LE 2

• the system does not lose or gain mass and no exchange of matter with its surroundings however it does exchange energy with its surroundings in the form of work and heat

  closed system
• amount of heat required to raise the temp of a 1mol of a substance by 1K or 1 deg C

  molar heat capacity
• When two objects at different temperatures are

  brought into contact, heat flows from the hotter to the colder body until the two are the same

  temperatures when they are in thermal equilibrium
• Heat capacity depends on whether the system is heated at constant pressure or at constant volume.
• We measure the quantity of heat released or absorbed by relating it to changes in temperature
• The measurement of heat flow is calorimetry, and a

  calorimeter is used to measure heat flow
• used to measure the heat released (or absorbed) by a physical or chemical process

  calorimeter
• The temperature change of a known amount of

  substance of known specific heat is measured.
• A coffee cup calorimeter is often used in laboratory classes to measure heats of reaction at constant pressure in aqueous solution
• Because the calorimeter is not sealed, the reaction occurs under the constant pressure of the atmosphere
• A heat of reaction, qrxn, is the quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system at constant temperature.
• One of the most common reactions studied is the combustion reaction. This is such a common reaction that we often refer to the heat of combustion when describing the heat released by a combustion reaction.
• The solution acts as the surrounding in the

  calculation
• Device that measure the amount of heat evolved or

  absorbed by a reaction occurring at constant volume

  Constant-Volume Calorimeter (Bomb Calorimeter)
• Commonly used to measure heat of combustion reactions

  Bomb Calorimeter
• The negative sign of ΔHrxn means that the enthalpy of the products is lower than that of the reactants. - This decrease in enthalpy appears as heat evolved to the surroundings. - exothermic
• When the products have a higher enthalpy than the reactants; ΔHrxn is positive. - heat is absorbed from the surroundings. – The reaction is

  endothermic.
• For gases, the standard state is the gas at 1.00 bar of pressure.
• For gaseous mixtures, the partial pressure must be 1.00 bar.
• For aqueous solution, the standard state is 1.00 M concentration.
• Refers to the H when the specified number of moles of reactants all at the standard state are converted completely to the specified number of moles of products all at the standard state

  Standard Enthalpy Chnage for a Reaction
• enthalpy change that occurs in the formation of

  one mole of the substance in the standard state from the reference forms of the elements in their standard states.

  Heat of Formation
• Most compounds have a negative Hf

  f – most compounds have exothermic formation reaction under standard condition
• Hess’s Law of Heat Summation states that the enthalpy change for a reaction is the same whether it occurs by one step or by any (even if hypothetical) series of steps.
• Bond energy is the amount of energy required to break one mole of bonds in a gaseous covalent substance to form products in the gaseous state at constant T and P
• The greater the bond energy, the more stable the bond is, the harder it is to break
• An average bond energy is the average of bond-

  dissociation energies for a number of different

  species containing the particular bond
• Nonspontaneous process will not occur unless some

  external action is continuously applied
• In any spontaneous process, the path between reactants and products is irreversible
• Spontaneity depend on temperature
• The dispersal of energy and matter is described

  by the thermodynamic state function entropy
• Melting, vaporization and sublimation – increases in entropy
• Freezing, condensation and deposition – decrease in entropy
• Entropy of any sample increases as its temperature

  increases
• Molecular covalent compounds dissolved in water generally have lower entropy than ionic compounds in water because there's no formation of ions dispersed in water
• In spontaneous changes the universe tends towards a state of greater disorder.
• The S univ of melting and freezing point is 0.
• At equilibrium, neither the forward nor the reverse reaction is spontaneous – no net reaction in either direction
• three types of motion or movement of molecules and atoms

  translational, vibrational, rotational
• If we keep on lowering the temperature, we can achieve a perfectly ordered state. This is defined by the 3rd law of thermodynamics