Ms. Capasso’s Awesome Unit 8 Review Sheet!

Energy is the ability to do work System: the part of the universe the scientist chooses to study Surrounding : the rest of the universe A system can interact with the surrounding!

All chemical reactions involve two types of energy:1. Potential Energy : The energy of position (stored in chemical bonds and

phase)2. Kinetic Energy : The energy of motion (temperature)

Heat is a transfer of energy. Always flows from an area with higher temperature to an area with lower temperature!

Types of energy: chemical, mechanical, electrical, thermal, etc. Many types of energy!

All phase changes require or give off energy!

The stronger the intermolecular forces, the higher the melting point and the boiling point!

Solid Liquid Gas: Changes are endothermic and require energy

Gas Liquid Solid: Changes are exothermic and release energy

Heating/Cooling Curves: In a single phase, increase/decrease KE and during phases changes increase/decrease PE

Heat and Phase Change Equations:

Specific Heat: of any substance is the amount of heat required to raise the temperature of that substance by one degree Celsius.

The stronger the intermolecular forces, the higher the specific heat for that substance!

Specific Heat (Single Phase) Phase Changesq = mC TΔ

Where… q= heat absorbed or released m = mass of substance T = change in temperatureΔ C = specific heat constant (different for each substance!)

q = Hfusion*m q = Hvaporization*m

Where…q = heat absorbed or releasem = mass of substanceHf or Hv = a constant for that phase change for that particular substance

Heat of Reaction - EnthalpyThe heat of reaction, abbreviated H or delta H, in the difference in enthalpyΔ (potential energy) between the products and the reactants

Enthalpy is the total energy of a system!

H = HΔ Products – HReactants

If heat is absorbed by the system at constant pressure, the enthalpy of the

system will increase ( H > 0, H is positive); reaction is endothermicΔ Δ If heat is released by the system at constant pressure, the enthalpy of the

system will decrease ( H < 0, H is negative); reaction is exothermicΔ Δ

Two Points To Remember!1. The energy absorbed/released in the forward reaction would be equal to the

energy released/absorbed in the backward reaction

H would have the same numerical value but different sign!!!Δ H2O (s) H2O (l) H = +6.01kJΔ H2O (l) H2O (s) H = -6.01kJΔ

2. The energy absorbed/released is proportional to the stoichiometric coefficients of a balanced equation!

2H2O (s) 2H2O (l) H = +12.02kJΔ

Systems in nature tend to go towards:1. Lower energy! (- H)Δ2. Higher disorder! (+ S)Δ

Writing Thermochemical Equations:Two Ways!

1. A + B + Energy C (writing energy as a product (exo) or reactant (endo))2. A + B C H = +45kJ (writing H after the equation with appropriate sign)Δ Δ

**Remember!! The H is proportional to the number of moles in the balancedΔ equation.

Table I tells you the H of a variety of chemical and physical reactions!Δ

Entropy: is the total disorder of a system! Abbreviated with S. S is positive, entropy is increased, more disorderΔ

Remember: If you reverse the reaction, you flip the sign.

If you multiple the coefficients by a number/divide by a number, you need to do the same to the

HΔ

S is negative, entropy is decreased, less disorderΔ

Ways to increase entropy:

1. Temperature: Increasing temperature, meaning particles are moving faster; increase random motion

2. Phase Change: Having a phase change. Gases have the most disorder, liquids in middle and solids the least disorder!

3. Structure: Having products that have simpler structures than reactants4. Moles: Having more products than reactants5. Solution: Having substances placed into a solution

Spontaneous Reactions!A spontaneous reaction is a reaction that can occur under specific circumstances without the application of external work.

1. If a reaction is spontaneous it will occur in the forward direction2. Spontaneous does not equal fast

Gibbs Free Energy Equation:G = H – T SΔ Δ Δ

If G is negative, the reaction is Δ spontaneous If G is positive, the reaction is Δ not spontaneous

Therefore, whether a reaction is spontaneous or not depends on:1. Change in enthalpy2. Change in entropy3. Temperature

Which reactions are spontaneous?

Collision TheoryIn order for any chemical reaction to occur, two important things must happen!

The particles of the reactants must collide with enough energy in the correct position in order for the reaction to occur!

Again, the particles must collide with:1. Enough energy2. In the right spatial orientation! This is called the collision theory

Chemical Reaction Steps: Chemical reactions occur by a series of intermediate steps between the initial

reactants and final products Transition state theory - there are intermediate products form that exist for

only brief periods of time while the atoms rearrange themselves The reason the particles must be positioned properly when they collide is so

that they can form the activated complex! The activated complex is a temporary, unstable arrangement of atoms that

occurs during a chemical reaction It is sometimes called a transition state During this state, old bonds are breaking and new bonds are forming In order for the activated complex to form, particles must collide with…you

guessed it! Enough energy, in the right orientation!

If particles collide with the wrong orientation, they simply bounce off of each other without a reaction happening.

Must come together to form the unstable transition state and have enough energy for old bonds to break and new bonds to form!

Potential Energy Diagrams

Show the potential energy of a reaction.

*For the exam you must be able to draw and label a PE curve!

Kinetics – The Speed of Reactions!

Six Factors That Affect The Speed of Reactions!

1. Nature of reactants: ionic compounds react faster than covalent2. Concentration: increase concentration, increase collisions, make reaction

faster3. Pressure: for gases only! Increase pressure, decrease volume and make

more collisions!4. Surface area: increase surface area, increase reaction

Labels:• A = Potential Energy of

Reactants• B = Activation Energy, forward

reaction• C = Activation Energy,

backwards reaction• D = Heat of Reaction (delta H)• E = Potential Energy of

Reactants • C + E = Potential energy of

activated complex

Endothermic: PE of reactants less than PE of products

Exothermic: PE of products less than PE of reactants

5. Temperature: the higher the temperature, the more collisions, faster reaction

6. Catalyst: lower the activation energy, increase rate of reaction

Catalysts work by decreasing activation energy!

Catalysts do:1. Lower activation energy of the forward (and reverse!) reactions2. Increase rate of reaction

Catalyst do not:1. Change the concentrations of reactant and product at equilibrium2. The PE of the reactants and products3. Delta H or delta S!

Reaction rate is measure as change in reactants or products over time:

Average Reaction Rate Equation:Average reaction rate = - [reactant]/ TΔ Δ

[ ] brackets mean concentration in mol/L

The reaction rate can also be written more generally as:

r = k [A]x[B]x etc.Where:r = reaction ratek = rate constant

Blue line: is reaction without catalyst

Red line: is reaction with catalyst

x = some power[A] = concentration of AReaction rates are determined experimentally and are due to the slowest step in the reaction.

Equilibrium

Many physical and chemical changes can reach a state where the concentrations of reactant and product are constant and the rates of the forward and reverse reaction are equal. This state is called equilibrium

During equilibrium:1. The rates of the forward and reverse reaction are equal2. The concentrations of the reactants and products are constant (CONSTANT

not EQUAL!)Since there is a constant concentration of reactants and products, we can right an equilibrium constant:

For the reaction: aA + bB ß cC + dDThe equilibrium constant is: Keq = [C]c [D]d

[A]a [B]b

Remember:1. Products are always divided by reactant2. Concentrations are raised to the power of the coefficient in a balanced

reaction3. The Keq has no units!4. **Do not include solids in the Keq!!**

Le Chatlier’s Principle!

Any change in temperature, pressure or concentration on equilibrium is called a stress

Le Chatlier’s Principle explains how the system reacts to this stress and comes to a new equilibrium

The principle states that the system will shift to undo the effect of the stress placed on it!

**To restore the stress on a system, the system will shift in the opposite direction!**

Increase DecreaseIncrease Temperature (Endothermic Reaction)

The rate of the forward reaction and concentration of products

Concentration of reactants

Temperature The rate of the reverse Concentration of products

(Exothermic Reaction) reacts and concentration of reactants

Increase Concentration of Reactants

The rate of the forward reaction and concentration of products

Concentration of reactants

Increase Concentration of Products

The rate of the backwards reaction and concentration of reactants

Concentration of products

Increase Pressure Concentration of the side with less moles of gas

Concentration of side with more moles of gas

Catalyst NO EFFECT! NO EFFECT!