The Flow of Energy Measuring and expressing enthalpy changes Thermochemistry The Flow of Energy Measuring and expressing enthalpy changes

Learning Targets You will contrast an exothermic and endothermic reaction. You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction. You will determine the enthalpy change in a reaction using stoichiometric factors.

Energy and Thermochemistry Energy is the capacity for doing work or supplying heat. Unlike matter, energy has neither mass nor volume. Energy is detected only because of its effects. Thermochemistry is the study of energy changes that occur during chemical reactions and changes in state.

Potential Energy and Heat Every substance has a certain amount of energy stored inside it. The energy stored in the chemical bonds of a substance is called chemical potential energy. The kinds of atoms and the arrangement of the atoms in a substance determine the amount of energy stored in the substance. Energy changes occur as either heat transfer or work, or a combination of both. Heat, represented by q, is energy that transfers from one object to another because of a temperature difference between the objects.

Endothermic and Exothermic Processes What happens to the energy of the universe during a chemical or physical process? Chemical reactions and changes in physical state generally involve either the absorption or the release of heat. You can define a system as the part of the universe on which you focus your attention. Everything else in the universe makes up the surroundings. Together, the system and its surroundings make up the universe. You will contrast an exothermic and endothermic reaction.

Law of Conservation of Energy The law of conservation of energy states that in any chemical or physical process, energy is neither created nor destroyed. During any chemical or physical process, the energy of the universe remains unchanged. If the energy of the system increases during that process, the energy of the surroundings must decrease by the same amount. If the energy of the system decreases during that process, the energy of the surroundings must increase by the same amount. You will contrast an exothermic and endothermic reaction.

You will contrast an exothermic and endothermic reaction. The direction of heat flow is given from the point of view of the system. Heat is absorbed from the surroundings in an endothermic process. Heat flowing into a system from its surroundings is defined as positive; q has a positive value. An exothermic process is one that releases heat to its surroundings. Heat flowing out of a system into its surroundings is defined as negative; q has a negative value. You will contrast an exothermic and endothermic reaction.

Exothermic or Endothermic? Your hand gets cold when you touch ice hand-surroundings ice-system flow is from the ice to the hand exothermic The ice melts when you touch it hand-surroundings ice-system flow is from the hand to the ice endothermic You will contrast an exothermic and endothermic reaction.

Exothermic or Endothermic? Ice cream melts container or cone-surroundings ice cream-system flow is from the container to the ice cream endothermic Two chemicals mixing in a beaker give off heat beaker-surroundings two chemicals-system flow is from the two chemicals to the beaker exothermic You will contrast an exothermic and endothermic reaction.

Common examples of exothermic and endothermic processes Burning Freezing condensation Endothermic Baking/cooking Melting Boiling Evaporation You will contrast an exothermic and endothermic reaction.

1 Calorie = 1 kilocalorie = 1000 calories Measuring Heat Flow Heat flow is measured in two common units: the calorie the joule A calorie (cal) is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1°C. The word calorie is written with a small c except when referring to the energy contained in food. The dietary Calorie is written with a capital C. One dietary Calorie is equal to one kilocalorie, or 1000 calories. 1 Calorie = 1 kilocalorie = 1000 calories The joule (J) is the SI unit of energy. You can convert between calories and joules using the following relationships: 4.184 J = 1 cal The heat capacity of an object depends on both its mass and its chemical composition. The greater the mass of the object, the greater its heat capacity. The specific heat capacity, or simply the specific heat, of a substance is the amount of heat it takes to raise the temperature of 1 g of the substance 1°C. You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction.

Table 17.1 – specific heats of common substances 4 You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction.

Calorimetry Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical processes. In a calorimetry experiment involving an endothermic process, the heat absorbed by the system is equal to the heat released by its surroundings. In an exothermic process, the heat released by the system is equal to the heat absorbed by its surroundings. The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter. You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction.

Enthalpy Most chemical reactions and physical changes carried out in the laboratory are open to the atmosphere and thus occur at constant pressure. The enthalpy (H) of a system accounts for the heat flow of the system at constant pressure. The heat absorbed or released by a reaction at constant pressure is the same as the change in enthalpy, symbolized as ΔH. The value of ΔH of a reaction can be determined by measuring the heat flow of the reaction at constant pressure. In this textbook, the terms heat and enthalpy change are used interchangeably. In other words, q = ΔH. You will determine the enthalpy change in a reaction using stoichiometric factors.

How to Calculate You can calculate the heat absorbed or released by the surroundings (qsurr) using the formula for the specific heat, the initial and final temperatures, and the heat capacity of your substance: Q = m · c · ΔT Q = energy (heat) required in J m = mass of the sample in grams c = specific heat capacity (sometimes symbolized as “s”) J g ℃ ΔT = change in temperature in °C You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction.

Example #1 Determine the amount of energy as heat that is required to raise the temperature of a 10.0 g sample of aluminum from 25.0°C to 58.0°C. Answer in joules and calories. (the specific heat capacity for aluminum is 0.890 J g ℃ ) 294 J; 70.2 cal Q = mc∆T Q = 10.0 g ∙ 0.890 J g ℃ ∙ (58.0℃ - 25.0℃) Q = 294 J 294 J ∙ 1 cal 4.184 J = 70.2 cal You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction.

Example #2 If 294 J of heat is transferred to a 10.0 g sample of silver at 25.0°C, what is the final temperature of the silver? (the specific heat capacity of silver is 0.240 J g ℃ ) 147.5 ℃ Q = mc∆T Q mc + Ti = Tf 294 J 10.0g ∙ .240 J g ℃ + 25.0 ℃ = 147.5 ℃ You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction.

Try on your own #1 A 2.80 g sample of pure metal requires 10.1 J of energy to change its temperature from 21.0°C to 36.0°C. What is this metal? (Use table 17.1) Silver (.240 J g ℃ ) You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction.

Try on your own #2 How many calories of heat were added to 5.0 x 102 g of water to raise its temperature from 25°C to 55°C? 15,000 cal You will use calorimetry to determine the amount of heat involved in the temperature change of a substance or chemical reaction.

Thermochemical Equations In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product. In the equation describing the exothermic reaction of calcium oxide and water, the enthalpy change can be considered a product. CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ A chemical equation that includes the enthalpy change is called a thermochemical equation. You will determine the enthalpy change in a reaction using stoichiometric factors.

Thermochemical Equations Each mole of calcium oxide and water that reacts to form calcium hydroxide produces 65.2 kJ of heat. CaO(s) + H2O(l) → Ca(OH)2(s) ΔH = –65.2 kJ In exothermic processes, the chemical potential energy of the reactants is higher than the chemical potential energy of the products. You will determine the enthalpy change in a reaction using stoichiometric factors.

Thermochemical Equations Baking soda (sodium bicarbonate) decomposes when it is heated. This process is endothermic 2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) + CO2(g) Remember that ΔH is positive for endothermic reactions. Therefore, you can write the reaction as follows: 2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g) ΔH = 85 kJ You will determine the enthalpy change in a reaction using stoichiometric factors.

Thermochemical Equations The amount of heat released or absorbed during a reaction depends on the number of moles of the reactant involved. The decomposition of 2 mol of sodium bicarbonate requires 85 kJ of heat. Therefore, the decomposition of 4 mol of the same substance would require twice as much heat, or 170 kJ. You will determine the enthalpy change in a reaction using stoichiometric factors.

Example #1 Calculate the amount of heat (in kJ) required to decompose 2.24 mol NaHCO3(s). 2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) CO2(g) 95.2 kJ 2.24 mol ∙ 85 kJ 2 mol NaHCO3 = 95.2 kJ You will determine the enthalpy change in a reaction using stoichiometric factors.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 890 kJ Example #2 Small amounts of natural gas within crude oil are burned off at oil refineries according to the following equation: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 890 kJ Calculate ΔH for a process in which a 5.8 g sample of methane is burned at constant pressure - 320 kJ 5.8 g CH4 ∙ 1 mol CH4 16.05 g CH4 ∙ −890 kJ 1 mol CH4 = - 320 kJ You will determine the enthalpy change in a reaction using stoichiometric factors.

4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s) + 1652 kJ Try on your own #1 The reaction that occurs in the heat packs used to treat sports injuries is 4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s) + 1652 kJ How much heat is released when 1.00 g Fe(s) is reacted with excess O2(g)? - 7.39 kJ You will determine the enthalpy change in a reaction using stoichiometric factors.

Try on your own #2 When 1 mol of sulfur dioxide reacts with excess oxygen to form sulfur trioxide at constant pressure, 198.2 kJ of energy is absorbed as heat. Show the thermochemical equation and calculate ΔH for a process in which a 12.8 g sample of sulfur dioxide reacts with excess oxygen at constant pressure. 2 SO2 + O2 + 396.4 kJ  2 SO3 39.6 kJ You will determine the enthalpy change in a reaction using stoichiometric factors.