Enthalpy change, of combustion

Cyclopropane possesses both significant angle and torsion strain. Strain in saturated cyclic hydrocarbons is determined from the heat of combustion, and comparisons are made per CH2 group in a molecule. For cyclopropane the molar enthalpy change of combustion is 2090 kJ mol-1, i.e. 696.5 kJ mol-1 per CH2 group. The corresponding figure for cyclohexane is 3948 kJ mol-1, i.e. 658 kJ mol-1 per CH2. If cyclohexane is taken to be strain free (which is a reasonable approximation), one can conclude that the strain in cyclopropane is (696.5 - 658) = 38.5 kJ mol-1 per CH2. It should be noted that these experimentally deter-... 

In the human body, foodstuffs are consumed slowly in a complicated series of chemical reactions. About 40% of the enthalpy change of combustion of a food is available to the body to be used in exercise. 

Figure 5.7 Simple apparatus used to measure enthalpy changes of combustion of liquids... 

Using an incorrect specific heat capacity in the calculation of the heat change. If a copper can is used as a calorimeter (Figure 5.7) during an enthalpy change of combustion investigation, then its specific heat capacity needs to be taken into account in the calculation. For example ... 

An example of the enthalpy change of combustion is the combustion of methane. The reaction can be described by the following thermochemical equation ... 

Enthalpy changes of combustion are always negative as heat is released during combustion processes. 

Measuring the enthalpy change of combustion of a liquid fuel using a flame combustion calorimeter... 

Use these results to calculate the molar enthalpy change of combustion. Compare the value with the literature value. 

The experimental literature value for the standard enthalpy change of combustion of methanol is... 

The error is due to the large heat losses that occur during the use of the flame combustion calorimeter. Heat losses to the surrounding air are relatively large, despite the use of heat shields. Also, heat energy from the flame heats up the material of the calorimeter itself, as well as the water. A correction can be made for the heat losses to the calorimeter if the specific heat capacity of copper is known or if the apparatus is standardized using a substance of known enthalpy change of combustion. 

The experimental literature values for standard enthalpy changes of combustion are obtained using a more accurate bomb calorimeter. These can be obtained for fuels as well as energy contents of dried foodstuffs. 

The enthalpy change of this reaction cannot be found direcdy by experiment because carbon dioxide is always formed when carbon reacts with even a limited amount of oxygen. This is an unavoidable reaction. However, the enthalpy changes of combustion of carbon and carbon monoxide can be found experimentally. 

Enthalpy changes of reaction from enthalpy changes of combustion... 

An enthalpy change can also be calculated from enthalpy changes of combustion. However, this can only be done for reactions in which the substances on botb sides of the equation can be burnt in oxygen (Figure 5.23). This method is widely used for organic compounds. 

CH2—, is -613 kJ mohh This type of simple calculation predicts that there should be an approximate linear relationship between the enthalpy change of combustion of an alkane and the number of carbon atoms (Figure 5.40). Experimental values confirm this prediction. 

The standard-state enthalpy change of combustion of methane at 298.15 K equals —890.36kJmoF, with hquid water as one of the products. Find the enthalpy change of formation of methane at 298.15 K using the enthalpy changes of formation of H2O and CO2. 

The molar enthalpy change of combustion of glycerol at 298.15 K is equal to —393.73 kcalmoU. Convert this to kJ mol and find the value of A[H° of glycerol at this temperature. 

What will be the rise in temperature of the calorimeter in the previous example if a pellet of anthracene of mass 1.345 g is burned The enthalpy change of combustion of anthracene is -7114.5 kJ mol . ... 

A steam engine has an efficiency that is 55% as large as that of a Carnot engine. If its boiler is at 250°C and its exhaust is at 100°C, calculate the height to which it can lift a 1000 kg mass near the earth s surface if it bums 5.00 kg of coal. Pretend that the coal is pure graphite and that its enthalpy change of combustion is equal to that at 25°C. 

The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions. The reactants and products must be in their standard states. 

The symbol for standard enthalpy change of combustion is AH. Enthalpy changes of combustion are always exothermic. The substances combusted can be either elements or compounds. 

We can find the enthalpy change of combustion by burning a known mass of substance and using the heat released to raise the temperature of a known mass of water. The apparatus used for this consists of a spirit burner and a metal calorimeter (Figure 6.6). 

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