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UCONN / Chemistry / CHEM 1127Q / What is the difference between heat and enthalpy?

What is the difference between heat and enthalpy?

What is the difference between heat and enthalpy?

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School: University of Connecticut
Department: Chemistry
Course: General Chemistry
Professor: Cady
Term: Fall 2016
Tags: General Chemistry, Exam 2, thermochemistry, and Enthalpy
Cost: 50
Name: Study Guide for Exam 2
Description: The next exam is Friday already! I'm not sure if everyone has even finished learning chapter 5, but here is a guide to the best of my ability. The test is only covering chapters 4 and 5, but it wouldn't hurt to look over our previous exams- we will be expected to know that material still. I included solutions this time, the harder ones with work as well, so that should help a bit. We also have no l
Uploaded: 10/24/2016
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Exam 2 Study Guide- Chapters 4 and 5


What is the difference between heat and enthalpy?



Chapter 4- Stoichiometry and Chemical Reactions 

Chemical Reactions

1) What are chemical reactions?

a) When two or more substances interact with each other to form something new 2) Evidence of chemical reaction

a) Precipitation- solid formed from a clear solution (cloudy)

b) Gas evolution- gas bubbles appear

c) Give off heat or light

d) Change in temperature or energy

e) Change of odor

3) Balancing chemical reactions

a) Everything in reactant has to end up in product.

b) Reactant(s) → Product(s)

4) Solubility and compounds dissolved in water

a) Many Ionic compounds dissolve in water… but some don’t


Rules governing oxidation numbers.



b) If things dissolve they are soluble, if they don’t they are insoluble

c) Water stabilizes Ions, makes them split from their pair to “hang” with water molecules 5) Precipitation Diagram

a) Table 4.1 in book- shows what normally does and does not dissolve, plus exceptions i) Memorize these 

6) Net ionic equation

a) Just like a chemical equation only we show all the ions separately and then eliminate ones thats are inactive/do not react.

b) The ones that react are called spectator ions.

c) Note- the subscript (aq) stands for aqueous, or dissolved in water

Acid Base Reactions

1) These reactions are also called neutralization reactions

2) The products are water and an ionic complex


Direction and sign of heat flow.



3) The ionic complex is also referred to as a salt.

4) Strong Acids and Bases

a) Strong Acids

i) Most halogens and a couple others

ii) Table 4.2 in book We also discuss several other topics like What happen in shays rebellion?

b) Strong Bases

i) Strong bases will be something that contains hydroxide

(1) (OH-)

(2) NaOH

(3) Ba(OH)2 

ii) Weak bases are everything else but mostly look out for ammonia (NH3).

c) Strong acids and bases split ions completely when dissolved

5) Acid base

a) You will see something that starts with H which is the acid.

b) The second reactant will almost always have an OH in it.

c) The products will be water (H2O) and an ionic complex.

6) Be able to identify single replacement, double replacement, acid-base reactions and precipitation reactions. (Aleks is a good tool for this). 

Oxidation-Reduction Reactions

1) Also known as Redox reactions. Reactions where electrons move from one species to another. 2) Oxidized = lose electrons, called Reducing Agent

3) Reduced = gains electrons, called Oxidizing Agent

4) Charge can also be known as Oxidation Number (#)

5) Oxidation # in the Multi Atom molecules

a) Oxidation number is the charge of each individual atom.

6) Rules Governing Oxidation Numbers

a) In book 

b) Ox # of an element is zero.

c) Ox # of an element in a monatomic ion is the charge on the ion. Don't forget about the age old question of What is sanctification?

d) Certain elements have the same oxidation number in most compounds

i) Group 1 metals are +1

ii) Group 2 metals are +2

iii) Oxygen is usually -2

iv) Hydrogen is usually +1

Stoichiometry

1) Molar ratio of reactants and products to each other

2) You need to know the number of moles/molecules for this to work. Anything else is not applicable.

a) For example, if you are given mass, you need to turn that into moles by dividing by Molar Mass, and then you find the moles of the other given element and turn that back into mass by multiplying by Molar Mass.

3) Important Information

a) Mass = MM × n

b) M = moles/liter or [X] We also discuss several other topics like Module key concepts in planning a healthy diet.

c) You can't destroy or create matter. Balance equations

d) When converting between molecules always convert through moles

4) Titrations

a) Acid react with base

b) Always H+ + OH- → H2O

c) Measuring the volume of a Standard Solution required to react with a measured amount of sample.

d) Equivalence Point: point at which the reaction is complete in a titration.

5) Limiting Reactants

a) Write out the balanced equation

b) Turn everything into moles if it isn’t already.

c) Check which reactant can make more product.

6) Yield

a) Theoretical Yield is how much product you should be able to make. Experimental Yield is how much you actually make.

b) Percent Yield is the Experimental Yield divided by the Theoretical Yield.

Chapter 5- Thermochemistry 

Reactions and Heat

1) Definitions Don't forget about the age old question of What are the three types of pigments?

a) Heat is abbreviated as q

b) Calories (cal)- energy unit

c) Joule (J)- SI energy unit

d) Heat capacity (C)- Energy required to heat something by 1 degree Celsius

e) Specific Heat capacity (c)- Energy required to heat 1 gram of something by 1 degree Celsius. Don't forget about the age old question of What is direct democracy?

2) Direction and Sign of Heat Flow

a) Heat is given the symbol, q

i) q is positive when heat flows into the system from the surroundings. This is called endothermic and would feel cold.

ii) q is negative when heat flows from the system into the surroundings. This is called exothermic and feels hot.

3) Magnitude of Heat Flow

a) In any process, we are interested in both the direction of heat flow and its magnitude. i) q is expressed in joules (or kilojoules)

ii) Alternate unit: calorie

(1) 1 calorie = 4.184 J

4) The Calorimetry Equation

a) q = C × Δ t

i) Δ t = t(final) - t(initial)

ii) C is the heat capacity of the system.

b) q = m × c × Δ t

i) C is the specific heat.

c) c depends on the identity and phase of the substance.

d) For a reaction performed in a Coffee-Cup Calorimeter use:

i) q (reaction) = -m (water) × 4.18 J /(g × ° C) × Δ t If you want to learn more check out What is a strategic business unit?

e) There are also Bomb Calorimeters

i) Use the same equation

ii) More versatile than coffee-cup

(1) Reactions involving high temperature

(2) Reactions involving gases

iii) The bomb is a heavy metal vessel that usually is surrounded by water (1) q(reaction) = -m (water) × 4.18 J /(g × ° C) × Δ t - C(bomb)Δ t

iv) Specific heat - there is a table in the book for common substances. We don’t have to memorize these, we should be given a table on the test as well. 

5) Enthalpy 

a) The heat flow at constant pressure it equal to the difference in enthalpy (heat content) between products and reactants 

b) The symbol for enthalpy is H 

c) We measure changes in enthalpy using a calorimeter and a reaction run at constant pressure 

i) Δ H = H(products) - H(reactants) 

6) Heat vs Enthalpy 

a) Enthalpy is only the temperature change. Enthalpy is a state function. It is reversible. b) Heat change involves change in volume. Heat is NOT as state function. The energy used to burst the can is not recovered. 

c) Enthalpy is a state property 

7) 3 Types of Equations 

a) Enthalpy in Thermochemical Equations 

i) A thermochemical equation is a chemical equation with the Δ H for the reaction included 

ii) Conventions 

(1) The sign ΔH indicates whether the reaction is endothermic or 

exothermic 

(2) The coefficients of the thermochemical equation represent the number of moles of reactant and product 

(3) The phases of all reactant and product species must be stated 

(4) This all applies when products and reactants are under 25° C 

iii) Rules 

(1) The magnitude of Δ H is directly proportional to the amount of reactant 

or product. 

(2) Δ H for the reaction is equal in magnitude but opposite in sign for ΔH 

for the reverse of the reaction 

(3) The value of Δ H is the same for whether the reaction occurs in one step 

in as a series of steps 

(a) This rule is a direct consequence of the fact Δ H is state variable: 

statement of Hess’s Law 

b) Enthalpy of Phase Changes 

i) Phase change involve enthalpy 

(1) There is no change in temperature during a phase change 

(2) Endothermic: melting or vaporization

(3) Exothermic: freezing or condensation 

(4) Pure substances have a value of Δ H that corresponds to melting 

(reverse, fusion) or vaporization (reverse, condensation). 

c) Enthalpy of Formation 

i) The standard molar enthalpy of formation, H°f Δ , is equal to the enthalpy change (1) For one mole of a compound 

(2) At constant pressure of atm 

(3) At a fixed temperature of 25°C 

(4) From elements in their stable states at that temperature and pressure. 

ii) Tabled in book or simply google. 

iii) Enthalpies of formation of elements and of H+(aq) 

(1) The enthalpy of formation of H+ (aq) is always zero 

iv) Calculations of Δ H° 

(1) H° = H°f products - H°f Δ Σ Δ Σ Δ reactants 

Note: It would be a good idea to go over the last exam, not only to review the material in case it reappears but also to study the format. Find your strengths and weaknesses, and study accordingly. This time I am attaching a solution guide, a few people brought it to my attention. Good luck everyone! 

Examples:

1) Balance the following chemical equations:

a) H2(g) + P4(s) → PH3(g).

b) CH3CH3(g) + O2(g) → CO2(g) + H2O(g).

2) Write the balanced equation for the interaction between HClO3 and KOH.

3) Identify the reducing and oxidizing agents in the the following equations (if there are any): a) 2Ca(s) + O2(g) → 2CaO(s)

b) FeO(s) + CO(g) → Fe(s) + CO2(g)

c) CO2(g) + 2H2O(l) → H3O+(aq) + HCO3-(aq)

4) Find the oxidation numbers for the following compounds: a) NH3 

b) ClO3-

c) LiH

d) Na(OH)2 

e) H2O

f) Cr2O72-

g) H2O2 

5) Find the half reactions of the following:

a) Cr (s)→ Cr+(s)

b) NO3- (aq) → NO(g)

c) O2(g) → O2(g)

6) Balance the Redox Equations in below:

a) MnO4−(aq) + I−(aq)→Mn2+(aq) + I2(s) (in an basic solution) b) HCL(aq) + HNO3(aq) → NO2(g) + CL2(g)

c) SO32-(aq) + MnO4-(aq) → SO4-(aq) + Mn2+(aq) (in acidic solution)

7) What volume of .750 hydrochloric acid solution can be prepared from the HCl produced by the reaction of 25.0g of NaCl with excess sulfuric acid? NaCl(s) + H2SO4(l) → HCl(g) + NaHSO4(s).

8) If 2.00g of propane is burned how much water is created in mL? C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l).

9) I2 is produced by the reaction of .4235 mol of CuCl2 according to the following equation: 2CuCl2 + 4KI → 2CuI + 4 KCl + I2.

a) How many molecules of I2 are produced?

b) What mass of I2 is produced?

10) You have 500 mL of Sulfuric Acid but you don’t know the concentration. You titrate the solution with 25 mL of 2M NaOH but overshoot the equivalence point. You then went back titrate with 3 mL of 0.1M HCL. What is the concentration of the sulfuric acid solution?

11) During a titration of 500 mL of an acid solution of unknown concentration you reach the equivalence point after adding 25mL of 1M Calcium Hydroxide. What is [H+] in the unknown solution?

12) Consider a 1.50g mixture of magnesium nitrate and magnesium chloride. After dissolving this mixture in water, 0.500M silver nitrate is added dropwise until the precipitation is complete. The mass of the precipitate formed is 0.641g.

a) Calculate the mass percent of magnesium chloride in the mixture.

b) What is the minimum volume of silver nitrate that must have been added to ensure the complete formation of the precipitate?

13) The monoprotic compound potassium acid phthalate KHC6H4O4, or KHP, is used in many labs. A .3420g sample of KHP reacts with 35.73mL of NaOH solution in a titration. What is the molar concentration of the NaOH?

KHC6H4O4 + NaOH → KNaC6H4O4 + H2O

14) 3g of Magnesium metal plus 1 mole of carbon dioxide react to make carbon and Magnesium Oxide.

a) What is the limiting reactant?

b) After we reacted the Magnesium with carbon dioxide we got 0.05g of Carbon. How does that compare to the calculated yield? (What is the percent yield?)

15) You are given a 1cm3 piece of sodium metal (0.968 g/cm3) and place it in 25 mL water. a) Which reactant is in excess?

b) How much of the excess reactant remains (in grams) after the reaction is completed?

c) How many grams of H2 were generated?

16) In a coffee cup calorimeter containing 50 mL of water a reaction takes place. The temperature of the water in the cup goes from 25°C to 75 °C. How much energy was released by the reaction?

17) In a Bomb Calorimeter we burned a Jelly bean. The calorimeter contained 100 g of water and changed temperature by 2°. How many nutritional calories did the jelly bean contain? The C for the Calorimeter is 0.5 J/°C and c of water is 4.18 J/°C. 

18) You place a 5g block of iron at 90°C into 100g of water at 15°C. What is the final temperature? The c of Fe = 0.446 J/g°C and water is 4.18 J/g°C. 

19) What is the change in enthalpy of the equation NH4NO3(s) → NH4+(aq) + NO3(aq) if the heat of the reaction is 351J for 1g of ammonium nitrate? 

20) If for the reaction Mg + 1/2O2 → MgO the H = -601.7 KJ and for C + O2 → CO2 Δ theΔ H = -393.5KJ, what is the H of Mg(s) + 1/2CO2 Δ (g) → MgO(s) + 1/2C(s)?

21) Measurements show that the energy of a mixture of gaseous reactants decreases by 377.kJ during a certain chemical reaction, which is carried out at a constant pressure. Furthermore, by carefully monitoring the volume change it is determined that −133.kJ of work is done on the mixture during the reaction. Calculate the change in enthalpy of the gas mixture during the reaction. 

22) During discharge of a lead-acid storage battery, the following chemical reaction takes place: Pb + PbO2 + 2H2SO4 ---> 2PbSO4 + 2H2O. Using the following two reactions: 

Pb + PbO2 + 2SO3 ---> 2PbSO4 ΔH° = -775 kJ 

SO3 + H2O ---> H2SO4 ΔH° = -113 kJ 

Determine the enthalpy of reaction for the discharge reaction above. 

23) Imagine 5g of water are being heated from -10°C to 120°C. What is the total change in heat? (C(ice)= 2.06 J/g°C, C(H2O) = 4.18 J/g°C, C(steam) = 2.01 J/g°C, Δ H(fusion) = 333.55 J/g, Δ H(vap) = 40.65 kJ/mole)

Solutions:

1) a) 6H2(g) + P4(s) → 4PH3(g).

b) 2CH3CH3(g) + 7O2(g) → 4CO2(g) + 6H2O(g).

2) This is an acid-base reactions (neutralization) so there will be water produced. HClO3 + KOH → KClO3 + H2O.

3) a) Ca is oxidized so it is the reducing agent, O2 is reduced so it is the oxidizing agent. b) Fe is reduced, so FeO it is the oxidizing agent. C is oxidized, so CO is the reducing agent. c) Tricky one. There is no exchange of electrons, so there is no oxidation taking place.

4) a) NH3 → N is -3, H is +1

b) ClO3- → Cl is +5, O is -2

c) LiH → Li is +1, H is -1. (Lithium is a metal, exception to hydrogen +1 rule.) d) Na(OH)2 → Na is +1, O is -2, H is +1

e) H2O → H is +1, O is -2

f) Cr2O72- ---slightly harder so I showed the work---

 2Cr# + 7O = -2

 2Cr# + -12 = -2

 2Cr# = +12

 Cr# = +6

g) H2O2-- rule breaker --

 2H# + 2O# = 0

 2H(+1) + 2O# = 0

 +2 + 2O# = 0

 O# = -1

5) a) Cr(s) → Cr+(s) + e 

b) NO3- (aq) → NO(g)

 NO3- (aq) → NO(g) + 2H2O

 NO3- (aq) + 4H+ → NO(g) + 2H2O

 NO3- (aq) + 4H+ +3e- → NO(g) + 2H2O

 c) O2 → O2- 

 O2 → 2O2- 

 O2 + 4e- → 2O2- 

6) a) MnO4−(aq) + I−(aq)→Mn2+(aq) + I2(s) ----I’ll show the work for these, they get confusing----

2I-(aq) → I2(s) MnO4-(aq) → Mn2+(aq)

2I-(aq) → I2(s) + 2e- MnO4-(aq) + 8H+(g) + 5e- → Mn2+(aq) + 4H2O(l) 2(MnO4-(aq) + 4H2(g) + 5e- → Mn2+(aq) + 4H2O(l))

+ 5(2I-(aq) → I2(s) + 2e-) 

2MnO4-(aq) + 16H+(g) + 10I-(aq) → 5I2(s) + 2Mn2+(aq) + 8H2O(l)

--remember it’s in a basic solution, we aren’t done---

2MnO4-(aq) + 16H+(g) + 16OH-(aq) + 10I-(aq) → 5I2(s) + 2Mn2+(aq) + 8H2O(l) + 16 OH-(aq) 2MnO4-(aq) + 16H2O(l) + 10I-(aq) → 5I2(s) + 2Mn2+(aq) + 8H2O(l) + 16 OH-(aq) 2MnO4-(aq) + 8H2O(l) + 10I-(aq) → 5I2(s) + 2Mn2+(aq) + 16 OH-(aq)

b) HCL(aq) + HNO3(aq) → NO2(g) + CL2(g)

 --- Because the first two elements are strong acids, this is an acidic solution. You should be able to check which type it is even if they don’t explicitly tell you----

HCl(aq) → Cl2(g) HNO3(aq) → NO2(g)

HCl(aq) → Cl2(g) HNO3(aq)→ NO2(g)

2HCl(aq) → Cl2(g) (H2O step) HNO3(aq) → NO2(g) + H2O 2HCl(aq) → Cl2(g) + 2H+(H+ step) H+ + HNO3(aq) → NO2(g) + H2O 2HCl(aq) → Cl2(g) + 2H+ + 2e- (e- step) 2(e- + H+ + HNO3(aq) → NO2(g) + H2O)

 2HCl → Cl2 + 2H+ + 2e- 

+ 2e- + 2H+ +2HNO3 → 2NO2 + 2H2O 

2HCl(aq) + 2HNO3(aq) → Cl2(g) + 2NO2(g) + 2H2O(l)

c) SO32-(aq) + MnO4-(aq) → SO4-(aq) + Mn2+(aq)

SO32-(aq) → SO4-(aq) MnO4- (aq) → Mn2+ (aq)

H2O + SO32-(aq) → SO4-(aq) + 2H+ + 2e- 8H+ + MnO4- (aq) + 5e-→ Mn2+ (aq) + 4H2O

 5(H2O + SO32-(aq) → SO4-(aq) + 2H+ + 2e-)

+ 2(8H+ + MnO4- (aq) + 5e-→ Mn2+ (aq) + 4H2O) 

5H2O(l) + 5SO32-(aq) + 16H+ + 2MnO4-(aq) → 5SO4-(aq) + 10H+ + 2Mn2+(aq) + 8H2O(l) 5SO32-(aq) + 6H2+ + 2MnO4-(aq) → 5SO4-(aq) + 2Mn2+(aq) + 3H2O(l)

7) What volume of 0.750M hydrochloric acid solution can be prepared from the HCl produced by the reaction of 25.0g of NaCl with excess sulfuric acid? NaCl(s) + H2SO4(l) → HCl(g) + NaHSO4(s).

25.0g of NaCl ÷ 58.443g/mol of NaCl = 0.42777 moles

0.42777 moles × 1 mole of HCl / 1 mole NaCl = 0.42777 moles

0.42777 moles ÷ 0.750 moles/L = 0.570L

8) If 2.00g of propane is burned how much water is created in mL? C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l).

2.00g of propane ÷ (3(12.001g/mol) + 8(1.008g/mol)) = 0.0454 moles C3H8

0.0454 moles C3H8(4H2O / 1 C3H8 × ) = 0.181 moles H2O

0.181 moles H2O × 18 g/mole = 3.266 g/mole H2O

3.266g/mole H2O÷ 1 g/mL = 3.266mL H2O

3.27mL H2O

9) a) .21175 moles of I2 

b) 57.74 g (remember, you have to double the molar mass since it’s a diatomic element)

10) You have 500 mL of Sulfuric Acid but you don’t know the concentration. You titrate the solution with 25 mL of 2M NaOH but overshoot the equivalence point. You then went back titrate with 3 mL of 0.1M HCL. What is the concentration of the sulfuric acid solution?

H+ + OH- → H2O

H2SO4 + HCL = NaOH

0.025L 2 moles/L NaOH = 0.05moles NaOH (1 OH- / 1 NaOH) = 0.05 moles OH- × × 0.003L 0.1 moles/L HCL = 0.0003 moles HCL (1H+ / 1 HCL) = 0.0003moles H+ × × 0.05 moles OH-(1H+ 1OH-) = 0.05 moles H+ total - 0.0003 moles H+ HCL = 0.0497 moles H+ × ÷ H2SO4 

H2SO4 → 2H+ + SO42-

0.0497 moles H+ (1 H2SO4 2 H+) = 0.02485 moles 1 H2SO4 × ÷ ÷ 0.500L = 0.0497M H2SO4 5. 10-2 × M

11) During a titration of 500 mL of an acid solution of unknown concentration you reach the equivalence point after adding 25mL of 1M Calcium Hydroxide. What is [H+] in the unknown solution?

H+ (?) + OH- (25 mL and 1M) → H2O

0.025L 1 mole/L Ca(OH)2 = 0.025 moles Ca(OH) × 2 

0.025 moles Ca(OH)2(2OH- 1 Ca(OH)2 ) = 0.05 moles OH- × ÷

0.05 moles OH-(1H+ 1 OH-) = 0.05moles H+ × ÷

0.05moles H+ 0.500L = 0.1 mole/L H+ ÷

12) Consider a 1.50g mixture of magnesium nitrate and magnesium chloride. After dissolving this mixture in water, 0.500M silver nitrate is added dropwise until the precipitation is complete. The mass of the precipitate formed is 0.641g.

a) Calculate the mass percent of magnesium chloride in the mixture.

b) AgNO3(aq) + Mg(NO3)2 → does not react, no precipitate

c) 2AgNO3(aq) + MgCL2 → 2AgCl(s) + Mg2+ + 2NO3-

i) AgCl(s) = 107.877g/mole + 35.45g/mole = 143.32g/mole

ii) 0.641g ÷ 143.32g/mole AgCl = 0.00447moles AgCl(s)

iii) 0.00447 moles AgCl (1 MgCl2 2 AgCl) = 0.002235 moles MgCl × ÷ 2

iv) 0.002235 moles MgCl2 95.21g/mole MgCl2 = 0.2128g MgCl × 2 

v) Mass % = (Mass MgCl2 ÷ total mass) × 100

vi) % = (0.2128g MgCl2 ÷ 1.50g) × 100 = 14.2%

d) What is the minimum volume of silver nitrate that must have been added to ensure the complete formation of the precipitate?

(1) 2AgNO3(aq) + MgCL2 → 2AgCl(s) + Mg2+ + 2NO3-

ii) 0.00447moles AgCl(s) (2AgNO3 × ÷ 2AgCl) = 0.00447moles of AgNO3 iii) 0.00447moles of AgNO3 ÷ 0.500moles/L (M) = 0.00894L AgNO3 

iv) 8.94mL of AgNO3 

13) The monoprotic compound potassium acid phthalate KHC6H4O4, or KHP, is used in many labs. A .3420g sample of KHP reacts with 35.73mL of NaOH solution in a titration. What is the molar concentration of the NaOH?

KHC6H4O4 + NaOH → KNaC6H4O4 + H2O

0.3420g ÷ 18-0.2016g = 0.00180979 moles KHP

0.00180979 moles KHP = 0.00180979 moles OH- 

0.00180979 moles NaOH = Mass NaOH × 35.73mL of NaOH

0.05M

14) 3g of Magnesium metal plus 1 mole of carbon dioxide react to make carbon and Magnesium Oxide.

a) What is the limiting reactant?

Mg(s) + CO2(s) → C(s) + MgO(s)

2Mg(s) + CO2(s) → C(s) + 2MgO(s)

3g / 24.305g/mol = .1234 moles of Magnesium

.1234 moles of Mg × 1 Carbons / 2 Magnesium = 0.0617 moles C

1 mole CO2 (1C / 1CO2 × ) = 1 mole C

Magnesium runs out first so it’s the limiting reactant. We can make 0.0617 moles of Carbon according to the theoretical yield.

b) After we reacted the Magnesium with carbon dioxide we got 0.05g of Carbon. How does that compare to the calculated yield?

.05g C ÷ 12.01 g/mole C = 0.004 moles C : experimental yield.

0.004 moles C / 0.05 moles C × 100 = 8%

15) You are given a 1cm3 piece of sodium metal (0.968 g/cm3) and place it in 25 mL water. a) Which reactant is in excess?

H2O is in excess, Na is limiting

b) How much of the excess reactant remains (in grams) after the reaction is completed?

24.24g left over

c) How many grams of H2 were generated?

0.0421g H2 

16) In a coffee cup calorimeter containing 50 mL of water a reaction takes place. The temperature of the water in the cup goes from 25°C to 75 °C. How much energy was released by the reaction? q = -m × c × Δ t

q = -10450J

17) In a Bomb Calorimeter we burned a Jelly bean. The calorimeter contained 100 g of water and changed temperature by 2°. How many nutritional calories did the jelly bean contain? The C for the Calorimeter is 0.5 J/°C and c of water is 4.18 J/°C. 

q = -m × 4.18 J /(g × °C) × Δ t - C(bomb)Δ t

q = -837 J × 1 cal/ 4.18 J = -200 cal 

q = -0.2 kcal 

18) You place a 5g block of iron at 90°C into 100g of water at 15°C. What is the final temperature? The c of Fe = 0.446 J/g°C and water is 4.18 J/g°C. 

-q (Fe) = q H2O → - (m (Fe) × c (Fe) × Δt) = m (H2O) × c (H2O) × Δt T(f) = 14.599°C 

19) What is the change in enthalpy of the equation NH4NO3(s) → NH4+(aq) + NO3(aq) if the heat of the reaction is 351J for 1g of ammonium nitrate? 

351 moles × 80.05g = 28097.55 0r 28.1 kJ 

Δ H = +28.1 kJ 

20) If for the reaction Mg + 1/2O2 → MgO the H = -601.7 KJ and for C + O2 → CO2 Δ theΔ H = -393.5KJ, what is the H of Mg(s) + 1/2CO2 Δ (g) → MgO(s) + 1/2C(s)? 

First we need to flip the second equation, so Δ H = 393.5KJ. 

We also need half the reaction, so 393.5KJ ÷ 2 = 196.75 

Finally we add the two together. -601.7 KJ + 196.75 = -404.95 

Total Δ H = -404.95

21) Measurements show that the energy of a mixture of gaseous reactants decreases by 377.kJ during a certain chemical reaction, which is carried out at a constant pressure. Furthermore, by carefully monitoring the volume change it is determined that −133.kJ of work is done on the mixture during the reaction. Calculate the change in enthalpy of the gas mixture during the reaction. Δ H = Δ E - W 

-244. kJ 

22) During discharge of a lead-acid storage battery, the following chemical reaction takes place: Pb + PbO2 + 2H2SO4 ---> 2PbSO4 + 2H2O. Using the following two reactions: 

Pb + PbO2 + 2SO3 ---> 2PbSO4 ΔH° = -775 kJ 

SO3 + H2O ---> H2SO4 ΔH° = -113 kJ 

Determine the enthalpy of reaction for the discharge reaction above. 

ΔH° = 2(-113)(-) = 226 kJ + -775 kJ 

ΔH° = -549 kJ 

23) Imagine 5g of water are being heated from -10°C to 120°C. What is the total change in heat? (C(ice)= 2.06 J/g°C, C(H2O) = 4.18 J/g°C, C(steam) = 2.01 J/g°C, Δ H(fusion) = 333.55 J/g, Δ H(vap) = 40.65 kJ/mole) 

5g / 18 g/mol = .2778 moles 

q(heating ice) = (5g × 2.06 J/g°C × (0° - (-10°)) = 103 J or .103 kJ 

q(melting) = (5g × (333.55J/g)) = 1667.75 J or 1.67 kJ 

q(heating H2O) = (5g × 4.18 J/g°C × (100°-0°) = 2090 J or 2.09 kJ 

q(boiling) = (.2778 moles × 40.65 kJ/mole) = 11.29 kJ 

q(heating vapor) = (5g × 2.01 J/g°C × (120° - 100°)) = 201J or .201 kJ 

Add them all up… q(total) = 15.354 kJ

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