# Chemistry Guide

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2 8 D 9 C 10 B 11 D 12 D 13 Work (example) Q = mc T 4.2 J 100 g 3.5 C g C Q = mol = J 1 mol = J H = kj/mol

3 14 Example of an appropriate procedure Calculating the change in the water temperature T = T f T i T = 34.0 C 24.0 C T = 10.0 C Calculating the heat absorbed by the water Q water = m water c water T water 4.19 J Q = 1000 g 10.0 C g C Q = J = 41.9 Q = absorbed Q released Quantity of paraffin burned Q n = H 41.9 n = /mol n = mol Molar mass of paraffin M = g/mol Mass of paraffin burned m = nm m = mol g/mol m = g Answer The mass of the paraffin burned: g.

4 15 Example of an appropriate procedure 3 C (s) + 4 H 2(g) C 3 H 8(g) H =? Reverse the «combustion of propane» equation and add to it the heats of formation of CO 2(g) and H 2 O (g) (Reverse) 4 H 2 O (g) + 3 CO 2(g) C 3 H 8(g) + 5 O 2(g) H = mol (mult 4) 4 H 2(g) + 2 O 2(g) 4 H 2 O (g) H = 968 mol (mult 3) 3 C (s) + 3 O 2(g) 3 CO 2(g) H = mol 3 C (s) + 4 H 2(g) C 3 H 8(g) H = 104 mol Answer : The heat of formation of propane is mol 16 Example of an appropriate procedure Application of Hess law CO 2(g) + 2H 2 O (g) 2 3 O2(g) + CH 3 OH (l) H = +726 C (s) + O 2(g) CO 2(g) H = 393 2H 2(g) + O 2(g) 2H 2 O (g) H = C (s) + 2H 2(g) + O2(g) CH 3 OH (l) 2 H = 151 Answer The molar heat of formation of methanol is 151 /mol.

5 17 Example of an appropriate procedure Find the mass of the ice cube in order to find the energy required to melt it. g ρ H 2 O(s) = cm at 0 C The volume of the ice cube = 5.00 cm 5.00 cm 5.00 cm = 125 cm 3 g The mass of the ice = cm 125 cm 3 = 115 g The heat of fusion of ice = g The energy required to melt the ice cube = Heat of combustion of ethanol = 29.7 g 115 g = 38.5 g Mass of ethanol required = g = 1.30 g Answer : The mass of ethanol required to produce the required energy is 1.30 g.

6 Chemistry Question Booklet Name : Group : Date : 1 The following table gives the Enthalpy changes associated with several reactions. Equations H (/mol) H 2(g) + 1/2 O 2(g) H 2 O (g) -242 C (s) + O 2(g) CO 2(g) -394 C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O (g) H 2(g) + 1/2 O 2(g) H 2 O (l) -286 Using Hess' Law, determine the molar heat of formation of propane, C 3 H 8(g). 3 C (s) + 4 H 2(g) C 3 H 8(g) H =? A) -104 /mol C) /mol B) 1410 /mol D) /mol 2 The following statements concern the enthalpy of substances : 1. When a chemical bond forms, the enthalpy decreases. 2. When a chemical bond is broken, the enthalpy decreases. 3. When a chemical bond is broken, the enthalpy increases. 4. When a chemical bond forms, the enthalpy increases. Which of the statements are true? A) 1 and 3 C) 2 and 3 B) 1 and 4 D) 2 and 4

7 3 Which diagram represents the following reaction? 2 2(g) Al(s) + 3/ 2 O Al2 O3(s) A) Enthalpy C) Enthalpy Al 2 O 3(s) Al (s) + 3/2 O 2(g) Al 2 O 3(s) Al (s) + 3/2 O 2(g) Progression of the reaction Progression of the reaction B) Enthalpy D) Enthalpy Al 2 O 3(s) 2 Al (s) + 3/2 O 2(g) Al 2 O 3(s) 2 Al (s) + 3/2 O 2(g) Progression of the reaction Progression of the reaction 4 A swimming pool contains L of water at a temperature of 20.0 C. The water absorbs of energy from the sun. What will be the FINAL temperature of the water? A) 8.0 C B) 23.2 C C) 28.0 C D) 31.6 C

8 5 While performing an experiment, you observed that energy was released as a result of the chemical reaction between solid magnesium and hydrochloric acid. After a fixed amount of magnesium decomposed, 50 of energy was released. Which graph best represents the change in enthalpy? Mg (s) + 2HCl (aq) MgCl 2(aq) + H 2(g)

9 6 You dissolve 4 g of sodium hydroxide in a solution of hydrochloric acid. The thermochemical equation for the reaction is: What is the heat of reaction? NaOH (s) + HCl (aq) NaCl (aq) + H 2 O (l) A) 10.5 J C) J B) 420 J D) J 7 Carbon, in the form of graphite, can be compressed at high temperatures and high pressures to produce carbon in the form of diamond. C (graphite) C (diamond) Which of the following is the heat of reaction, H, that will produce one mole of diamond from one mole of graphite, based on the equations below? C (diamond) + O 2(g) CO 2(g) C (graphite) + O 2(g) CO 2(g) A) H = C) H = B) H = D) H =

10 8 From a table, a student writes the following thermochemical equations for the reaction of iron and oxygen. Fe(s) +1/ 2 O2(g) FeO(s) Fe(s) + 3/ 2 O2(g) Fe2 O3(s) Based on the above equations, calculate the heat liberated by the formation of one mole of Fe 2 O 3(s) according to the following equation: 2 2(g) FeO(s) +1/ 2 O Fe2 O 3(s) A) liberated B) liberated C) liberated D) liberated 9 You are given two unknown liquids. You mix 100 ml of liquid A at 80 C with 100 ml of liquid B at 20 C. The resulting mixture has a final temperature of 60 C. What can you conclude from these results? A) The specific heat capacity of liquid A is less than the specific heat capacity of liquid B. B) The specific heat capacity of liquid A is equal to the specific heat capacity of liquid B. C) The specific heat capacity of liquid A is more than the specific heat capacity of liquid B. D) Liquid A and liquid B are identical in both nature and concentration.

11 10 According to the diagram, which statements below is correct? Enthalpy (/mol) Progress of the reaction A) The forward reaction is endothermic. B) The activation energy of the forward reaction is less than the activation energy of the reverse reaction. C) The enthalpy of the reactants is less than the enthalpy of the products. D) The activation energy of the forward reaction is zero. 11 You are given the diagram for the following hypothetical, reaction: A + B AB. Enthalpy (/mol) 35 0 A + B -135 AB Progress of reaction According to this diagram, what is the activation energy of the reverse reaction? A) 35 C) 135 B) 100 D) 170

12 12 When 4.0 g of potassium hydroxide, KOH, is dissolved in ml of water in a calorimeter, the temperature increases from 25.0 C to 31.5 C. Calculate the molar heat of solution of the potassium hydroxide. A) +5.4 /mol B) -5.4 /mol C) +76 /mol D) -76 /mol 13 An experiment to find the heat of solution of NH 4 Cl (s) gave the following results : mass of NH 4 Cl (s) dissolved volume of water initial temperature of the water final temperature of the water specific heat of water 5.35 g 100 ml 22 C 18.5 C 4.2 J g C Calculate the molar heat of solution of NH 4 Cl (s). Show all your work. 14 Below are the results of an experiment that involved burning paraffin, C 25 H 52(s). Initial temperature of the water in the calorimeter Final temperature of the water in the calorimeter Volume of the water in the calorimeter 24.0 C 34.0 C 1.00 L Burning one mole of paraffin releases What was the mass of the paraffin burned? Show all your work.

13 15 Some automobiles and buses are equipped to burn propane gas, C 3 H 8, as a fuel. The complete combustion of propane is shown by the following chemical equation: C 3 H 8 ( g ) + 5 O 2 ( g ) 3CO 2(g) + 4H 2 O (g) H = mol Given the following heats of formation. H 2(g) O2(g) H 2 O (g) H = mol C (s) + O 2(g) CO 2(g) H = mol What is the heat of formation of propane? 3 C (s) + 4 H 2(g) C 3 H 8(g) H =? 16 Since the molar heat of formation of methanol, CH 3 OH, from its elements cannot easily be measured, chemists prefer to calculate this value using the following thermochemical equations : CH 3 OH (l) O2(g) CO 2(g) + 2H 2 O (g) C (s) + O 2(g) CO 2(g) H 2(g) O2(g) H 2 O (g) H = 726 H = 393 H = 242 The formation of methanol from its elements is represented by the following equation : C (s) + 2H 2(g) O2(g) CH 3 OH (l) Given this data, what is the molar heat of formation of methanol? Show all your work.

14 17 Ethanol is a liquid that combusts relatively easily. In a laboratory investigation, a sample of ethanol was placed into an alcohol burner and ignited. The heat from the combustion of the alcohol was used to melt a cube of ice placed into a beaker as shown below. ice cube ethanol Some properties of ethanol and ice are listed below: Heat of combustion of ethanol = 29.7 /g Heat of fusion of ice = /g Density of ice = g/cm 3 at 0 C Given this information, what mass of ethanol is needed to melt a cube of ice that is 5.0 cm on each side? (Assume 100% of the heat produced from the ethanol is transferred to the ice cube.)

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