
5. Insert the stopper assembly in to the beaker
6. Bring the water level up to the top of the pipette
7. Fill a large plastic ice bath bucket halfway with ice.
8. Place the calorimeter beaker inside

1. Read and record water level in the pipette and monitor its rate of fall as it reaches phase equilibrium

1. When the water level holds steady, add the magnesium strips to the reaction test tube
2. Start the timer
3. Continue recording until the rate of fall equals the rate initially observed

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Calculations:

ΔV = Ybefore - Yafter, x= 480s

ΔV = 3x10-4x + 0.4891

ΔV = 0.6331

ΔV= mass water x [dice + dwater] / (dice + dwater)

(0.6331)(.917) = mass water x .083

=7.452

qice = (mass ice melted x 333J/g)/1000

qice = (7.452 x 333)/1000

= 2.4815

qice = -qrxn

2.4815 = -(2.4815)

(qrxn / (1.0 x 10-2)) x (1 kJ/mol/103J)

= 469.3 KJ/mol

Percent error= ([ΔHobserved – Δexpected]/ Δexpected) x 100

Percent error= (469.3 – 466.9)/466.9

x 100

= .5%