Thermochemistry!

AP Chapter 5

Temperature vs. Heat

• Temperature is the average kinetic energy of the particles in a substance.

• Heat is the energy that is transferred from one object to another.

• Heat always flows from the hotter object to the colder object.

Energy!

• Energy is the ability to do work.

• Kinetic Energy - the energy of motion

• Potential Energy – the energy that an object has as a result of its composition or its position with respect to another object.

Units of Energy

• 1 Joule = 1 kg m2/s2 (1 kJ is 1000J)• Used to calculate the energies associated with chemical

reactions.

• Calorie – Amount of energy required to raise the temperature of 1 gram of substance 1 °C. (This is specific heat!)1 calorie will raise the temperature of 1 g of H2O

from 14.5 °C to 15.5 °C.

1 calorie is equal to 4.184 Joules (exactly!)

Systems and Surroundings

• System – the portion used in a study.– It can be an open system or a closed system.

• Open system – matter and energy can interact with the surroundings.

• Closed system – the matter cannot interact with the surroundings.

First Law of Thermodynamics

•Energy Is Conserved!

Internal Energy

• Internal Energy is the sum of all the kinetic and potential energies of all its components.

ΔE = Efinal - Einitial

ΔE

• A positive value for ΔE is when Efinal > Einitial

• If energy has been absorbed from its surroundings, it is endothermic.

• If energy is given off to the surroundings, it is exothermic.

Initial state refers to the reactants, while final state refers to the products.

Endothermic reaction Exothermic reaction

A system composed of H2 (g) and O2 (g) has greater internal energy than a system composed of H2O (l).

Gases have greater kinetic energy and must lose some of that energy to change states back to the liquid state.

Internal energy is a function of state.

a) If a battery is shorted out and loses energy to the environment only as heat, no work is done.

b) If a battery is discharged and loses energy as work (to make the fan run) it also loses heat energy.

c) The value of ∆E is the same.

Enthalpy

• The change in enthalpy for a reaction (∆H) is the overall measure of energy that is absorbed to break bonds and the energy that is released when new bonds form.

• A reaction is said to be spontaneous if it occurs without being driven by an outside force. (driving forces are enthalpy & entropy)

• ∆H = ΣH(products) - ΣH(reactants)

In an endothermic system where it absorbs heat, ∆H will be positive (∆H > 0).

In an exothermic system, where heat is given off, ∆H will be negative (∆H < 0).

Enthalpy Diagrams

• Enthalpy is an extensive property – it depends on how much you have. If 1mol of CH4 and 2 mol O2 yield -890 kJ, then 2 mol CH4 and 4 mol O2 would yield double that.

• The enthalpy change for a reaction is equal in magnitude, but opposite sign, for a reverse reaction.

Calorimetry

• This is a measure of the amount of energy that is needed or lost when a certain mass of a substance changes temperature.

• q = mC∆T• q is the amount of energy (J)• m is the mass of the substance (g)• C is the specific heat capacity of the substance• ∆T is the change in temperature

Calorimeters

• Calorimeters are devices that measure the transfer of heat from one object to another.

Heat of Formation (∆H°f)

• The heat change that occurs when one mole of a compound is formed from its elements at 1 atm pressure.

• Generally, the standard enthalpy of formation for any element in its most stable form is 0. (i.e. O2 gas would have a standard enthalpy of 0.)

• Remember Appendix C!

Standard Enthalpy Changes

• The standard enthalpy change can be calculated from the standard enthalpies of formation of the reactants and products in the reaction (see Appendix C for values.)

• ∆H°rxn = Σ n∆H°f (products) - Σm ∆H°f (reactants)

• The n and m refer to the molar coefficients in the chemical equation.

Also refer to Appendix C!

Hess’s Law

• If you can break a chemical reaction into several steps, add up all of the ∆H’s for each step to get the overall ∆H for the reaction.

Entropy• Entropy is a measure of randomness or

disorder of a system. The greater the disorder, the greater the entropy.

• In terms of entropy, gases>liquids>solids.• When pure substance dissolves in a liquid, its entropy

increases.• When gas molecules escape a solvent, entropy increases.• Entropy increases with molecular complexity.• Reactions that increase the number of moles of particles

often increase the entropy of the system.

Predict!

• Na+ (aq) + Cl- (aq) → NaCl (s) ∆S is negative

• NH4Cl (s) → NH3 (g) + HCl (g) ∆S is positive