General Chemistry (Part 3): Thermochemistry

For all students of XI grade, read the following questions carefully, choose one question, give your answer here but once the question has been answered, right or wrong, you can not answer it. Just write a final answer, you don't need to write stepwise. For the last five answers, you may change your answer twice, if the first answer is false.

1) What is thermochemistry exactly?

2) Why do we learn thermochemistry?

3) Give 2 examples of thermochemistry applications in daily life.

4) State a thermochemical equation if ammonium chromate (solid) formation releases 1806.7 kJ/mole of energy.

5) 130 grams acetylene (C2H2) can raise 20 liters of water from 200C to 1000C. Calculate enthalpy change of water and state its thermochemical equation if known density of water = 1 gr/ml and heat capacity of water = 1 J/gr.0C.

6) Known the enthalpy change of ethylene and ethane formations in the manner are 51,8 kJ and 64,4 kJ, what heat will be released by ethylene produces ethane?

7) By using bond energy data in your text book, calculate how much heat will be included to react ethylene and chlorine form ethyl chloride and chloric acid? Please write the bond energy of each substance.

8) Known the following thermochemical equations:
XO2 + CO -® XO + CO2 DH = - 20 kJ
X3O4 + CO -® 3 XO + CO2 DH = + 6 kJ
3 X2O3 + CO -® 2 X3O4 + CO2 DH = - 12 kJ
Calculate the enthalpy change for the following equation:
2 XO2 + CO -® X2O3 + CO2

9) Explain latent heat and sensible heat.

10) What is 1 Btu (British Thermal Unit)? Calculate the number of joules in 1 Btu of heat.

11) What does Hess' Law state and give one example to explain your answer (you may use an example from daily life or even everywhere).


12) The natural gas in methane reacts with oxygen to give carbon dioxide and water. Calculate the heat given off when 0.010 mole of methane reacts with excess oxygen in a bomb calorimeter if the temperature of 1.00 kilogram of water in the bath surrounding the bomb increases by 1.918 0C.

13) How do we measure the enthalpy of a reaction? Explain your answer sharply.

14) What are differences between an exothermic reaction and an endothermic reaction?

15) If a reaction can not go spontaneously what you will do? Explain your answer by giving an example of each way.

16) Predict which of the following substances should have an enthalpy of formation equal to zero:
(a) Hg (l)
(b) Br2 (g)
(c) H (g)
Give your reason.

17) What value of heat will be included in the following blank enthalpy change?



















18) How much heat is given off when 1 mole of nitrogen reacts with 2 moles of oxygen to give 2 moles of nitrogen oxide gas, if for the given balanced reaction is 33.2 kJ/mole of nitrogen oxide?

19) Explain why there is only one value for the standard enthalpy of reaction for a chemical reaction whereas there can be many different values for the enthalpy of reaction.

20) Use the standard enthalpies of formation in your text book to determine whether heat is given off or absorbed when limestone (calcium carbonate) is converted to lime (calcium oxide) and carbon dioxide. Write the heat value included.

21) Use bond dissociation enthalpies to estimate the enthalpy of reaction for the combustion of carbon sulfide in the phase gas to produce carbon dioxide and sulfur dioxide.

22) Calculate the standard enthalpy change for magnesium in the solid phase reacts with fluorine in the gas phase to create magnesium fluoride in the solid phase, assuming that 1.00 gram of magnesium gives off 46.22 kJ of heat when it reacts with excess fluorine.

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Does Stoichiometry Suck Your Life?

Oh God if I knew what the easiest way to teach stoichiometry for my students, I would run that way! I do don't know other way to explain stoichiometry after trying all ways I'd known. For me personally stoichiometry is the best part of chemistry. The most I loved in chemistry when I was at school was stoichiometry. I'd take the highest score in this part but not in the characteristic of Periodic System because this part might ask you to memorize too much thighed statements as I didn't like to memorize. For me at that time when a kinda natural lesson asked you much to memorize, the lesson should go to a social science. No wonder then why I loved stoichiometry more than other parts of chemistry.

But, oh my, I still need to face the worst world of mine after years I've struggled it. With no good mood of the weakest heart I put my students' papers on the floor as I was sitting around and checked all points of each paper. I hoped I'd find something wrong with my check point on some papers, hoped I was blank when I checked one or more papers and I needed to recheck the score. But I couldn't find something to increase even one score of the papers.

It's the second exam of stoichiometry. I needed to rerun the same field because nobody had taken a good score in last stoichiometric exam. Every single person should take minimally 70 as her/his score to pass the exam. In the last exam, the highest score was 42 and the lowest one was 14. And now I'm rechecking results of the second exam. The highest score is 48 and the lowest is 8. There are some students take 8 and some others take 16.

If I'd be a uncared teacher or I'd leave this part after leaving the chemistry class, I'd love it but how I could because I'd accept students as my sons and daughters. Will a mommy leave her kids alone when they need her? So I'm here with my students' papers and just face their papers on as my paled face going much paler. I don't think my students too stupid to answer such stoichiometry questions. Yes they all were A-level questions but we'd discussed such questions in the class. When I asked them why they couldn't answer the questions and many didn't answer essay questions, they said they didn't understand the questions because they were in English and I used new vocabularies in many questions. Of course I won't give same questions with what we'd studied in the class. All vocabularies I used in those questions were just common vocabularies and as I used Cambridge tutorial to check the British English style on the questions.

I'm curious what's wrong with my students. Do I use wrong ways when I'm teaching stoichiometry? Should I change the way I teach them? Can someone out there tell me what better way i should run on?

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General Chemistry (Part 2): Questions for X Level

The following questions are about:

• All mater
• Elements, compounds, and mixtures
• Atomic structure and Periodic Table
• Bonding and structure
• Chemical calculation
  

1) Which one is a compound?

A. A leaf

B. A paper

C. A pocket of dry ice

D. A tube of toothpaste

2) Which of the following electronic configurations represents an element that will form possible one simple ion with a charge of –3?

A. 1s² 2s² 2p⁶ 3s² 3p¹

B. 1s² 2s² 2p⁶ 3s² 3p³

C. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹

D. 1s² 2s² 2p⁶ 3s² 3p¹ 4s² 3d³

3) Which of the following ions contains unpaired electrons?

A. Fe²⁺ (Z = 26)

B. Mg²⁺ (Z = 12)

C. Ti⁴⁺ (Z = 22)

D. Zn²⁺ (Z = 30)

4) The successive ionization energies, in kJ.mol^–1, of an element X are 870; 1800; 3000; 3600; 5900; 7000; 13200. What is X?

A. ₃₃As

B. ₃₅Br

C. ₅₃I

D. ₅₂Te

5) The atoms X and Y have the electronic configurations shown below:

X: 1s² 2s² sp⁶ 3s² 3p¹ 4s² 3d¹⁰

Y: 1s² 2s² 2p⁴

Which one of the following compounds are they likely to form?

A. XY

B. XY₂

C. X₂Y

D. X₂Y₃

6) The electronic configuration of A and B are shown below:

A: 1s² 2s² sp⁶ 3s² 3p¹ 4s²

B: 1s² 2s² sp⁶ 3s² 3p⁵

What is the bonding type of the compound formed?

A. Ionic bonding

B. Covalent bonding

C. Covalent double bonding

D. Covalent coordinate bonding

7) In absence of catalyst, ammonia burns in oxygen to produce steam and nitrogen. What is gas and its volume remaining when 100 cm³ of ammonia is burned in 125 cm³ of oxygen? (All volumes are being measured at the same temperature and pressure).

A. 33.3 cm³ of ammonia

B. 66.7 cm³ of ammonia

C. 50 cm³ of oxygen

D. 75 cm³ of oxygen

8) How many grams of sodium hydroxide do you need to add into 0.2 liters of magnesium chloride 1 M to produce 5.8 grams of settled magnesium hydroxide? (Ar H = 1, O = 16, Na = 23, Mg = 24)

A. 4 grams

B. 8 grams

C. 16 grams

D. 32 grams

9) Which solution will have the highest electricity conduction hence they all are 1 M?

A. HCl

B. HNO₃

C. H₂SO₄

D. H₃PO₄

Write your answer in dark blue or black pen.

Do not use highlighters, glue, or correction fluid on your answer sheet.

10) There are 3 elements: A, B, C. They three are in same period of period system.

A and B are metals.

B is an explosive metal when it is burned with some water.

C is a polyatomic element which is in right side-neighbor of A.

a) Determine reactivity of those elements with oxygen from the highest to the lowest.

b) Suggest a real atomic symbol for each element.

11) A number of aluminum is added into aqueous sulfuric acid. A half of volume of gas produced is passed through a nitrogen tube and heated to produce ammonia. The ammonia is measured and it shows 2.24 liters of volume. How many grams of initial aluminum should be added? (Ar Al = 27)

Good Luck!

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General Chemistry (Part 1): Questions of Basic Stoichiometry

The following questions are about the basic stoichiometry. You may think why I wrote chemistry field again? It ain't a fun part of chemistry. Because I know many students get hard with stoichiometry so I put this field once again. But here are questions only. You need to read 'Let's Try to Learn Stoichiometry' at first and continue reading this part then. Here we go....

1. Calculate the empirical formula for a compound with the following percent composition: 23.29% of Mg, 30.72% of S and 45.99% of O.
   
   
2. A saturated hydrocarbon contains 82.66% of carbon. what is its empirical formula and molecular formula?
   
   
3. Calculate of the mass of chloride ion in 1.00 liter of each of the following solutions: a) 10% of NaCl (1.07 g/ml of density); b) 1.00 M NaCl; c) Calculate the molarity of the first solution.
   
   
4. Express the following gas pressures in atmospheres: a) 300 cmHg; b) 380 torr.
   
   
5. Calculate the molarity of each type of ion remaining in solution after 20.0 ml of 6.00 M hydrochloric acid is mixed with 50.0 ml of 2.00 M barium hydroxide.
   
   
6. How would one prepare exactly 300 ml of 5.00 M HCl solution by diluting sufficient 12.00 M stock solution.
   
   
7. Calculate the mass of CuS produced and the concentration of hydrogen ion produced by bubbling excess sulfuric acid into 1.00 liter of 0.10 M copper (II) chloride solution.
   
   
8. A refrigeration tank is holding 5.00 liters of freon gas (C₂C_l2F₂) at room temperature and 3.00 atm of pressure develop a leak. When the leak was discovered and repaired, the tank had lost 76.0 grams of the gas. What was the pressure of the gas remaining in the tank at the room temperature above?
   
   
9. In an auto engine with no pollution controls, about 5% of the fuel (assume 100% octane, C₈H₁₈) is unburnt. Calculate the relative masses of CO and C₈H₁₈ emitted in the exhaust gas. Calculate the relative volumes of these gases.
   
   
10. When 0.75 mole of a solid "C₈" and 2 moles of gaseous oxygen are heated in a seated vessel (bomb), completely using up the reactants and producing only one compound, it is found that when the temperature is reduced to the initial temperature, the contents of the vessel exhibit a pressure equal to half the original pressure . What conclusions can be drawn from these data about the product of the reaction?
    

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Let's Try to Learn Stoichiometry

You may think it's funny for you when you check the tittle of this post. 

'Is it a view? Why don't you leave this post and let other blogs to create this?' you may ask me. Let's me explain. As I'm a chemistry teacher and I've created this blog for my students especially so it's no wonder why I create an explanation of lessons as a view. I hope such posts will be used by other students of other senior high schools especially in Indonesia. Indonesia has a specific curriculum called national curriculum which adopts O-level curriculum and A-level curriculum and adds some new fields for X, XI, and XII grades. As I've been teaching in a senior high school with plus curriculum for about 11 years, I'm experienced enough to know what kinda chemistry problems for Indonesian students. I hope they may find some solutions for their chemistry problems here. This post will explain the stoichiometry concept only because it's commonly the hardest part of chemistry for Indonesian students but it will relate with many parts of chemistry and it's the basic of chemistry calculations. What's the basic of Stoichiometry? The basic concept is mathematics. Once you understand math well you can learn stoichiometry easier for sure. To make this field being narrower, here are the equations you need to get closer with. General equations Molar mass of an atom is equal to the molecular weight of the given atom. Molar mass of a molecule is equal to the molecular weight of the given molecule.

For all conditions

• mass = the number of moles x molar mass

m = n . mm

• the number of molecules = the number of moles X Avogadro number

X = n . L

• the number of moles = Molarity x Volume

n = M. V For specific conditions

• At standard temperature and pressure

Volume = the number of moles x Molar Volume V = n . Vm V = n x 22.4 liter/moles

• At specific temperature and pressure

P V = n R T where P = pressure (atm) n = the number of moles R = the ideal gas constant (0.082 liter.atm/mol.K) V1 . n2 = V2 . n1 where V1 and V2 = volume of gases n1 and n2 = the number of moles of gases What are steps to solve a stoichiometry problem? Here are the steps and do them in the manner:

• Read and understand the problem and list the known stuffs. Don't mention the earlier word of a given number but the later one of the given number.

Here's an example of a part of an unfinished question: What is the number of mass of 2.24 liters of ammonia.... So the known stuff is 'volume of ammonia or V' not 'the mass of ammonia or m'

• Determine if the problem needed a chemical equation or not. Write a balanced chemical equation if it needed it. Go to the next point if it didn't.

Example:

200 mL of nitrogen gas is flown into a tube of 200 mL of hydrogen gas and they are heated. In this case, you need to write a chemical equation and then balance it. Just use a balanced equation to solve any chemistry problems.

• List the equation(s) which you need to solve by using the known stuffs. Circle the known numbers and leave the unknown ones. Solve equations as many as possible.

• Add the new known number(s) into the equation(s) which you must solve.
• Convert the equation into the asked stuff in the problem and run it.
• You just solve a chemistry problem!

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I Hate Chemistry

It's a specific title of my unfinished-book, but here I don't wanna talk about it. You must be wondering as I'm a chemistry teacher but why I title it so? I loved chemistry but less than physic or math when I was at school. I just liked stoichiometry as the calculation part of chemistry field. It includes general mole concepts, buffer calculations, hydrolysis components, and all things of basic calculation in chemistry. Because I loved math so much, no wonder I loved stoichiometry only at young age.

But nowadays I know how to learn easily chemistry. When we wanna learn something new, we should find the key first and then the door will open widely, right? Here are the keys.
First, use your logical math to replace chemistry problems. It's easier then to solve it. For example, it says a student adds a number of mixture of two kinda metals into a HCl solution, the new mixture releases 1 liter of hydrogen gas at rtp, and you must calculate how many grams of each metal. For solving the given problem, you should know which metal will react or if they two will react with HCl solution. If one metal only will react, it will be easier to solve it because you can assume that all 1 liter of hydrogen gas as the product are produced from one reaction only. Hence another one doesn't react with HCl, it doesn't produce hydrogen gas. Base on the given reaction, you can determine the mole number of hydrogen gas and also other substances. Once you've known the number of moles, you can calculate all things by converting the number of moles into the given questions.
If the two metals will react with HCl solution, just think that the hydrogen gas is produced by two reactions between metals and HCl solution. Hence we have two reactions, we need to assume that we have X liter of hydrogen gas from one reaction and (1 - X) liter of hydrogen gas from another one. You further must convert them into the number of moles of each hydrogen. By that way, you can calculate the number of moles of other substances of each reaction. So you can go further to solve the problem.

Second, you must put the problem as it's your daily activity only. For example, when you're asked to prepare 1 liter of potassium chloride solution1 M, put the job as your mommy is asking you to prepare a glass of water with some sugar. You'll run some questions. How sweet is the solution your mum want you to prepare? What do you do first, add water or sugar in the glass? By that way you can go to calculate the number of mass of potassium chloride. Find a specific relation between molarity (as it's asked) and mass (as it's the real thing you have at laboratory). Convert molarity into mass. Then do your job because your mum has been waiting for her water!

Third, put chemistry as a real case in your life. So no other way you may learn chemistry easily and clearly but do experiments as it's impossible to face a hard life if you're just sitting alone in your own emptied-room. For this case, I can't suggest you something as we all have known what kinda laboratory we have at our school. And about how poor laboratories in national schools in our country, we should run this question at the government. I myself don't wanna place you in a fire seat in this case but you can make your comment here freely. It's our space to speak freedom as our foundations had proclaimed our independence day years ago. If there's no something wrong in our country, people should be able to speak freely, right?

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