CHM031L Experiment 2: Heats of Combustion

Vocabulary flashcards reviewing concepts, equations, apparatus components, and procedural parameters for Experiment 2 (Heats of Combustion) in CHM031L.

Combustion

A chemical reaction between substances, usually including oxygen and usually accompanied by the generation of heat and light in the form of flame.

Adiabatic Bomb Calorimeter

An apparatus used to analyze heat transfer in combustion reactions under constant volume conditions, surrounded by an adiabatic shield so that no heat enters or leaves the calorimeter.

Bomb

A strong closed reaction container inside a calorimeter where combustion occurs under constant volume conditions.

Motorized Stirrer

A component of a bomb calorimeter that continuously stirs the water bath to ensure heat is distributed evenly throughout the calorimeter.

First Law of Thermodynamics

The principle stating that energy can be converted from one form to another through heat, work, and internal energy, but cannot be created or destroyed, represented as $\Delta U = q + w$.

Expansion and Compression Work

Work done by or on a gas given by $w = -p\Delta V$, which equals zero in a bomb calorimeter because the volume is constant ($\Delta V = 0$).

Change in Internal Energy ($\Delta U$)

The total change in internal energy of a system, which in a constant-volume bomb calorimeter equals the heat of reaction ($\Delta U = q = q_{rxn} = q_{sys}$).

Enthalpy Change Equation for Bomb Calorimetry

The equation relating enthalpy change to heat transferred and gas mole change: $\Delta H = q_{sys} + RT\Delta n_{gas}$.

Change in Moles of Gas ($\Delta n_{gas}$)

The change in gaseous moles calculated as $\Delta n_{gas} = n_{gas,products} - n_{gas,reactants}$, counting only gaseous species.

Benzoic Acid

A standard compound with a molar mass of $122.12\,g\,mol^{-1}$ used in the first operation of bomb calorimetry to determine the calorimeter constant ($C_{cal}$).

Pellet Press

A lab device (pelletizer) used to press approximately $0.6\,g$ of a powder sample into a compact pellet prior to placement in the sample cup.

Ignition Wire

A fine fuse wire (approximately $10\text{--}11\,cm$ long) attached to electrodes and touching the sample to initiate combustion inside the bomb.

Vent Cap

A valve cap on the bomb head assembly that is opened to expel air during assembly and screwed shut tightly to prevent oxygen leaks during operation.

Calorimeter Constant ($C_{cal}$)

The heat capacity of the calorimeter assembly, calculated using benzoic acid standard via $C_{cal} = \frac{n q_{std,surr} + m_{wire} q_{wire,surr}}{\Delta T}$.

Heat of Combustion per Gram of Wire ($q_{wire,surr}$)

A constant value of $5858\,J\,g^{-1}$ representing the heat released per gram of burned fuse wire.

Temperature $T_b$

The temperature corresponding to when $60\%$ of the total temperature change is achieved, calculated as $T_b = T_a + 0.60(T_c - T_a)$.

Corrected Temperature Change ($\Delta T$)

The total temperature change adjusted for heat transfer rate corrections, calculated as $\Delta T = T_c - T_a - r_1(a - b) - r_2(c - b)$.

Sucrose

A compound with a molar mass of $342.3\,g\,mol^{-1}$ whose heat of combustion ($\Delta H_c$) is determined in the second operation of the experiment.

Combustion Reaction of Sucrose

The balanced chemical reaction $C_{12}H_{22}O_{11}(s) + 12O_2(g) \rightarrow 12CO_2(g) + 11H_2O(l)$, for which $\Delta n_{gas} = 0\,mol$ and $\Delta \bar{H} \approx \Delta \bar{U}$.