CHE114: TUT 5 (2021)
1.
The equation for the combustion of butane, C4H10, is:
2 C4H10 (g) + 13 O2 (g) → 8 CO2 (g) + 10 H2O (g)
Which one of the following situations generates the smallest amount of heat? Motivate
your answer.
2.
3.
a)
Burning one mole of butane.
b)
Reacting one mole of oxygen with excess butane.
c)
Producing one mole of carbon dioxide by burning butane.
d)
Producing one mole of water by burning butane.
The process of dissolving ammonium nitrate, NH4NO3, in water is an endothermic process.
a)
What is the sign of q?
b)
If you were to add some ammonium nitrate to water in a flask, would you expect the
flask to feel warm or cool?
c)
Commercial cold packs consist of solid ammonium nitrate and water. NH4NO3
absorbs 3,30 x 102 J of heat per gram dissolved in water. In a coffee-cup calorimeter,
4,40 g NH4NO3 is dissolved in 100,0 g of water at 21,0 °C. What is the final
temperature of the solution? Assume that the solution has a specific heat capacity of
4,18 J/g·K.
Hydrogen cyanide is a highly poisonous, volatile liquid. One way in which it can be
prepared is by the reaction:
CH4 (g) + NH3 (g) → HCN (g) + 3 H2 (g)
Use Hess’ law and the following thermochemical equations to find the heat of the above
reaction at constant pressure.
N2 (g) + 3 H2 (g) → 2 NH3 (g); ∆H = –91,8 kJ
C (graphite) + 2 H2 (g) → CH4 (g); ∆H = –74,9 kJ
H2 (g) + 2 C (graphite) + N2 (g) → 2 HCN (g); ∆H = 270,3 kJ
4.
Suppose two 45-g ice cubes are added to a glass containing 500 cm3 of cola at 20,0°C.
When thermal equilibrium is reached, all the ice will have melted, and the temperature of
the mixture will be somewhere between 20,0°C and 0°C. Calculate the final temperature
of the mixture. You may assume that the cola has a density and specific heat capacity the
same as water namely:
Density = 1,0 g/cm3 and specific heat = 4,184 J/gK
The heat of fusion (of water) is 333J/g.
[Hint: You should take into account that the 90g of water formed when the ice melts, must
be warmed to the final temperature.]
5.
Suppose four 30-g ice cubes are added to a glass containing 200 cm3 of orange juice at
20,0°C.
You may assume that the orange juice has a density and specific heat capacity the same
as water namely: Density = 1,0 g∙cm–3 and specific heat = 4,184 J/gK
The heat of fusion (of water) is 333 J/g.
Your task is twofold. You must determine:
6.
a)
Whether or not all the ice will melt; and
b)
If your finding is that some ice will be left in the juice, you must calculate what mass
of ice will be present the moment when thermal equilibrium is reached.
What is the enthalpy of reaction, ΔH, for the formation of tungsten carbide, WC, from its
elements?
W (s) + C (graphite) → WC (s)
The enthalpy change for this reaction is difficult to measure directly, because the reaction
occurs at a very high temperature. However, the reaction enthalpies of the following
reactions are known:
2 W (s) + 3 O2 (g) → 2 WO3 (s); ΔH = – 1685,8 kJ
C (graphite) + O2 (g) → CO2 (g); ΔH = – 393,5 kJ
2 WC (s) + 5 O2 (g) → 2 WO3 (s) + 2 CO2 (g); ΔH = – 2391,8 kJ
7.
Calculate ∆H° for the following combustion reaction from the data given in the table below.
SiH4 (g) + 3 O2 (g) → SiO2 (s) + 2 H2O ()
Compound
∆H°f (kJ∙mol–1)
SiH4 (g)
34,3
SiO2 (s)
– 910,9
H2O ()
– 285,8