ThermodynamicsThermodynamicsHeating/Cooling CurvesHeating/Cooling Curves

ThermodynamicsThermodynamicsHeating/Cooling CurvesHeating/Cooling Curves

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Credits

• Some information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

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What is energy?

• One interesting definition of energy is that which is needed to oppose natural attractions (for example, gravity and electrostatic attractions)

• We will define energy as the capacity to do work or to produce heat.

• The law of conservation of energy states that energy can be converted from one form to another but can neither be created nor destroyed.

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More Definitions

• Potential energy is energy due to position or composition.

• The kinetic energy of an object is energy due to the motion of the object and depends on the mass of the object, m, and its velocity, v: KE = 1/2mv2

• Heat involves the transfer of energy between two objects due to a temperature difference.

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Yup, Even MORE Definitions

• The system is the part of the universe on which we wish to focus attention.

• The surroundings include everything else in the universe.

• When a reaction results in the evolution of heat, it is said to be exothermic; that is, energy flows out of the system.

• Reactions that absorb energy from the surroundings are said to be endothermic.

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It seems easy, but…

• All of this is counterintuitive!– If a reaction is exothermic, a beaker will feel hot

to you (the surroundings) but the temperature is actually dropping (in the system)!

– If a reaction is endothermic, a beaker will feel cold to you (the surroundings) but the temperature is actually rising (in the system)!

• The variable for heat is q, and when q = -x, the rxn is exothermic. When q = +x, the rxn is endothermic.

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What is enthalpy?

• A less familiar property of a system is its enthalpy, H, which is defined as H = E +PV where E is the internal energy of the system, P is the pressure of the system, and V is the volume of the system.

• BUUUUUUUT, a change in enthalpy is equal to heat (q) at constant pressures.– And when do we change the pressure in the lab?

Never! So for us, enthalpy and heat are the same thing. (And therefore, +H means endothermic, and –H means exothermic)

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What is calorimetry?

• The device used experimentally to determine the heat associated with a chemical reaction is called a calorimeter.

• Calorimetry, the science of measuring heat, is based on observing the temperature change when a body absorbs or discharges energy as heat.

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Heating/Cooling Curves

http://library.thinkquest.org/C006669/media/Chem/img/Graphs/HeatCool.gif

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Uhhhh, OK…?

• Notice that there are two features to the graph on Slide 9, pieces with flat slopes and pieces with positive, linear slopes

• Flat slopes occur during phase changes.• Upward slopes occur during temperature

changes.• YES, this means that temperature does not

change during a phase change!

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Why Not?

• It’s easiest through an example: water boiling– For water to boil, the particles must go from a

state where the particles have strong IMFs between them which holds them somewhat together into a state where the particles are totally free of any IMFs and act independently.

– To break all of those attractions for every single molecule takes a lot of extra energy. ALL the molecules have to change state before the temperature will rise again!

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Enthalpy of Vaporization

• The enthalpy of vaporization is the energy required to help a substance change from a liquid to a gas (+Hvap) or from a gas to a liquid (-Hvap). – It is usually given in kJ/mol, and therefore, the

heat required to vaporize a substance can be calculated by q = nHvap, where n is moles and Hvap is the enthalpy of vaporization

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Enthalpy of Fusion

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• The enthalpy of fusion is the energy required to help a substance change from a liquid to a solid (-Hfus) or from a solid to a liquid (+Hfus). – It is usually given in kJ/mol, and therefore, the

heat required to melt or freeze a substance can be calculated by q = nHfus, where n is moles and Hfusis the enthalpy of fusion

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So what about when temp. IS changing?

• Different substances have different abilities for heating up quickly (or not)

• The heat capacity, C, of a substance, which is a measure of this property, is defined as C = heat absorbed/increase in temperature

• If the heat capacity is given per gram of substance, it is called the specific heat capacity, and its units are in J/g°C

• If the heat capacity is given per mole of the substance, it is called the molar heat capacity, and it has the units J/mol °C

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And how do I use this “specific heat”?

• During a temperature change ( NOT a phase change), the amount of heat being transferred is calculated as q = mCΔT, where m is the mass of the substance, C is the specific heat capacity, and ΔT is the change in temperature.

• NOTE: The same substance has different C values for the different states of matter! Cice ≠ Cwater ≠ Csteam

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Homework

• REMEMBER: There are two equations! One if for phase changes, and one is for temperature changes. Some questions will require a combination of both, and/or the use of the law of conservation of energy.

• USE the internet to find values for the specific heat capacity and enthalpy of fusion or vaporization for each substance.

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Homework Questions

• 1) How many joules are required to heat 250 grams of liquid water from 0C to 100C?

• 2) How many joules are required to melt 100 grams of copper?

• 3) How many joules are given off when 120 grams of water are cooled from 14C to

-45C?

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