Thermochemistry Chapter 17

Chapter 17: Terms to Know Thermochemistry Chemical potential energy Heat System Surroundings Law of Conservation of Energy Endothermic processes Exothermic processes Heat capacity Specific heat Calorimetry Calorimeter Enthalpy Thermochemical equation Heat of reaction Heat of combustion

Introduction Thermochemistry is the chemistry associated with heat. Heat (q) is a form of energy that flows. Heat flow is a measurable quantity. Objects have different rates at which they absorb heat. Objects also store and release a specific amount of heat, that is measurable. Calorimetry is the technique used to measure heat flow.

The Flow of Energy – Heat and Work (Section 17.1) Energy Transformations Exothermic & Endothermic Processes Units for Measuring Heat Flow Heat Capacity and Specific Heat

I.) Energy Transformations Energy = capacity to do work or supply heat. Energy can be changed from one form to another. There are different types of energy transformations.

Chemical bonds also have potential energy. During a chemical reaction a substance changes into another substance with a different PE in its bonds. Thermochemistry is the study of energy changes during a chemical reaction and a change in state. Energy changes occur as either heat transfer or work or a combination of both.

What is heat? Heat (q): Energy that transfers from one object to another because of temperature difference between them. Heat always flow from a warmer object to a colder one until both objects are at the same temperature.

II.) Exothermic & Endothermic Processes Chemical reactions or changes in physical states involve either the absorption or release of heat. Though energy moves from one place to another, overall energy does not change. Law of Conservation of Energy We can describe the movement of heat as either exothermic or endothermic depending upon our perspective.

Chemical perspective: System and surroundings. System: The part of the universe on which you focus your attention. Surroundings: Everything else in the universe. System + Surroundings = Universe

Endothermic Processes A process in which the system absorbs heat from the surroundings. In these processes the system heats up while the surroundings cool down.

Exothermic processes A process in which the system releases heat to the surroundings. In these processes the system loses heat as the surroundings heat up.

Determining direction of heat flow. Heat flowing out of a system = -q Heat flowing into a system = +q

Sample problem On a sunny winter day, the snow on a rooftop begins to melt. As the melt-water drips from the roof, it refreezes into icicles. Describe the direction of heat flow as the water freezes. Is the process endothermic or exothermic?

III.) Units for Measuring Heat Flow Heat flow is measured in two common units, the calorie and the joule. calorie(cal): The quantity of heat needed to raise the temperature of 1 g. of pure water 1oC. Joule (J): The SI unit of energy. 1 J = 0.2390 cal 4.184 J = 1 cal

IV.) Heat Capacity and Specific Heat Heat capacity: The amount of heat required to increase the temperature of an object exactly 1oC. Heat capacity is dependent upon the mass and chemical composition of an object. The greater the mass of an object the greater its heat capacity.

Specific heat. Substance J/(g x oC) Cal/(g x oC) Water 4.18 1.00 Aluminum 0.90 0.21 Iron 0.46 0.11 Silver 0.24 0.057 The specific heat capacity of a substance is the amount of heat it takes to raise the temperature of 1 g. of the substance 1oC.

Calculating specific heat. q C = m x ∆T C = specific heat q = heat (joules or calories) M = mass (grams) ∆T = Change in temperature (Tf – Ti) Tf = Final temperature Ti = Initial temperature

Sample problem The temperature of a 95.4 g. piece of copper increases from 25oC to 48.0oC when the copper absorbs 849 J of heat. What is the specific heat of copper?

Measuring and Expressing Enthalpy Changes (Section 17.2) Calorimetry Thermochemical Equations

I.) Calorimetry The precise measurement of the heat flow into and out of a system for chemical and physical processes. These measurements can be done at constant pressure or constant volume.

Constant pressure calorimetry: coffee cup calorimetry Most chemical reactions and physical changes are carried out under constant pressure. The heat content of a system at constant pressure is the same as the enthalpy (H) of a system.

Calculating enthalpy changes (∆H). The heat released or absorbed by a reaction at constant pressure is the same as the change in enthalpy. Therefore q = ∆H. If q = m x C x ∆T, And qsystem = -qsurroundings, Then qsystem = ∆H = -qsurroundings = -m x C x ∆T +∆H = endothermic reactions - ∆H = exothermic reactions

Sample problem When 25 mL of water containing 0.025 mol HCl at 25oC is added to 25.0 mL of water containing 0.025 mol NaOH at 25.0oC in a foam cup calorimeter, a reaction occurs. Calculate the enthalpy change (in kJ) during the reaction if the highest temperature observed is 32.0oC. Assume the densities of the solutions are 1.00 g/mL.

Constant volume calorimetry: bomb calorimetry. In this type of calorimetry a sample is burned at high pressure. The heat released warms the water surrounding the chamber. Measuring the temperature difference allows for the calculation of heat released.

Thermochemistry Chapter 17 The End