5. Energetics and thermochemistry SL

1.What energy changes occur when chemical bonds are formed and broken?

2. The temperature of a 2.0 g sample of aluminium increases from 25°C to 30°C. How many joules of heat energy were added? (Specific heat of Al = 0.90 J g–1K–1)

3. Using the equations below: C(s) + O2(g) → CO2(g)    ∆H = –390 kJ Mn(s) + O2(g) → MnO2(s)     ∆H = –520 kJ what is ∆H (in kJ) for the following reaction? MnO2(s) + C(s) → Mn(s) + CO2(g)

4. Which statements about exothermic reactions are correct? I. They have negative ΔH values. II. The products have a lower enthalpy than the reactants. III.    The products are more energetically stable than the reactants.

5. A sample of a metal is heated. Which of the following are needed to calculate the heat absorbed by the sample? I. The mass of the sample II. The density of the sample III.    The specific heat capacity of the sample

6. The average bond enthalpies for O—O and O==O are 146 and 496 kJ mol–1 respectively. What is the enthalpy change, in kJ, for the reaction below? H—O—O—H(g) → H—O—H(g) + ½O==O(g)

7. When the solids Ba(OH)2 and NH4SCN are mixed, a solution is produced and the temperature drops. Ba(OH)2(s) + 2NH4SCN(s) → Ba(SCN)2(aq) + 2NH3(g) + 2H2O(l) Which statement about the energetics of this reaction is correct?

8. Using the equations below Cu(s) + O2(g) → CuO(s)∆Hο = –156 kJ 2Cu(s) + O2(g) → Cu2O(s)∆Hο = –170 kJ what is the value of ∆Hο (in kJ) for the following reaction? 2CuO(s) → Cu2O(s) + O2(g)

9. Which of the quantities in the enthalpy level diagram below is (are) affected by the use of a catalyst?

10. Consider the following equations. Mg(s) + O2(g) → MgO(s)    ∆Hο = –602 kJ H2(g) + O2(g) → H2O(g)      ∆Hο = –242 kJ   What is the ∆H° value (in kJ) for the following reaction? MgO(s) + H2(g) → Mg(s) + H2O(g)

11. For which of the following is the sign of the enthalpy change different from the other three?

12. Separate solutions of HCl(aq) and H2SO4(aq) of the same concentration and same volume were completely neutralized by NaOH(aq). X kJ and Y kJ of heat were evolved respectively. Which statement is correct?

13. Which statements are correct for an endothermic reaction? I. The system absorbs heat. II. The enthalpy change is positive. III.    The bond enthalpy total for the reactants is greater than for the products.

14. The mass m (in g) of a substance of specific heat capacity c (in J g–1 K–1 ) increases by t°C. What is the heat change in J?

15. The average bond enthalpy for the C―H bond is 412 kJ mol–1. Which process has an enthalpy change closest to this value?

16. The following equation shows the formation of magnesium oxide from magnesium metal. 2Mg(s) + O2(g)→2MgO(s)    ΔHӨ = –1204kJ Which statement is correct for this reaction?

17. The following equations show the oxidation of carbon and carbon monoxide to carbon dioxide. C(s) +O2(g) → CO2(g)    ΔHӨ = –x kJ mol–1 CO(g) + O2(g)→ CO2(g)    ΔHӨ = –y kJ mol–l What is the enthalpy change, in kJ mol–1, for the oxidation of carbon to carbon monoxide? C(s) + O2(g)→ CO(g)

18. A simple calorimeter was used to determine the enthalpy of combustion of ethanol. The experimental value obtained was –920 kJ mol–1. The Data Booklet value is –1371 kJ mol–1. Which of the following best explains the difference between the two values?

19. For the reaction 2H2(g) + O2(g) → 2H2O(g) the bond enthalpies (in kJ mol–1) are

H–H	x
O=O	y
O–H	z

Which calculation will give the value, in kJ mol–1, of ΔHӨ for the reaction?

20. Which statement about bond enthalpies is correct?

21. An equation for a reaction in which hydrogen is formed is CH4 + H2O → 3H2 + CO    ΔHӨ = +210 kJ Which energy change occurs when 1 mol of hydrogen is formed in this reaction?

22. The equations and enthalpy changes for two reactions used in the manufacture of sulfuric acid are: S(s) O2(g) → SO2(g)    ΔHӨ = –300 kJ 2SO2(g) + O2(g) → 2SO3(g)    ΔHӨ = –200 kJ What is the enthalpy change, in kJ, for the reaction below? 2S(s) + 3O2(g) → 2SO3(g)

23. Approximate values of the average bond enthalpies, in kJ mol–1, of three substances are:

H–H	430
F–F	155
H–F	565

What is the enthalpy change, in kJ, for this reaction? 2HF → H2 + F2

24. The standard enthalpy change of formation values of two oxides of phosphorus are: P4(s) + 3O2(g) → P4O6(s)    ΔHӨf= –1600 kJ mol–1 P4(s) + 5O2(g) → P4O10(s)    ΔHӨf= –3000 kJ mol–1 What is the enthalpy change, in kJ mol–1, for the reaction below? P4O6(s) + 2O2(g) → P4O10(s)

25. Which statement is correct for an endothermic reaction?

26. Which statement is correct about the reaction shown? 2SO2(g) + O2(g) → 2SO3(g)    ΔH = –196 kJ

27. Which statements are correct for all exothermic reactions? I. The enthalpy of the products is less than the enthalpy of the reactants. II. The sign of ΔH is negative. III.    The reaction is rapid at room temperature.

28. Consider the specific heat capacity of the following metals.

Metal	Specific heat capacity / J kg–1 K–1
Cu	385
Ag	234
Au	130
Pt	134

Which metal will show the greatest temperature increase if 50 J of heat is supplied to a 0.001 kg sample of each metal at the same initial temperature?

29. Consider the following reactions. S(s) + O2(g) → SO3(g)    ΔHӨ = −395 kJ mol−1 SO2(s) + O2(g) → SO3(g)    ΔHӨ = −98 kJ mol−1 What is the ΔHӨ value (in kJ mol–1) for the following reaction? S(s) + O2(g) → SO2(g)

30. Which statement is correct for an endothermic reaction?

31. According to the enthalpy level diagram below, what is the sign for ΔH and what term is used to refer to the reaction?  

	ΔH	reaction
A.	positive	endothermic
B.	negative	exothermic
C.	positive	exothermic
D.	negative	endothermic

32. When 40 joules of heat are added to a sample of solid H2O at –16.0°C the temperature increases to –8.0°C. What is the mass of the solid H2O sample? [Specific heat capacity of H2O(s) = 2.0 J g–1K–1]

33. The ΔHӨ values for the formation of two oxides of nitrogen are given below. N2(g) + O2(g) → NO2(g)    ΔHӨ = –57 kJ mol–1 N2(g) + 2O2(g) → N2O4(g)    ΔHӨ = +9 kJ mol–1 Use these values to calculate ΔHӨ for the following reaction (in kJ): 2NO2(g) → N2O4(g)

34. How much energy, in joules, is required to increase the temperature of 2.0 g of aluminium from 25 to 30°C? (Specific heat of Al = 0.90 J g–1 K–1).

35. Which combination is correct for a chemical reaction that absorbs heat from the surroundings?

	Type of reaction	ΔH at constant pressure
A.	Exothermic	Positive
B.	Exothermic	Negative
C.	Endothermic	Positive
D.	Endothermic	Negative

36. Using the equations below: C(s) + O2(g) → CO2(g)    ∆Hο = –394 kJ mol–1 Mn(s) + O2(g) → MnO2(s)    ∆Hο = –520 kJ mol–1 What is ∆H, in kJ, for the following reaction? MnO2(s) + C(s) → Mn(s) + CO2(g)