1

Test 1 Fall

covering Energy, Enthalpy, Specific Heat and Hess’ Law

January 26-27, 2011

Form A

Show work for up to 25% credit even if you get the wrong answer. STP is 1 atm at 0C. F= 9/5C + 32. R is 0.08206 L atm/mol K 2.54 cm/in 453.59 g/lb 1.06 qt/L

1)A sample of water is heated from 10 °C to 15 °C by the addition of 30 calories of heat. What is the mass of the water?

a)5.0 g

b)6.0 g

c)30 g

d)150 g

2)When 100 calories of heat energy is added to 10 grams of water at 20°C the final temperature of the water will be

a)10 °C

b)30 °C

c)40 °C

d)100 °C

3)When 5 grams of a substance is burned in a calorimeter 3 kilocalories of energy is released. The energy released per gram of substance is

a)600 cal

b)1,700 cal

c)5,000 cal

d)15,000 cal

4)How many kilocalories of heat energy are absorbed when 100 grams of water is heated from 20 degrees Celsius to 30 degrees Celsius?

a)1 kcal

b)10 kcal

c)100 kcal

d)0.1 kcal

5)The temperature of a sample of water in the liquid phase is changed from 15 degrees Celsius to 25 degrees Celsius by the addition of 500 calories. What is the mass of the water?

a)10 grams

b)50 grams

c)100 grams

d)5,000 grams

6)When 25 grams of water is cooled from 20 °C to 10 °C the number of calories of heat energy released is

a)10

b)25

c)200

d)250

7)Given the equilibrium H2O(s) ↔ H2O(l) at a pressure of 1 atmosphere. The temperature of the ice-water mixture must be

a)0 °C

b)32 °C

c)110 °C

d)212 °C

8)How many calories of heat energy are released when 50 grams of water are cooled from 70 °C to 60 °C?

a)10 calories

b)50 calories

c)500 calories

d)1,000 calories

9)Which of the following best describes exothermic chemical reactions?

a)They never release heat

b)They always release heat

c)They never occur spontaneously

d)They always occur spontaneously

10)The number of calories needed to raise the temperature of 10 grams of water from 20 °C to 40 °C is

a)10

b)20

c)100

d)200

Questions 11-14 refer to the following enthalpies

A)Hf

B)Hfusion

C)Hcombustion

D)Hvaporization

E)Hneutralization

11Equals zero for an element in its standard state.

12CH4(g) + 3 O2(g)  CO2(g) + 2 H2O(g)

13H2O(l)  H2O(g)

14 N2(g) + H2(g)  NH3(g) at 25C and 1 atm

Directions: Each group of questions or incomplete statements below is followed by a five suggested answers or completions. Select the one that is best in each case and fill in the corresponding letter on the answer sheet.

15Which of the following statements are correct?

I)A superscript of zero, such as H, indicates a specified temperature of 0C.

II)For an element the Standard Enthalpy of Formation is 0 kJ/mol

III)For a pure gas, the standard state is the gas at a pressure of one atmosphere

.

A)I only

B)II only

C)III only

D)II and III only

E)I, II and III

  1. Given the following standard entropy, S,determine the entropy change, Srxn, for the following reaction.

Cu2O (s) + H2(g) 2 Cu(s) + H2O(g)

Substance / S()
Cu2O (s) / 93
H2(g) / 131
Cu(s) / 33
H2O(g) / 189
  1. -88
  2. -31
  3. -5
  4. 5
  5. 31
  1. An acid dissolves in water and the solution becomes quite hot. Which of the following are true about the dissolution of the acid?

I)The process is exothermic.

II)The solubility will be greater in warmer water.

III)H for the reaction is positive.

A)I only

B)II only

C)III only

D)II and III only

E)I, II and III

  1. For which of these is Hfonot equal to zero?
  2. Br2(l)
  3. Fe(s)
  4. I2(s)
  5. O3(g)
  6. Given the following experimental data

Mg(s) + 2 HCl(aq)  MgCl2(aq) + H2(g)H= -328

MgO(s) + 2 HCl(aq)  MgCl2(aq) + H2O(l)H= -45.0

2H2(g) + O2(g)  2 H2O(l)H= -572

determine the enthalpy change for the following reaction.

Mg(s) + O2(g)  MgO(s)

  1. -945
  2. -546
  3. 289
  4. 569
  5. 945
  1. Given the following standard energies of formation, Hf, and the enthalpy of the reaction, Hrxn, for the following reaction, determine the appropriate standard enthalpy of formation for the ethane gase, C2H4.

C2H4(g) + 3 O2(g) 2 CO2(g) + 2 H2O(l)Hrxn= -1411

Substance / Hf()
C2H4(g) / ???
CO2(g) / -394
H2O(l) / -286
  1. -2800
  2. -2100
  3. -50
  4. 50
  5. 2100

Each free response problem is worth 10 points. Show your work for partial credit.

  1. 58.00g of a metal alloy is heated to 250°C then cooled by pouring into 200g of water at 25°C. When equilibrium is established both the metal and the water are at 65°C. What is the Specific heat capacity for the metal alloy?
  2. Given the thermochemical equations:

Br2(l) + F2(g) 2BrF(g)H° = -188 kJ

Br2(l) + 3F2(g)  2BrF3(g)H° = -768 kJ

determine H° for the reaction

BrF(g) + F2(g)  BrF3(g)H° = ?

  1. When 2.74 g of Ba(s) reacts with O2(g) at 298 K and 1 atm to form BaO(s), 11,100 J of heat is released. What is ΔHf° for BaO(s) in kJ·mol–1?
  1. H°f (kJ/mol)

Cr3+(aq) -143

Ni2+(aq) -54

For the reaction shown, which is closest to the value of H?

2Cr3+(aq) + 3Ni(s) 2Cr(s) + 3Ni2+(aq)