AP Chemistry

Name: _________________________ PHASE AND CHEMICAL CHANGES

Phase Change 

a change in the state of matter without any change in the chemical composition of the system.



These changes can be either:  exothermic - energy is _________________ from the system to the surroundings as new, stronger, attractions between particles form  endothermic – energy is _________________ from the surroundings by the system in order to overcome attractions between particles

Common Phase Changes: Endo/Exothermic Endo Exo Endo Exo Endo Exo 

Phase Change Melting (fusion) Freezing (solidification) Vaporization Condensation Sublimation Sublimation

State Change sl ls lg gl sg gs

Example Ice cube melting Water freezing Water boiling Dew forming on grass Dry ice: CO2(s)→ CO2(g) I2(g) → I2(s)

Phase changes always involve energy changes but never involve temperature changes during the actual change in state.  ie. all the particles undergoing a phase change remain at the same temperature until all the particles have changed phase. Thus there is no change in the average kinetic energy of the particles for a phase change. 

A change in ________________ ______________ is taking place due to changes in the attractions between particles (IMF for molecular substances).

Chemical Change   

A change in the composition of the system as reactants are converted to new substances (products). Include combustion, double replacement, single replacement, formation (synthesis) and decomposition. Energy changes involve changes in bonding thus the type of energy involved is potential energy.  Changes in potential energy are due to changes in the covalent and ionic bonds between atoms and ions, respectively.  involve more energy than phase changes because the ionic or covalent bonds are much stronger than IMF involved in phase changes.

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AP Chemistry  



Name: _________________________

For all reactions, energy is absorbed for bonds to break and is released when new bonds form. These changes can be either:  Exothermic: more energy released by the system than absorbed.  Endothermic: more energy absorbed by the system than released.

No change in temperature of the system occurs, thus no change in avg EK

Enthalpy and Enthalpy Changes When a system undergoes a change in potential energy _______________ a change in temperature, the system undergoes an enthalpy change Enthalpy(H):  the total internal energy of a substance at constant pressure  it cannot be measured directly, rather scientists measure the change in enthalpy Enthalpy Change (ΔH):  a measure of the difference in potential energy of the _______________ and the potential energy of the reactants in a phase or chemical change Molar Enthalpy (∆H):  the enthalpy change involving one mole of a substance  units = J/mol or kJ/mol  each physical or chemical change has it’s own enthalpy notation (∆H) symbol: Symbol ∆Hfus ∆Hsolid ∆Hvap ∆Hcond ∆Hsol’n ∆Hcomb ∆Hf ∆Hdecomp

Representation ∆H of fusion ∆H of solidification ∆H of vaporization ∆H of condensation ∆H of solution ∆H of combustion ∆H of formation ∆H of decomposition

Type of Change Physical (s  l) Physical (l  s) Physical (l  g) Physical (g  l) Physical (s  aq) Chemical Chemical Chemical

+ve or –ve value* +ve -ve +ve -ve +ve or –ve -ve -ve (most) +ve (most)

*Note: +∆H = endothermic process -∆H = exothermic process 

many of these values are tabulated values  Standard Molar Enthalpy: Ho

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AP Chemistry

Name: _________________________

 The enthalpy change per mole of a substance, in kJ/mol, undergoing a change under standard (SATP) conditions of 25oC and 100 kPa. Examples: 1. ∆Hovap for water = 40.7 kJ/mol (+ve value = endothermic) 2. ∆Hocond for water = - 40.7 kJ/mol (-ve value = exothermic) 

For opposite phase changes the magnitude of ∆H is conserved, but the sign is reversed:  Vaporization and ____________________  Fusion (melting) and solidification (freezing)  Formation (synthesis) and decomposition

Representing Enthalpy Changes 

enthalpy changes for phase or chemical changes can be represented by thermochemical equations, enthalpy notation or enthalpy diagrams

A. Thermochemical Equations: 

A balanced chemical equation that includes the net amount of _____________ produced or absorbed by the given reaction i.e. includes the H value



Can be expressed in 2 ways:

1. As a term in the equation:  consider H as a reactant for an endothermic process and as a product for an exothermic process Examples: a)

H2O(g)  H2O(l) + 40.7 kJ

Exothermic

b)

CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ

Exothermic

c)

H2O(l) + 285.8 kJ  H2(g) +

/2 O2(g)

Endothermic

or

2 H2O(l) + 571.6 kJ  2 H2(g) + O2(g)

Endothermic

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Note: Since H values are given in kJ/mol, for the decomposition of 2 moles of water: H = 2 mol x 285.8 kJ/mol = 571.6 kJ

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AP Chemistry

Name: _________________________

2. In H notation:  the symbol and value is given after the equation  negative value for exothermic; positive value for _____________________ Examples:

Hcond = -40.7 kJ

a)

H2O(g)  H2O(l)

b)

CaO(s) + H2O(l)  Ca(OH)2(s)

b)

H2O(l)  H2(g) +

or

2 H2O(l)  2 H2(g) +

Note:

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Hf = -65.2 kJ

/2 O2(g) Hdecomp = + 285.8 kJ O2(g) Hdecomp = + 571.6 kJ

Addition of a catalyst will not affect the molar enthalpy of the reaction.

B. Enthalpy Diagrams  

diagrams to represent the change in enthalpy of a reaction as the reaction ________________ over time can be drawn to represent endothermic or exothermic processes

Examples: 1. For the formation reaction: CaO(s) + H2O(l)  Ca(OH)2(s)

Hf = -65.2 kJ

CaO(s) + H2O(l)

H (kJ)

H = -65.2 kJ Ca(OH)2(s)

Reaction Progress

  

The arrow pointing downward indicates that the change is exothermic. The potential energy of the reactants is greater than that of the products The enthalpy, H, of the system decreases as energy is released to the surroundings as heat

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AP Chemistry 2.

Name: _________________________

For the decomposition reaction: H2O(l)  H2(g) +

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/2 O2(g) Hdecomp = + 285.8 kJ

H2(g) + 1/2 O2(g)

H (kJ)

H2O(l)

H = + 285.8 kJ

Reaction progress

  

The arrow pointing upward indicates that the change is ____________________ The potential energy of the products is greater than that of the reactants The enthalpy of the system increases as energy is absorbed from the surroundings

Standard Molar Enthalpy of Formation: The quantity of energy that is absorbed or released when one mole of a compound is formed directly from its elements in their standard states. Question 1: Write a balanced thermochemical equation to represent the standard molar enthalpy (∆H) of formation of each substance below. Include heat terms in the equation. a) H2O(g)

b) CaCl2(s)

c) CH4(g)

d) C6H6(l)

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AP Chemistry

Name: _________________________

Question 2: Write a balanced thermochemical equation to represent the standard molar enthalpy (∆H) of decomposition of each substance below. Include heat terms in the equation. a) NaCl(s)

b) HCl(g)

c) C6H12O6

d) NO

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