5.2 Calorimetry and Enthalpy

Measuring Energy Changes Calorimetry is the experimental process used to measure the transfer of thermal energy. – A bomb calorimeter is a closed system in which a chemical reaction occurs, usually combustion. – Often used to determine the energy associated with food. 2

Bomb Calorimeter 3

SMCHS Reaction Calorimeter MIT Reaction Calorimeter 4

Measuring Energy Changes The heat lost or gained (Q) in a system is related to: – The mass of the sample (m) – The change in temperature in that sample (  T= T 2 – T 1 ) – The material’s capacity to absorb energy. The specific heat capacity (c), the quantity of energy required to change the temperature of one gram of matter by one Kelvin (or Celsius degree) Refer to tables for these values. (Pg 292 Table 1) Q = mc  T 5

Calculating Energy Changes A 250 mL sample of water has a temperature of 20.0 o C when it is placed in the freezer. After 15 minutes the temperature of the water is 17.5 o C. Determine the loss of energy (heat) to the surroundings. 6

Calorimetry Calculation Assumptions Any thermal energy transferred form the calorimeter to the outside environment is negligible Any thermal energy absorbed by the calorimeter itself is negligible All dilute, aqueous solutions have the same density (1.00g/ml) and specific heat capacity (4.18 J/(g°C) as water

Terminologies Enthalpy (H) – the total amount of energy in a substance due to nuclear, chemical and physical sources of energy. – This is a theoretical value that can not be experimentally determined. Heat (q or Q) – the amount of thermal energy that is transferred between substances. (i.e. – system  surroundings) Q=mc  T Enthalpy change (  H) – the difference in enthalpies of reactants and products during a system change. (physical, chemical, nuclear)  H system  Q system  – This value can be determined through calorimetry experiments. 8

Molar Enthalpy The molar enthalpy, ∆H x, is the enthalpy change associated with a physical, chemical or nuclear change involving one mole of a substance. ∆H=n∆H x - subscript x indicates the kind of change that is occurring - n denotes the number of moles - most reactions, involve more or less than one mole and we can use the following formula to calculate the molar enthalpy of the reaction

Quick.., what’s a mole? A mole is defined as the quantity of a substance, which contains the same number of chemical units, as there are atoms in exactly 12g of carbon-12. It is the mass of any substance, which contains Avogadro’s number (6.023 × ) of chemical units of the substance.

Cruisin’ down the mole highway!

Ex. 1 How many grams are in 2.0 moles of HCl?

Using Molar Enthalpies in Heat Calculations ∆ H=n∆H x Remember Exothermic reactions (feel hot) are given by a negative sign. Endothermic reactions (feel cold) are given by positive sign

Ex. 1. What amount of methanol would vaporize while absorbing 120.0kJ of heat?

9.0 grams of charcoal (C) were completely consumed in a bomb calorimeter. If we assume that the 2.0 L of water absorbed all of the heat released by the charcoal, and if the temperature of the water increased from o C to o C, what is the molar enthalpy of carbon? Is the reaction exothermic or endothermic?

Ways of Representing Change