Energetics (Triple) quiz

1. Assuming bond energies in kJ/mol: H-C 412, C-C 348, O-H 463, C-O 360, C=C 612. Calculate the molar enthalpy change for the reaction: ethanol → ethene + water

+45 kJ/mol (Some workings as follows. Breaking bonds 3231 kJ/mol. Making bonds 3186kJ/mol. Breaking - making = 3231-3186 = +45 kJ/mol)

-45 kJ/mol (Some workings as follows. Breaking bonds 1583 kJ/mol. Making bonds 1487/mol. Making - breaking = 1487-1583 = -96 kJ/mol)

-45 kJ/mol (Some workings as follows. Breaking bonds 3231 kJ/mol. Making bonds 3186kJ/mol. Making - breaking = 3186-3231 = -45 kJ/mol)

+96 kJ/mol (Some workings as follows. Breaking bonds 1583 kJ/mol. Making bonds 1487/mol. Breaking - making = 1583-1487 = +96 kJ/mol)

Question 1 of 16

2. In a combustion calorimetry experiment, 0.78g of ethanol (C₂H₅OH) produced 12,540 J of heat energy. Calculate the molar enthalpy change.

-738 kJ/mol (Amount = 0.78/46 = 0.017 mol. Answer = 12540/1000/0.017 = 738 kJ/mol)

-425 kJ/mol (Amount = 23/0.78 = 29.5 mol. Answer = 12540/29.5 = 425 kJ/mol)

-213 kJ/mol (Amount = 46/0.78 = 59.0 mol. Answer = 12540/59.0 = 213 kJ/mol)

-369 kJ/mol (Amount = 0.78/23 = 0.034 mol. Answer = 12540/1000/0.034 = 369 kJ/mol)

Question 2 of 16

3. In a chemical reaction, 250 kJ/mol is taken to break all the bonds and 280 kJ/mol is released when new bonds are formed. What is the overall molar enthalpy of the reaction?

-530 kJ/mol

-30 kJ/mol

+30 kJ/mol

+530 kJ/mol

Question 3 of 16

4. Are combustion reactions exothermic or endothermic?

Depends on the temperature of the surroundings

Depends on the reagents

Exothermic

Endothermic

Question 4 of 16

5. In a calorimetry experiment to investigate the heat energy released by the combustion of ethanol, why should the water in the calorimeter be stirred?

To allow the heat to spread evenly through the copper container

To ensure the container does not get too hot

To ensure the heat energy is mixed evenly throughout all the water

To prevent the water from evaporating

Question 5 of 16

6. What is meant by the term exothermic?

A reaction which will only be spontaneous at warm temperatures (above 100⁰C)

A reaction which will only be spontaneous at cold temperatures (below 0⁰C)

A reaction in which heat energy is taken in (surroundings get colder)

A reaction in which heat energy is given out (surroundings get hotter)

Question 6 of 16

7. Explain, in terms of making and breaking bonds, why a reaction could be exothermic

The energy needed to break the bonds is less than the energy released to make the bonds

Breaking the bonds is less favourable than making them

The energy needed to break the bonds is more than the energy released to make the bonds

The bonds in the reagents are stronger than the bonds in the products

Question 7 of 16

8. State the units of molar enthalpy change.

J/⁰C/mol

mol/⁰C

kJ/mol

kJ/⁰C

Question 8 of 16

9. What does this diagram represent?

Reaction profile for an endothermic reaction

Reaction profile for an exothermic reaction

Energy level diagram for an endothermic reaction

Energy level diagram for an exothermic reaction

Question 9 of 16

10. Explain, in terms of making and breaking bonds, why a reaction could be endothermic

Breaking the bonds is more favourable than making them

The energy needed to break the bonds is more than the energy released to make the bonds

The energy needed to break the bonds is less than the energy released to make the bonds

The bonds in the reagents are weaker than the bonds in the products

Question 10 of 16

11. Explain why experimental values of enthalpy change differ from theoretical values

Different equipment produces different results. Not all the reactants are used up

Different equipment produces different results. Heat energy is lost to the surroundings. Not all the reactants are used up

Heat energy is lost to the surroundings. Not all the reactants are used up.

Different equipment produces different results. Heat energy is lost to the surroundings

Question 11 of 16

12. What is meant by the term endothermic?

A reaction which will only be spontaneous at warm temperatures (above 100⁰C)

A reaction in which heat energy is taken in (surroundings get colder)

A reaction which will only be spontaneous at cold temperatures (below 0⁰C)

A reaction in which heat energy is given out (surroundings get hotter)

Question 12 of 16

13. What does the symbol ΔH mean

exothermic

heat energy

enthalpy (energy) change

specific heat capacity

Question 13 of 16

14. Are displacement reactions exothermic or endothermic?

Endothermic

Depends on the reagents

Exothermic

Depends on the temperature of the surroundings

Question 14 of 16

15. Use Q=mcΔT and c=4.18J/°C/g. A spatula of sodium fluoride is put into a boiling tube with 25cm³ of water. The temperature drops from 22.0°C to 18.6°C. What is the total heat energy change?

1944 kJ

355 kJ/mol

1944 J

355 J

Question 15 of 16

16. Use Q=mcΔT and c=4.18J/°C/g. A strip of magnesium is added to a beaker with 200cm³ of copper (II) sulfate. The temperature starts at 21.7°C and rises to a maximum of 23.1°C. What is the total heat energy change?

2.153 kJ

1170 J

1247 J

1944 kJ

Question 16 of 16

Energetics (Triple) quiz

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