Chemistry/Physical Setting The Flow of Energy-Heat and Work Br. Jabreal

Aim: How can heat and energy flow be generally described? Do Now: State the direction of heat flow between the air in an oven and muffin batter when the muffin batter is first placed in the preheated oven at 200oC. From the oven air to the muffin batter

Energy Transformations Thermochemistry: Study of energy changes occurring during chemical reactions and changes in state Chemical Potential Energy: Energy stored in chemical bonds Thermal Energy: Energy associated with random molecular motions Internal energy is total energy in a system and is made up of chemical and thermal energy.

Heat Energy transfers in chemical reactions and state changes often involve transfer of heat Heat Energy that transfers from one object to another because of temperature differences Represented by “q” Heat always flows from warmer object to cooler object Heat flows from warm object to cold object until temperature of both objects is the same

Exothermic and Endothermic Processes In studying energy changes/heat flow, important to define the system and its surroundings: System: Part of universe chemist is focusing on Surroundings: Everything else in universe apart from system Total Energy = Energy in system + Energy in Surroundings

System and Surroundings Example: Ice-water Chemist may be observing heat transfers in ice-water as the ice melts. The water is the system here. Everything else is part of surroundings.

Law of Conservation of Energy In any chemical or physical process, energy is neither created nor destroyed If energy in the system decreased during a process (such as becoming colder), it must have gone somewhere  to the surroundings. If energy increases in system, it must have came from the surroundings. Total energy (system + surroundings) is constant, in other words

Extra Credit Name the three things conserved in chemical reactions.

Extra Credit In a chemical reaction, there is conservation of Energy, volume, and mass Energy, volume, and charge Mass, charge, and energy Mass, charge, and volume

Endothermic Processes An Endothermic Process is one that absorbs heat from the surroundings System gains heat as the surroundings cool down Heat (q) is defined as positive in this case Heat/energy is a reactant in this case

Exothermic Processes An Exothermic process is one that releases heat to its surroundings System loses heat as the surroundings heat up Heat (q) is defined as negative in this case. Heat/energy is a product in this case

Practice 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 this process endothermic or exothermic?

Practice A container of melted paraffin wax is allowed to stand at room temperature until the wax solidifies. What is the direction of heat flow as the liquid solidifies? Is the process exothermic or endothermic?

Extra Credit Base your answer to the question on the following equation: AB + energy  A + B Is this chemical reaction endothermic or exothermic? Explain.

C(s) + O2(g)  CO2(g) Is this reaction endothermic or exothermic C(s) + O2(g)  CO2(g) Is this reaction endothermic or exothermic? Explain.

Units for Measuring Heat Flow Heat flow measured in the joule (j) or the calorie (c). We will primarily use the joule (j). calorie vs. Calorie 1 Calorie = 1 kilocalorie = 1000 calories

Heat Capacity and Specific Heat Heat Capacity: The amount of heat needed to increase temperature of an object 1oC. Depends on both mass and composition Ex. Takes longer (more energy) to boil more water than less water Ex. Iron heats up faster than water. Will you get burnt touching water that has been left out a few hours in the sun like if you touched iron that was out in the sun that long?

Specific Heat Specific Heat Capacity (Specific Heat): Amount of heat it takes to raise the temperature of 1 g of a substance 1oC. *Specific Heat value of water listed in Reference Table B.

Specific Heat Calculations Calculations involving raising the temperature of a substance or finding a substance’s specific heat can be addressed using the following equation: q = mCΔT *q is heat. m is mass. C is specific heat capacity. ΔT is change in temperature

Example How much heat is needed to raise the temperature of 20.0 grams of liquid water from 5.0oC to 20.0oC?

Example The temperature of a 95.4-g piece of copper increases from 25.0oC to 48oC when the copper absorbs 849 J of heat. What is the specific heat of copper?

Example What is the temperature change in degrees Celsius when 11.7 J of heat is added to 6.0 grams of water?

Practice How much heat is needed to raise the temperature of 45 grams of water by 30oC?

Practice What quantity of heat is released when 50.0 grams of water is cooled from 70oC to 60oC?

Practice When 10.0 grams of water at 20oC absorb 418 joules of heat, the temperature of water increases by how many degrees Celsius?

Practice What is the maximum number of grams of water at 10oC that can be heated to 30oC by the addition of 84 joules of heat?

Homework Page 535 Questions 45, 46, 55