UNIT 14 - Reaction Energy & Reaction Kinetics...

1

Chemistry- Shields Unit 15/16

UNIT 15 - Reaction Energy & Reaction Kinetics

I. Thermochemistry:

A. Heat equation (change in temperature): Q = m . Cp . ΔT 1. Q = (unit is Joules) 2. m = (unit is grams) 3. Cp = (unit is J/g.oC) 4. ΔT = (unit is oC) B. Heat equation (phase change): Q = m . Hfus (Hvap) 1. Q = (unit is Joules) 2. m = (unit is grams) 3. Hfus = (unit is J/g) ~ used when phase change occurs between 4. Hvap = (unit J/g) ~ used when phase change occurs between II. Enthalpy (H): A. Heat of Reaction (ΔH):

1. Positive value for ΔH means __________________ reaction

ENDOTHERMIC REACTIONS: Reactants + ENERGY --> Products ~ Energy is absorbed during the reaction. ~ The heat content of the products is higher than that of the reactants. 2. Negative value for ΔH means __________________ reaction

EXOTHERMIC REACTIONS: Reactants --> Products + ENERGY ~ Energy is released during the chemical change. ~ The heat content of the reactants is higher than the heat content of the products.

V. Reaction Pathway Diagram

A =

B =

C =

D =

E =

~ This diagram represents an (endothermic / exothermic) reaction.

2


~ An endothermic reaction’s reaction pathway diagram would look like:

F. Activation energy (Ea):

G. Catalyst:

III. Entropy (S):

According to the Second Law of Thermodynamics, nature is always proceeding to a state of higher entropy.

A. Solids have (high / low) entropy compared to gases. B. Gases have (high / low) entropy compared to solids.

Determine whether the following reactions show an increase or decrease in entropy. 1.) 2 KClO3 (s) --> 2 KCl (s) + 3 O2 (g)

2.) H2O (l) --> H2O (s)

3.) N2 (g) + 3 H2 (g) --> 2 NH3 (g)

4.) NaCl (s) --> Na+1 (aq) + Cl-1 (aq)

5.) KCl (s) --> KCl (l)

6.) CO2 (s) --> CO2 (g)

7.) H+1 (aq) + C2H3O2-1 (aq) --> HC2H3O2 (l)

8.) C (s) + O2 (g) --> CO2 (g)

9.) H2 (g) + Cl2 (g) --> 2 HCl (g)

10.) Ag+1 (aq) + Cl-1 (aq) --> AgCl (s)

11.) 2 N2O5 (g) --> 4 NO2 (g) + O2 (g)

12.) 2 Al (s) + 3 I2 (s) --> 2 AlI3 (s)

13.) H+1 (aq) + OH-1 (aq) --> H2O (l)

14.) 2 NO (g) --> N2 (g) + O2 (g)

15.) H2O (g) --> H2O (l)

3


IV. Gibb's Free Energy (G): A. Free energy equation: ΔG = ΔH - TΔS 1. ΔG = units are 2. ΔH = units are 3. T = units are 4. ΔS = units are B. If ΔG is ________________________, the reaction is spontaneous.

A negative value for ΔH means that the reaction is _________________________ (favored). A positive value for ΔH means that the reaction is __________________________ (not favored) .

A negative value for ΔS means that the products are _________ ordered than the reactants (not favored). A positive value for ΔS means that the products are _________ ordered than the reactants (favored). *If ΔH and ΔS are not both favored (or not favored), the reaction will depend on the ______________________.

Practice-

Fill in the blanks. 1. ΔG will always be negative when ΔH is ___________________ and ΔS is ___________________. 2. ΔG will always be positive when ΔH is ___________________ and ΔS is ___________________.

Consider the following reactions. Determine the signs for ΔH and ΔS. Then decide whether the reaction is sometimes, always, or never spontaneous. 3. NaOH (s) --> Na+1 (aq) + OH-1 (aq) + energy

ΔH = _____ ΔS = _____ ΔG = _____ __________________ spontaneous

4. energy + 2 H2 (g) + O2 (g) --> H2O (l)


5. energy + H2O (s) --> H2O (l)


4


Collision Theory = in order for reactions to occur, particles of a substance must collide and interact with each other

o The most “effective” collisions produce the fastest reactions. o What makes a collision “effective”?

RATE OF REACTIONS

Rate of reaction:

FACTORS INFLUENCING THE RATE OF REACTION: ~ NATURE OF _____________________: - Some substances are just more _______________________ than others. ~ Iron will oxidize much faster than zinc. ~ Fluorine gas is more reactive than chlorine gas. (Activity Series)

~ EFFECT OF ______________________: - Usually, raising the temperature _____________________ the reaction rate. ~ Increasing temperature increases the kinetic energy and therefore the frequency and the momentum of colliding particles also increases.

~ EFFECT OF _______________________: - Cramming more particles into a smaller space _________________ the chance of collisions. ~ Therefore, increasing the concentration of the reactants increases the rate at which a reaction occurs.

~ EFFECT OF ________________________: - The __________________ the particle size, the larger the surface area. - Increasing the surface area ____________________ the rate of reaction. ~ Powdered iron filings would rust faster than a solid chunk of iron of the same mass.

~ EFFECT OF ________________________: - Catalysts ____________________ the rate of reaction without taking part in the reaction.

5


UNIT 16 – NUCLEAR CHEMISTRY

Radioactivity: spontaneous disintegration of the nucleus of certain atoms accompanied by the emission (release) of particles and/or energy

I. TRANSMUTATION: A. can be caused by radioactive decay (called transmutation) 1. Particles that are emitted during radioactive decay a. alpha particle ():written as 42He; low energy; blocked by __________________________ 21084 Po 20682 Pb + 42 He

b. beta emission ():written as 0-1 e or 0-1blocked by _______________________________ 146 C 147 N + 0-1

c. positron emission: written as 0+1 e or 0+1

3819 K 38

18 Ar + 0+1

d. gamma radiation () – no mass or charge, just energy blocked by _________________________________________

B. can be caused by bombardment by subatomic particles (called __________________ transmutation) 1. particle accelerator: subatomic particles are accelerated to cause nuclear transmutations a. proton (a hydrogen nucleus) 94 Be + ______ 63 Li + ______

b. alpha particle (a helium nucleus)

2713 Al + ______ 3015 P + ______

c. only charged particles can be accelerated – neutrons cannot be accelerated in ordinary particle accelerator.

Balancing Nuclear Equations

- Nuclear equations are difference than regular equations because the atomic number and mass number are given along with the symbol for the element

- To balance a nuclear equation, the sum of the mass numbers on one side of the equation must equal the sum of the mass numbers on the other side of the equation. The same is true for atomic numbers.

Examples: 63 Li + 10 n --> 42 He + ______

23090Th _______ + 42He

14261Pm + ________ 14260Nd

6


NUCLEAR ENERGY: according to Einstein, E = mc2

A. FISSION REACTIONS: when a large, unstable nucleus becomes two or more ___________________________ - only elements with _________ atomic numbers that are unstable can be used in fission reactions - chain reaction: neutrons released during fission reaction cause other fission reactions; can become out of control is not contained

B. NUCLEAR REACTOR: nuclear fission reactions produce energy used to generate electricity - most produce energy from the fission of uranium-235 - naturally-occurring uranium is mostly uranium-238, only 0.7 % uranium-235

C. FUSION REACTIONS: two small nuclei combine to form a __________________________ - only occur at extremely high temperature and pressure - proceed very slowly - problems with containing energy released in fusion reactions - nuclear fusion is not a practical source of energy at this time D. USES OF RADIOISOTOPES 1. In the laboratory as a ______________ to follow steps in a complex chemical reaction 2. In medicine – 3. In industry – 4. In geology and archaeology –

E. RADIOACTIVE WASTES 1. Why are radioactive wastes so dangerous?

2. What is done with solid, liquid, and gas waste that is radioactive? a. SOLID/LIQUID: b. GAS:

BALANCING NUCLEAR REACTIONS

Predict the missing product or reactant in the following nuclear reactions.

1. 4219 K --> 0-1 e + ______

2. 23994 Pu --> 42 He + ______

3. 23592 U --> ______ + 231

90 Th

4. 11 H + 31 H --> ______

5. 63 Li + 10 n --> 42 He + ______

6. 2713 Al + 42 He --> 30

15 P + ______

7. 94 Be + 11 H --> ______ + 42 He

8. 3719

K --> 0+1

e + ______

9. ______ + 10 n --> 142

56 Ba + 91

36 Kr + 3 1

0 n

10. 23892

U + 42 He --> ______ + 1

0 n

11. 146 C --> 14

7 N + ______

12. 18775

Re + ______ --> 18875

Re + 11 H

7


Half-life:

Given Asked for Steps to solve: Half-life (h) Elapsed time (T) Initial amount (Ai)

Amount remaining (AR) 1. calculate number of half-lives in elapsed time (T/h) 2. Divide original amount by 2 for the number of half-lives calculated in step 1

Half-life (h) Initial amount (Ai) Amount remaining (AR)

Elapsed time (T)

1. Determine how many half-lives elapsed in order to get original amount to amount remaining (how many times did you divide by 2?) 2. Multiply half-life by the number from step 1

Elapsed time (T) Initial amount (Ai) Amount remaining (AR)

Half-life (h)

1. Determine how many half-lives elapsed in order to get original amount to amount remaining (how many times did you divide by 2?) 2. Divide elapsed time by number for half-lives from step 1

A. 32P has a half-life of 14.3 days. If 32 grams were present to start, how many grams remain after 71.5 days?

B. 131I has a half-life of 8.07 days. In how many days will a 12 gram sample of 131I decay, leaving a total of 1.5 grams of the original isotope?

C. In 6.20 hours, a 100. gram sample of 112Ag decays to 25.0 grams. What is the half-life of 112Ag (in hours)?

Practice:

1.) What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 grams in 21.6 seconds? 2.) All isotopes of technetium are radioactive, but they have widely varying half-lives. If an 800.0 gram sample of technetium-99 decays to 100.0 g of technetium-99 in 639,000 years, what is its half-life? 3.) A 208 g sample of sodium-24 decays to 13.0 g of sodium-24 within 60.0 hours. What is the half-life of this radioactive isotope?

8


4.) If the half-life of iodine-131 is 8.10 days, how long will it take a 50.00 g sample to decay to 6.25 g? 5.) The half-life of hafnium-156 is 0.025 seconds. How long will it take a 560 g sample to decay to one-fourth its original mass? 6.) Chromium-48 has a short half-life of 21.6 hours. How long will it take 360.00 g of chromium-48 to decay to 11.25 g? 7.) Potassium-42 has a half-life of 12.4 hours. How much of an 848 g sample of potassium-42 will be left after 62.0 hours? 8.) Carbon-14 has a half-life of 5730 years. How much of a 144 g sample of carbon-14 will remain after 1.719 × 104 years? 9.) If the half-life of uranium-235 is 7.04 × 108 years and 12.5 g of uranium-235 remain after 2.82 × 109 years, how much of the radioactive isotope was in the original sample?

10.) Germanium-66 decays by positron emission, with a half-life of 2.5 hours. Write an equation for the decay. How much 66Ge remains from a 25.0 gram sample after 10.0 hours?

UNIT 14 - Reaction Energy & Reaction Kinetics...

Documents

Transcript of UNIT 14 - Reaction Energy & Reaction Kinetics...