Thermochemistry

Thermochemistry

• C.11A Understand energy and its forms, including kinetic, potential, chemical, and thermal

• energies.• Supporting

Standard

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Thermochemistry

• C.11B Understand the law of conservation of energy and the processes of heat transfer.

• Supporting Standard• C.11C Use thermochemical

equations to calculate energy changes that occur in chemical

• reactions and classify reactions as exothermic or endothermic.

• Readiness Standard

Thermochemistry

• C.11D Perform calculations involving heat, mass, temperature change, and specific heat.

• Supporting Standard• C.11E Use calorimetry to calculate

the heat of a chemical process.• Supporting Standard

Vocabulary of Instruction:

• energy• kinetic energy• potential energy• thermal energy• specific heat• law of conservation of energy• phase change• heat transfer• calorimetry

Key Understandings and Guiding Questions:

• Energy is conserved when heat is transferred.• What is the law of conservation of energy?• What is the difference between heat and

temperature?• What are the three types of energy transfer?• List the forms of energy you know about. What is an

example for each?• How is a calorimeter used to measure energy

transfer?

Energy• Energy is the capacity to do work.• Work is defined as force moving through a

distance. • In chemistry we consider the gas pressure

against a piston moving it upward as work.

• Energy in motion is called ____.• This refers to atoms and molecules moving

in gases, liquids, and solids. • What is the other type of energy?

Forms of Energy

• Light/Heat(Thermal)---Radiant-from electromagnetic waves

• Electricity-from flow of electric charges• Magnetism-from alignment of charged

particles which causes attractions or repulsions

• Mechanical-from movement of objects• Chemical-energy stored in bonds• Nuclear-energy stored in the nucleus• Sound-from vibrations

Energy

• We cannot see energy, only its effects.

• Heat is the measure of the total amount of kinetic energy in a system.

• It is measured in joules or calories.

• It is measured with a calorimeter.

Energy

• Temperature is a measure of the effects of heat. It measures the average kinetic energy of a system.

• It is measured in oC or K.

• It is measured with a thermometer.

Energy

• Consider two beakers of water. • One has 100 g of water at 80oC.• One has 100 g of water at 20oC.• Which one has more mass? • Which one has more energy?• Which one has the greater kinetic

energy? • What will happen if we pour the two

together into a third beaker?

Energy

• What will be the mass of the water in the third beaker?

• What will be the temperature of the water in the third beaker?

• Did the cooler water cool the warm water or did the warmer water warm the cool water? In other words, which way did the heat flow?

Heat Flow

• Energy flows from a warmer object to a cooler one, never the reverse.

• There really isn’t such a thing as cold; there is just a lack of heat.

• The heat of a closed system is maintained (not lost to surroundings).

• In fact, those are the only two parts to the universe---systems and surroundings.

• If energy is lost by the system, it is gained by the surroundings and vice versa.

Methods of Heat Transfer

• Convection• Conduction• Radiation

Law of Conservation of Energy

• Energy is never lost or gained, only transferred or transformed.

• Heat lost = Heat gained• q = m x C x ∆t where q is in joules

or calories, m is in grams, C is the specific heat, and t is in Celsius.

Hess’ Law of Constant Heat Summation

• The sum of the heats of formation of the products minus the sum of the heats of formation of the reactants equals the heat of reaction.

• The heat of formation is the amount of energy needed to make a compound from its elements.

Gibb’s Free Energy• Gibb’s free energy tells the amount of

useful work that can be done by a system.

• Because it is work done by a system, it is going out of the system and so is given a negative sign if the reaction is spontaneous.

• ∆G = ∆H-T∆S where G is Gibb’s free energy, H is the enthalpy of reaction and S is the entropy of the reaction. T is in Kelvins.

Gibb’s Free Energy

• Entropy is the randomness of a system.

• More randomness is better. • Gases are more random than

liquids which are more random than solids.