Name: ____________________________________Unit 10: Kinetics and EquilibriumHow do Reactions Occur and How fast?

I. If two or more reactants are present, a reaction does NOT always occur

For a reaction to occur, the reactant

particles must _________________

II. Rate of Reaction… How fast a reaction takes to occur

Measured in the change in concentration of reactants or products over time

[Reactants] means concentration of reactants (or whatever else is inside brackets)

TimeIII. Reaction Mechanism: pathway or steps that a reaction goes through to make the products

*lots of steps or a very slow or long step, reaction takes a ____________ timerate of reaction is ______________

*a few steps or short steps, reaction takes a ____________ timerate of reaction is ______________

Change the mechanism, change the rate of reaction… One way is to use a catalyst or an inhibitor.Catalyst: speeds up a reaction by lowering the energy needed to start a reaction (called Activation energy) without being used up. It provides a shorter alternate mechanism.Ex. _____________________________________________________________Inhibitor: slows a reaction by increasing the energy needed to start a reaction (called Activation energy) without being used up. It provides a longer alternate mechanism.Ex. _____________________________________________________________Why improve a mechanism?? ___________________________________________

_________________________________________________________________

r[R

eact

ants

] [P

rodu

cts]

1

_________________________________________________________________Mechanism Analogy

IV. What is Needed for a Reaction to Occur?

The same factors which are necessary to sink the “7 ball in a game of billards are required for a reaction to occur. Draw your shot in!

2

To Sink the 7 Ball For a reaction to Occur1) 1)

2) 2)

3) 3)

Effective Collisions lead to the reaction occurring. For Effective collisions, the reacting particles must

1) __________________________________________________________

2) __________________________________________________________

To increase the rate of reaction, you must increase the number of collisions so that there is more effective collisions (correct energy and correct orientation)Reaction between hydrogen and iodine to make hydrogen iodide:

__________________________________________________________________

3

V. Factors Which Affect the Rate of ReactionMake the following predictions:

1) Which will occur faster in hot coffee, dissolving a) a packet of sugar b) sugar cube

Chem factor: _______________________________________________2) Which will occur faster dissolving a sugar packet in a) cold tea b) hot tea

Chem factor: _______________________________________________3) Which has a greater chance of a collision on the dance floor a) 2 couples b) 20 couples

Chem factor: _______________________________________________4) Which will take a shorter time to make a) bread made with baking powder b) bread made with yeast

Chem factor: _______________________________________________

These are the same factors, which affect chemical reactions! Change these and you change the rate of reaction. If you increase the rate of reaction, then the reaction will take less time to occur! If you decrease the rate of reaction then the reaction will take __________ time to occur!Just like travelling in a car, faster your rate (speed), less time the trip takes.

1) Surface Area: (For the same amount or # of grams) more the surface area, more chance of collisions occurring between reactants, greater the chance the collisions will be effective and lead to the reaction occurring. Smaller the particle, the ______________________ the surface areaRate of reaction will _______________________2) Temperature: increasing the temperature means there is more kinetic energy, more particles with the correct amount of energy, faster moving particles greater chance of collisions, greater chance the collisions will lead to a reaction occurringHigher the temperature, ___________________Rate of Reaction

3) Concentration (how much): more moles of reactions, more chance of collisions occurring, between reactants, greater chance the collisions will lead to the reaction occurringex 6 moles of HCl will react faster than 1 mole of HCl with Zincex Increasing pressure on a gas, increases the concentration of the gas. (More molecules in a smaller volume.More moles of reactants, ________________ Rate of Reaction

4) Nature of the reactants

4

Reactions between ions: breaking only a few bonds and making only a few bonds, leads to a faster rate. Ex. Forming a precipitate Reactions involving Covalent compounds: breaking and making lots of bonds lead to a slower rate. Ex burning glucose for energy: 140 steps to the reaction in our body Which is faster (circle)? Reaction involving: ions or covalent compounds

5) Catalyst: speeds up a reaction by lowering the energy needed to start a reaction (called Activation energy) without being used up. It provides a shorter alternate mechanism.Inhibitor: slows a reaction by increasing the energy needed to start a reaction (called Activation energy) without being used up. It provides a longer alternate mechanism. For a faster reaction: Use a catalyst or inhibitor Guided Practice: Identify if the rate of reaction will increase (I), decrease (D) or remain the same (R).

_________1) Powdered zinc is used instead of large chunks in the reaction with hydrochloric acid._________ 2) The reaction is run at 25oC instead of 60 oC._________ 3) The pressure is increased in a container with 2 reacting gases._________ 4) A catalyst is added to a test tube containing decomposing hydrogen

peroxide._________ 5) Pressure is increased in a container containing two reacting solutions._________ 6) If the reaction rate increases, the time that the reaction takes to occur will _____ .Which reactions or processes will occur faster:

1) Reaction between: ionic solutions or covalent compounds in solution2) Dissolving: large pieces or small pieces of sodium carbonate3) Reaction using a inhibitor or a catalyst

List three factors, which would increase the rate of reaction1) ________________________________________________________________2) ________________________________________________________________3) ________________________________________________________________

List three factors, which would decrease the rate of reaction

5

1) ________________________________________________________________2) ________________________________________________________________3) ________________________________________________________________

VI: Thermodynamics (heat motion)Thermodynamics: the study of the energy changes and the heat flow in chemical reactionsLaw of Conservation of Energy: energy can neither be created nor destroyed in a chemical reaction, only transferredENTHALPY (HEAT): (H) Definition: total energy in a reaction, stored in the bonds and energy of motionA. The Role of Energy in a Reaction

1) Activation Energy (AE):_____________________________2) Energy needed or absorbed to break OLD bonds3) Energy released when NEW bonds are formed.

2 &3 measured by the change in Enthalpy __________ called Heat of REACTION

B. Potential Energy DiagramsGraphs ,which show how the potential energy changes, during the course of a chemical reaction.I. TERMS & SYMBOLS USED with PE Diagrams

Heat of ReacTION (H): enthalpy (H) changes between the enthalpy (heat) of the products (Hp) and the enthalpy (heat) of the reactants (Hr) (middle)

Equation:**Must Memorize**

Hp: enthalpy (heat or potential energy) of the products (right side)

Hr: enthalpy (heat or potential energy) of the reacTANTS (left side)

AE: activation energy, energy needed to start a reaction (from Hr to top)- a catalyst will lower this value by providing an alternate mechanism

Activated Complex: The intermediate between reactants and products (highest point)

HAC: enthalpy (heat or potential energy) of the activated complex

Eabsorbed > Ereleased : ____________________ ex. ___________________Products @ higher E than reactants

6

Ereleased > Eabsorbed : ____________________ ex. ___________________Reactants @ higher E than products

7

II. DIRECTION OF REACTIONS WITH ENERGY IN THE REACTION:

Forward Reaction Reverse ReactionReaction as Written (L R) Reaction as Written (R L)Exothermic (Exo) : Reverse Reaction is:

Endothermic (Endo): Reverse Reaction is:

Memory Hook: Energy always goes on the (LOW OR HIGH ) of a PE diagram!!!

III. Effect of a Catalyst: Which values are affected by a catalyst? ___________________________________

Which values are NOT affected by a catalyst? _______________________________

C. Reference Table I: Heat of Reactions at 101.3 KPa and 298 K

I. This reference table lists the energy either released (exo) or absorbed (endo) when these reactions (H) occur under ordinary conditions. Units for H are kilojoules (kJ).

8

*The note at the bottom of the table reminds you that exothermic reactions have a -H.

The first 7 reactions are known as combustion (burning) reactions. These reactions are all _______________. Energy is released.

Which of these reactions releases the most energy?

___________________________________________________________

The next 12 reactions are known as synthesis reactions. These reactions have elements as reactants. The reverse of these reactions, are known as decomposition reactions.

Those reactions with -H, the product is more stable than the reactants, which made it. The more energy released, the more stable the product.

Which of these reactions releases the most energy and, therefore, forms the most stable product? _____________________

If the reaction makes 2 moles of product, how much energy is released if only one mole is made? _____________________________

Those reactions with +H, the product is less stable than the reactants, which made it. The more energy absorbed, the less stable the product.

Which of these reactions absorbs the most energy and, therefore, forms the least stable product? _____________________

The next 7 “reactions” have H2O written over the arrow, these are equations showing how ionic compounds dissolve and how much energy is released or absorbed.

Those with a -H, could be hot packs. Which one would be the hottest? _______

Those with a +H, could be cold packs. Which one would be the coldest? _______

The last reaction is the neutralization reaction, which occurs between an acid and a base. Is it exothermic or endothermic? _____________________

II. Writing reactions with the energy included

In the endothermic reaction, +H is a reactant and is written on the left side, the low side on the endothermic PE diagram.

Example:

_________________________________________________________________

9

In the exothermic reaction, -H is a product and is written on the right side, the low side on the endothermic PE diagram. HOWEVER, the absolute (+) value is written in the equation.Example:

_________________________________________________________________

Guided Practice:1) Look up the H and identify the following reactions as either as exo or endo2) Write the reaction with the energy included.

1) N2 + 2 O2 2NO2 exo or endo H = _____________

_________________________________________________________

2) C + O2 CO2 exo or endo H = _____________

_________________________________________________________

3) 2C + H2 C2 H2 exo or endo H = _____________

_________________________________________________________

4) CH4 + O2 CO2 + 2 H2O exo or endo H = _____________

_________________________________________________________

5) 4 Al + 3O2 Al2O3 exo or endo H = _____________

_________________________________________________________

III. Forward and Reverse Reactions: Remember, if the forward reaction is exothermic then the reverse will be endothermic.

Forward Reaction is exothermic: C + O2 CO2 + 393.5 kJReverse Reaction is endothermic: CO2 + 393.5 kJ C + O2

Determine whether the forward and reverse reactions are exo or endo.

1) N2 + 3 H2 2 NH3 Forward: ____________ Reverse: _____________

2) H2 + I2 2HI Forward: ____________ Reverse: _____________

3) NH4Cl NH4+ + Cl- Forward: ____________ Reverse: _____________

10

Memory HookGas

Liquid

Solid

D. Entropy(Nature’s Excuse for a Messy Room)

When does ice melt? _________________When does water freeze? _____________When does ice not melt? ______________When does water not freeze? ____________What are two things that are different about these situations?1) _______________________ 2) _________________________Nature favors lower energy ____________ lower energy, ___________stable

Most exothermic reactions or changes are spontaneous – once started continue but not always. Some endothermic reactions or changes are spontaneous even though not favored from an energy point of view. Another competing factor, which depends on temperature, is involved. That factor is Entropy

Entropy-a measure of the randomness , disorder, messiness of a “system”“system” what you are looking at- a reaction in a beaker, a phase change, your notebook, your room etc- the greater the disorder, the better, and the higher the _________________

- Symbol: S = change in entropy

- Keeping the E words straight: entHalpy has an H so its H, no H in entropy so it’s S

In questions, it is often easier to answer if your replace the words with the symbols (H & S)

- **Nature favors low energy __________ and high disorder _____________**- It is expected that systems will often go from conditions of

greater order (low entropy) to conditions of greater disorder (high entropy)

- Higher the Temperature, the greater the entropy!

11

NaCl(s) + H2O(l) NaCl (aq) S = ___________

HCl(g) + H2O(l) HCl (aq) S = ___________

2 Mg (s) + O2 (g) 2 MgO (s) S = ___________

More moles of gas, higher the entropy: C3H8(g) + 5 O2 3CO2(g) + 4 H2O(g) S = ___________

1.) Predict the sign of S for the following physical and chemical changes:(+S = high entropy - disorder) (- S = low entropy - order)PRACTICE…

a. CO2(s) CO2(g) S = ___________

b. H2O(l) H2O(s) S = ___________

c. NaCl(s) Na+(aq) + Cl-(aq) S = ___________

d. 2 NH3(g) N2(g) + 3H2(g) S = ___________

e. 2 H2(g) + O2(g) 2 H2O(l) S = ___________

f. H2O(l) @ 25°C H2O(l) @ 50°C S = ___________

2. Which change in a sample of water is accompanied by the greatest increase in entropy?

a) H2O(l) at 100°C is changed to H2O(g) at 200°Cb) H2O(g) at 100°C is changed to H2O(g) at 200°Cc) H2O(s) at -100°C is changed to H2O(s) at 0°Cd) H2O(l) at -100°C is changed to H2O(l) at 0°C

3. What occurs when a sample of CO2(s) is changed to CO2(g)?a) The gas has greater entropy and less orderb) The gas has greater entropy and more orderc) The gas has less entropy and less orderd) The gas has less entropy and more order

12

4. As NaCl(s) dissolves according to the equation: NaCl(s) Na+(aq) + Cl-(aq) the entropy of the systema) increases b) decreases c) remains the same d) depends on the pressure

5. Which change results in an increase in entropy?a) H2O(g) H2O(l) c) H2O(s) H2O(l)b) H2O(l) H2O(s) d) H2O(g) H2O(s)

6. Spontaneous reactions (in nature) go towards:a) endothermic with high entropy c) exothermic with high entropyb) endothermic with low entropy d) exothermic with low entropy

13