Molar Enthalpy and Calorimetry

1. Molar Enthalpy and Calorimetry SCH4U0

2. Calorimetry • Calorimetry is a technique used by chemists to measure the energy changes in a chemical system. • Temperature changes are used to calculate the quantity of heat (q) exchanged between the system and its surroundings. • The temperature change is dependent on the amount of heat exchanged, the mass of the substance being measured, and the type of substance being measured.

3. Specific Heat Capacity • The heat absorbed/released by a substance can be defined by the following equation • q = quantity of heat transferred to the surroundings • m = the mass of the substance being measured • c = the specific heat capacity of the substance being measured • ΔT = The temperature change of the substance

4. Specific Heat Capacity • Specific Heat Capacity: The amount of energy (in joules) required to raise the temperature of 1g of a substance by 1°C. • Every substance has its own unique specific heat capacity. Eg: water (liquid) = 4.19 J/g°C water (solid) = 2.01 J/g°C Aluminum = 0.900 J/g°C Iron = 0.444 J/g°C

5. Heat Capacity • In some cases we lump the mass in with the specific heat capacity to make the heat capacity • Heat capacity (c) is the heat required to raise the temperature of a specific object by 1°C • We only do this for specific objects that aren`t changing in mass and are used in calorimetry experiments (eg: the case in which we conduct the experiment) • The equation then becomes;

6. Example Homework = pg 281 #1-10 • If the same amount of heat were added to 1 g samples of water, methanol, and aluminum, which substance would undergo the greatest temperature change? c(water) = 4.19 J/g°C c(methanol) = 2.918 J/g°C c(aluminum) = 0.900 J/g°C

7. Example • When 600 mL of water in an electric kettle is heated from 20°C to 85°C to make a cup of tea, how much heat flows into the water? q = ? m = c = ΔT = 600 g 4.19 J/g°C 85°C - 20°C = 65°C

8. Practice • What will be the final temperature of 10.0 g of methanol if 250.0 J of energy were transferred into it? The initial temperature of the methanol is 20.0°C. The specific heat capacity of methanol is 2.918 J/g°C

9. Calorimetry • Calorimetry experiments are often used to determine the enthalpy change of a reaction • The heat released or absorbed by a system undergoing a chemical reaction is the same as the heat absorbed or released by the surroundings

10. Calorimetry Example • What is the enthalpy of solution when ionic solid X(s) is dissolved in water if 1.0 mol of X(s) is dissolved in 250 mL of water, raising the water temperature from 24.0ºC to 35.0ºC?

11. Calorimetry Practice • What is the enthalpy of solution when ionic solid J(s) is dissolved in water if 1.0 mol of J(s) is dissolved in 350 mL of water, lowering the water temperature from 23.5ºC to 4.0ºC?

12. Molar Enthalpy • Enthalpy changes are dependent on the quantity of material involved in the change (more reactants = more energy released in an exothermic reaction). • We use molar enthalpies to describe the enthalpy change in a reaction (or phase change) per mol of reactant. • Molar Enthalpy (ΔH): The enthalpy change per 1.0 mol of the substance undergoing change. Units = kJ/mol

13. Molar Enthalpy Or

14. Molar Enthalpy • In phase changes the potential energy of the molecules does not change, and neither does the temperature. • The enthalpy change results only from the breaking and forming of intermolecular forces.

15. Molar Enthalpy • What is the enthalpy change when 50.0 g of ice is converted at 0°C to water at 0°C? • Since we know the molar enthalpy of fusion, find the moles of water:

16. Calorimetry w/ Molar ΔH • What is the molar enthalpy of solution of potassium chloride if dissolving 7.46g of KCl in 100.0mL of water lowers the temperature of the water from 24.1°C to 20.0°C? The heat lost by the water equals the heat gained by the KCl First, find moles of KCl

17. Calorimetry w/ Molar ΔH

18. Homework • Molar Enthalpy pg 299 #11 – 20 • Calorimetry pg 305 #21 – 30 • Heat Exchange pg 309 #31 – 40