Thermochemistry PDF: Heat Capacity, Calorimetry Practice Questions

Heat Capacity, C Calorimetry is the measurement of heat changes. The apparatus used is called calorimeter. There are different types of calorimeter. The simplest one would be the coffee cup calorimeter as shown Figure 1.5. Other calorimeter includes bomb calorimeter as in Figure 1.6 and differentia...

Heat Capacity, C Calorimetry is the measurement of heat changes. The apparatus used is called calorimeter. There are different types of calorimeter. The simplest one would be the coffee cup calorimeter as shown Figure 1.5. Other calorimeter includes bomb calorimeter as in Figure 1.6 and differential scanning calorimeter (DSC). Figure 1.5: Coffee cup calorimeter.  28 | Physical and Organic Chemistry Figure 1.6: Bomb calorimeter. IMPORTANT TERMS IN CALORIMETRY Heat (q) is thermal energy changes that is measured using a thermometer. Specific heat capacity (c) is the amount of heat required to raise the temperature of 1 g of the substance by 1oC (unit: J g–1 oC–1 or Jg–1 K–1). Heat capacity (C) is the amount of heat required to raise the temperature of a given quantity of the substance by 1oC (unit: J oC─1 or J K─1). The basic principle in calorimeter is, Heat released by a reaction = Heat absorbed by surroundings Surroundings may refer to the  calorimeter itself or;  the water and calorimeter. There are three equations that can be used here: q = mc∆T q = C∆T q= mc∆T + C∆T where q = heat released by substance m = mass of substance ∆T= temperature change c = specific heat capacity C = heat capacity of a calorimeter Physical and Organic Chemistry 7 x 10.indb 28 23/01/2021 10:08 AM  Thermochemistry | 29 The calorimeter works on the principle: Heat lost by one object will be gained by another. [Heat lost = Heat gained] EXAMPLE 1.5 25.0 cm3 hydrochloric acid, HCl 1.0 M and 25.0 cm3 sodium hydroxide aqueous solution, NaOH 1.0 M, both at initial temperature of 30.0°C are mixed in a calorimeter. The maximum temperature of the solution is observed to be 36.9°C. Given the specific heat capacity of water is 4.184 Jg–1 °C–1. (a) Write the balance equation for the reaction above. (b) Calculate the number of mol of acid and base used. (c) Determine the enthalpy of neutralisation for the reaction. PRACTICE 1.2 1. In an experiment, 0.100 g of H2 were compressed into a 1.00 L bomb, which then placed into a calorimeter that has a heat capacity of 9.08 × 104 J oC─1. The initial and final temperature of the calorimeter is 25.0oC and 25.15oC, respectively. Calculate the enthalpy of combustion for H2. 2. A sample of 0.14 g magnesium, Mg metal was combusted in a bomb calorimeter containing 3.0 × 102 g of water at 25.0°C. The maximum temperature recorded was 26.0°C. If the heat capacity of the calorimeter is 1769 J oC─1 and the specific heat capacity of water is 4.184 J g─1 oC─1, a) calculate the enthalpy of combustion of magnesium. b) write the thermochemical equation for the reaction above. 3. A student mixes 200.0 mL of 0.20 M RbOH(aq) with 100.0 mL of 0.40 M HBr(aq) in a coffee cup calorimeter. If the temperature of each of the two solutions was 24.40°C before mixing, and the temperature raised to 26.18°C, what is the ∆H associated with the thermochemical equation? Write the thermochemical equation. (1. –285 kJ mol─1, 2. –524.92 kJ mol─1, 3. –55.86 kJ mol─1) Physical and Organic Chemistry 7 x 10.indb 29 23/01/2021 10:08 AM