# Calorimetry Prelab

Topics: Enthalpy, Thermodynamics, Thermochemistry Pages: 6 (929 words) Published: November 29, 2012
Experiment 12

Calorimetry and Heat of Reactions
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PERFORMANCE GOALS:
1. To learn how to use of a calorimeter
2. To learn how to collect and manipulate data in the computer 3. To calculate the calorimeter constant
4. To use Hess' Law to find the heat or formation of magnesium oxide

CHEMICAL OVERVIEW:

Enthalphy: (ΔH) : when chemical or physical changes occur at a constant pressure.
Calorimeter: is an instrument with insulating walls where the reaction happens. Eq. 1: q rxn = -qsurrounding

Heat of capacity of the calorimeter: "Cp" must be calculated at the beginning of every calorimeter experiment in Joules/ °C

Heat Capacity of the Calorimeter:
The calorimeter constant is easily found by adding a fixed amount of hot water to a known amount of cold water and the change in temp for each recorded, due to the Law of Energy Conservation the amount of heat released by the hot water should be equal to the amount of heat absorbed by the cold water: Eq. 2 q released( Hot Water) = -q absorbed (Cold Water)

If there is a discrepancy between these two values use the following equation:

Eq. 3q= m x spht x ΔT

Where spht is the specific heat of the substance in J/ g°C
ΔT is the temperature change in °C
andm is the mass in grams

Mass of Cold Water|51.20 g|
Initial temperature of cold water|20.3 °C|
Mass of hot water|49.82 g|
Initial temp of hot water|98.2 °C|
Final temp of the mixture|58.3 °C|

Eq. 4qHot = mHot x sphtWater x ΔTHot
qHot = (4.184 J/ °C-g)(49.82g)(58.3 °C-98.2 °C) = -8317 J

Eq. 5qCold = mCold x sphtWater x ΔTCold
qCold = (4.184 J/ °C-g)(51.20g)(58.3 °C-20.3 °C) = 8142 J

8317-8142 = 175 joules

Cp = ( 175 J ) / (58.3 °C - 20.3 °C)
Cp = 4.6 J/ °C

HEAT OF REACTIONS:
q released = -q absorbed

Eq. 6 q released = - (q solution + q calorimeter )

Eq. 7 qSolution = mSolution x sphtWater x ΔTSolution

sphtsolution= 4.184 J/ g °C

Eq. 8q calorimeter= Cp x ΔT

Eq. 9 q reaction = ΔHreaction

Hess's Law
Hess's Law states that the enthalpy of a reaction is independent of the steps that it takes to get from reactants to products because enthalpy of reaction is a state function.

State Function-
depends on initial and final state but not on the path taken
Temperature
Volume
Pressure
Energy

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Mg (s) + 1/2 O2 (g) --> MgO ( s)

PRE-LAB ASSIGNMENT

1. Predict the product, balance the questions and write the net ionic equations for the reactions:

a. Mg (s) + HCI (aq) --->

b. MgO (s) + HC: (aq) --->

2. Write the reaction that represents the enthalpy of formation ( ΔHfor ) of water.

3. Use the table of the thermodynamic data in your text book to calculate the ΔH for each of the three reactions

REMEMBER
Eq. 10 ( ΔH °rxn = Σ ( nΔH °for )prod - Σ ( nΔH °for )react

1)

2)

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4. Use Hess's Law combining the three molecular equations to calculate the ΔHrxn for the reaction of the formation of MgO.

PROCEDURE

A. CALIBRATION OF THERMISTOR

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B. DETERMINATION OF THE HEAT CAPACITY OF THE CALORIMETER

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C. REACTION OF MgO AND HCI

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D. REACTION OF Mg AND HCI

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E. DATA AND CALCULATIONS

A. Calorimeter Constant
Mass of Styrofoam cup with lid + spin bar (g)||
Mass of cup with lid + spin bar + 50mL of room temp. water (g)|| Initial Temp of Room Temp. Water ( °C )||
Initial Temp of Hot Water( °C )||
Total mass at the end (g)||
Calculated Heat released by Hot Water...

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