CHM1311 lab

Topics: Thermodynamics, Enthalpy, Energy Pages: 12 (2683 words) Published: November 1, 2014
Experiment 3: Enthalpy of Various Reactions
Introduction
A coffee cup calorimeter is an apparatus that is used to measure the quantity of thermal energy gained or lost in a chemical reaction. This experiment utilizes this apparatus, which is made from two styrofoam cups with plastic lids and a thermometer, to measure changes in thermal energy of various reactions. When using this type of apparatus, it is assumed that no heat is transferred between the calorimeter and the surroundings, and that no heat is absorbed or released by the cup. This allows for determination of enthalpy change, which will then allow for the calculation of heat absorbed or released.

The specific heat capacity of a substance is the amount of thermal energy required to heat one gram of that substance by one degree. It is an intensive property, as opposed to the heat capacity, which is an extensive property that depends on the amount of substance present. In this experiment, the heat capacity is determined by measuring the change in temperature of the cold water when a hot metal (that does not react with the water) is placed in it. It is assumed that the specific heat capacity is constant in the temperature range, although in reality it does vary with temperature.

q = mcΔT

    The formula above is used to determine the specific heat capacity (c). M represents mass and ΔT represents the change in temperature. The heat lost by the metal when placed in cold water is gained by the water. Thus, the heat gain of water must equal the heat loss of the metal.     

-q (metal) = q (water)

The specific heat capacity can be used to determine the molar mass of a metal using the equation below. The derivation of this equation was largely based off of an assumption by Dulong and Petit in 1819, that one mole of all metals have roughly the same capacity to absorb heat. They found that the heat capacity per weight of many substances hovered around a certain constant, and using this derived the following equation. cmet x MMmet = 25J/mol◦C

The molar enthalpy can then be determined by the following equation, using the amount of heat released in the reaction. ΔH = q = -mcΔT
n n

    This equation can be used for all three parts of the experiment.     
Procedure
As described in the lab manual (Enthalpy of Various Reactions, Dr. Rashmi Venkateswaran, 2013, Experiment 3, p. 34-38).

    

Observations/Data/Results

Table 1: Specific Heat Capacity of a Metal (Zinc)

DataTrial 1Trial 2
Mass of Zinc (g)10.5110.77
Mass of Empty Calorimeter (g)9.188.81
Volume of Distilled Water in Calorimeter (mL)20.0020.00
Mass of Calorimeter with Water (g)28.5427.52
Temperature of Metal in Boiling Water (◦C)100.00100.00
Change in Temperate of the Water (◦C)3.604.10
Mass of Water (g)19.3618.71
Energy Gained by Water (J)291.61320.96
Change in Temperature of Zinc (◦C)-75.0-73.9
Specific Heat Capacity of Zinc (J/g◦C)0.369950.40327
Actual Molar Mass of Zinc (g/mol)67.3161.74
Percent Error for Specific Heat Capacity (%)95.59104.20
Percent Error for Molar Mass (%)102.9594.43
Observations:
-The zinc metal was shiny and silver. It was broken up into square like pieces, and was very light. It was malleable, and easily breakable into smaller pieces. -The water in the beaker on the hot plate took some time to boil. -The thermometer was moved straight from the hot beaker to the calorimeter, resulting in a quick temperature change. -It took about four to four and a half minutes for the final solution to settle at a constant temperature.

Table 1.1: Specific Heat Capacity of a Metal (Zinc); Temperature (ᵒC) vs Time (min); Trial 1 (left) and Trial 2 (Right) Time (minutes)Temperature (ᵒC)Time (minutes)Temperature (ᵒC) 0:3021.40:3021.9

1:0021.41:0022.0
1:3021.21:3022.0
2:0021.32:0022.0
2:3021.42:3022.1
3:0021.43:00...
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