3.2 Calorimetry and Enthalpy

Transcription

1 3.2 Calorimetry and Enthalpy Heat Capacity Specific heat capacity (c) is the quantity of thermal energy required to raise the temperature of 1 g of a substance by 1 C. The SI units for specific heat capacity are J/g C. It takes a lot of thermal energy to raise the temperature of a substance with a high specific heat capacity, such as water, but relatively little to raise the temperature of a substance with a low specific heat capacity, such as sand. Calorimetry and Thermal Energy Transfer When methane reacts with oxygen in a lab burner, enough heat is transferred to the surroundings to increase the temperature and even to cause a change of state. How is this amount of heat measured? The experimental technique is called calorimetry and it depends on careful measurements of masses and temperature changes. When a fuel like methane burns, heat is transferred from the chemical system into the surroundings (which include the water in the beaker). If more heat is transferred, the observed temperature rise in the water is greater. Similarly, given the same amount of heat, a small amount of water will undergo a greater increase in temperature than a large amount of water. Finally, different substances vary in their ability to absorb amounts of heat. A calorimeter is a device used to measure energy changes during a physical or chemical change. The design of calorimeters varies:

2 Calorimetry Calculations In calorimetry, the total amount of thermal energy absorbed or released by a chemical system is given the symbol q. The magnitude of q depends on three factors: the mass of the substance the specific heat capacity of the substance the temperature change experienced by the substance as it warms or cools The three factors mass (m), temperature change (ΔT), and type of substance are combined in an equation to represent the quantity of heat (q) transferred: m is the mass of the substance, c is the specific heat capacity of the substance, and ΔT is the change in temperature of that substance. The change in temperature is the difference between the final temperature, T final, and the initial temperature, T initial, which can be written as If q has a negative value, the system transfers thermal energy to its surroundings and the change is exothermic. The temperature of the water (the surroundings) in the calorimeter increases. If q has a positive value, the system absorbs thermal energy from its surroundings and the change is endothermic. The temperature of the water in the calorimeter decreases.

3 q < 0 exothermic q = 0 no exchange of thermal energy q > 0 endothermic Since energy cannot be destroyed, the total thermal energy of the system and its surroundings remains constant. Therefore, the sum of q system plus q surroundings should equal zero, since these symbols represent equal quantities but with opposite signs: Calculating Quantity of Heat When 600 ml of water in an electric kettle is heated from 20 C to 85 C to make a cup of tea, how much heat flows into the water? First, use the density formula to calculate the mass of water. M = dv = 1.00 g/ml x 600 ml = 600 g Use the heat formula, q = mcδt, to calculate the quantity of heat transferred. q =? m = 600 g c = 4.18 J/(g C) ΔT = 85 C - 20 C = 65 C Example What would the final temperature be if J of heat were transferred into 10.0 g of methanol initially at 20.0 C? Solution

4 Heat Transfer and Enthalpy Change Chemical systems have many different forms of energy, both kinetic and potential. These include the kinetic energies of moving electrons within atoms; the vibration of atoms connected by chemical bonds; and the rotation and translation of molecules that are made up of these atoms. More importantly, they also include the nuclear potential energy of protons and neutrons in atomic nuclei; and the electronic potential energy of atoms connected by chemical bonds. Researchers have not yet found a way to measure the sum of all these kinetic and potential energies of a system. For this reason chemists usually study the enthalpy change, or the energy absorbed from or released to the surroundings when a system changes from reactants to products. An enthalpy change is given the symbol ΔH, pronounced delta H, and can be determined from the energy changes of the surroundings. A useful assumption that will be applied in more detail later in this chapter is that the enthalpy change of the system equals the quantity of heat that flows from the system to its surroundings, or from the surroundings to the system. This assumption applies as long as there is no significant production of gas, which is the case in most reactions you will encounter.

5 This idea is consistent with the law of conservation of energy energy may be converted from one form to another, or transferred from one set of molecules to another, but the total energy of the system and its surroundings remains the same. For a chemical reaction, the enthalpy change, ΔH, is given by the equation Determine the enthalpy of the reaction in which magnesium metal, Mg (s), reacts with an aqueous solution of hydrochloric acid, HCl (aq), according to the net ionic equation

6 Molar Enthalpy Change The molar enthalpy change (ΔH r ) of reaction for a substance is the energy change that occurs when 1 mol of that substance undergoes a physical, chemical, or nuclear change. The SI unit for molar enthalpy change is joules per mole (J/mol). In the symbol ΔH r, the subscript r indicates the chemical reaction type. The quantity of energy involved in a change (the enthalpy change, ΔH, expressed in kj) depends on the quantity of matter that undergoes the change. For example, it takes twice the quantity of energy to convert 1 L of liquid water to water vapour as it does to convert 500 ml. To calculate an enthalpy change, ΔH, for some amount of substance other than 1 mol, you need to obtain the molar enthalpy value, ΔHr, from a reference source, and then use the formula Sample Problem 1: Calculate H for Vaporization Reactions Ethanol, CH 3 CH 2 OH (l), is used to disinfect the skin prior to an injection. If a 1.00 g sample of ethanol is spread across the skin and evaporated, what is the expected enthalpy change? The molar enthalpy of vaporization of ethanol is 38.6 kj/mol.

7 Sample Problem 2: Calculate ΔH for Dissolution Reactions An energy change called enthalpy of solution, Hsol, occurs when a substance dissolves in water. A student places 125 g of liquid water, H2O(l), at 24.2 C into a coffee-cup calorimeter, and then adds 10.5 g of solid potassium bromide, KBr(s), also at 24.2 C. He stirs the liquid until the potassium bromide dissolves, and then determines that the temperature has changed to 21.1 C. Calculate the molar enthalpy change for this dissolution reaction, Hsol. Assume that the specific heat capacity, c, of the liquid in the calorimeter is the same as the specific heat capacity of water, 4.18 J/(g C). Representing Molar Enthalpy Changes All chemical reactions undergo a change in enthalpy. It is often difficult to tell whether a reaction is exothermic or endothermic. Therefore, it is important to have a clear way of communicating this information in the chemical equation for the reaction. A chemical equation that describes the enthalpy change of a reaction is called a thermochemical equation. For example, the synthesis of water from its elements is an exothermic reaction and can be represented by the two thermochemical equations

8 You can also represent the enthalpy change of a reaction using a potential energy diagram. In a chemical reaction, both reactants and products have potential energy. In a potential energy diagram, you express the potential energy (y-axis) as a function of the reaction progress (x-axis). Sample Problem 1: Thermochemical Equations with Energy Terms The combustion of methane gas, CH4(g), is an exothermic reaction. When 1 mol of methane burns, kj of energy is released. Write the thermochemical equation both by representing the energy change as a H value, and by representing the energy change as an energy term in the equation.

9 Sample Problem 2: Drawing Potential Energy Diagrams Draw potential energy diagrams for the following reactions:

10 Types of Enthalpy Changes Worksheet 3.2: Calorimetry and Enthalpy p. 306, Q 2, 3, 4, 5, 6