Tiêu đề Topic: Energetics - Unit: Enthalpy ✅

92 I n our study of energetics to this point we have mainly been concerned with thermochemistry—the study of energy transfers between reacting chemicals and their surroundings. We have found that the enthalpy of a reaction can be determined by experimental and some theoretical means. Energetics also encompasses the study of thermodynamics—the scientific study of the relationships between heat, work and energy. Work is done on an object when you transfer energy to that object. The laws of thermodynamics are fundamental laws of physics that bind together our studies of chemistry. First law: Energy can neither be created nor destroyed; it is conserved. This law is fundamental to the balancing of chemical equations. Second law: The entropy of an isolated system (when not in equilibrium) will tend to increase over time. We will investigate this law in our studies of entropy in section 4.3. Third law: As a system approaches absolute zero (0 K) of temperature, all processes cease and the entropy of the system approaches a minimum value (zero). The effect of temperature and entropy on the free energy of a reaction (its spontaneity) will be investigated in section 4.4. A chemical reaction occurring in the laboratory does not exist in isolation. It has surroundings to which it may release heat (an exothermic reaction) or from which it can absorb heat (an endothermic reaction). The reaction being studied can be referred to as ‘the system’ and it is separated from the surroundings by a boundary. Together the system and its surroundings can be referred to as the ‘universe’. If heat is unable to escape or enter the system, this must be because the system is thermally insulated. In the case of a closed system, matter is unable to enter or leave the system. The heat flow into or out of a system at constant pressure is known as enthalpy and is represented by the symbol H. Since the absolute enthalpy of a substance cannot be determined, we measure the change in enthalpy, ∆H. ∆H = H(products) − H(reactants) When the enthalpy change is measured under standard conditions at 298 K and 101.3 kPa pressure, it is referred to as a standard enthalpy change. This is written as ∆H ʅ. Enthalpy changes can be identified specifically by the type of reaction for which they are measured. The enthalpy change for a combustion reaction such as CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) measured under standard conditions is known as the standard enthalpy change of combustion, ∆H̳. Notice that all reactants and products are in their standard state—the state in which they would be found at 298 K and a pressure of 1.01 × 102 kPa. 4.1 STANDARD ENTHALPY CHANGES OF REACTION 15.1.1฀ Define฀and฀apply฀the฀terms฀ standard฀state,฀standard฀ enthalpy฀change฀of฀formation (∆H฀̲)฀and฀standard฀enthalpy฀ change฀of฀combustion฀(∆H฀̳).฀ ©฀IBO฀2007

94 Worked example 1 Write the chemical equation, including state symbols, representing the standard enthalpy of formation of nitric acid. Solution The standard enthalpy of formation of a compound is the enthalpy change that results when one mole of a compound is formed from its elements at a pressure of 1.01 × 102 kPa and 298 K, so this equation should have the reactants H2, N2 and O2 and the product HNO3. All states should be those that occur under standard conditions. Note that this is a theoretical equation only. 1 2 H2(g) + 1 2 N2(g) + 1 1 2 O2(g) → HNO3(l) Standard enthalpy changes of formation can be used to calculate the standard enthalpy change of reaction, ∆H ʅrxn. This is defined as the enthalpy change of a reaction when carried out at 298 K and at a pressure of 1.01 × 102 kPa. Let us consider a hypothetical reaction aA + bB → cC + dD in which a mol of A reacts with b mol of B to make c mol of C and d mol of D. The standard enthalpy of reaction, ∆H ʅrxn can be calculated using the ∆H ̲ values for the reactants and products in a similar way to which we defined the enthalpy change for a reaction: ∆H = H(products) − H(reactants) ∆H ʅrxn = [c∆H ̲(C) + d∆H ̲(D)] – [a∆H ̲(A) + b∆H ̲(B)] =฀∑n∆H ̲(products) –฀∑m∆H ̲(reactants) where ∑ (sigma) means ‘the sum of’ and m and n represent the stoichiometric coefficients for the reactants and products. Although standard enthalpies of formation are often tabulated and readily available, it may sometimes be necessary to determine the standard enthalpy of formation of a compound before it can be used elsewhere. This may be done using the equation for the formation of that compound from its elements and other data such as enthalpy of combustion. For example, carbon dioxide is formed when graphite burns in oxygen according to the equation: C(graphite) + O2(g) → CO2(g) ∆H ̳ =฀−394 kJ mol−1 This equation illustrates the standard enthalpy of formation of carbon dioxide and is also the equation for the combustion of graphite. We can use this equation and the standard enthalpy change of combustion of graphite, ∆H ̳(C(graphite)), to find the standard enthalpy of formation of carbon dioxide. Both oxygen and graphite are the most stable allotropic forms of the elements (oxygen and carbon), so their standard enthalpies of formation are both equal to zero. 15.1.2฀ Determine฀the฀enthalpy฀change฀ of฀a฀reaction฀using฀standard฀ enthalpy฀changes฀of฀formation฀ and฀combustion.฀©IBO฀2007