calorimetry - measurement of heat flow
- calorimeter - device used to measure heat flow
- heat capacity - amount of heat needed to raise temperature by 1°C
- molar heat capacity - heat capacity of 1 mol of a substance
- specific heat - heat capacity of 1g of a substance
- specific heat = quantity of heat transferred / (grams of substance x temperature change)
- = q / (m x DT)
- q = specific heat x grams of substance x DT
Find amount of heat needed to warm 250 g of water from 22°C to 98°C
- Given:
- specific heat of water = 4.18 J/g-K
- change in K same as change in °C
- q = specific heat of water x grams of water x DT
- = 4.18 J/g-K x 250g x 76K
- = 7.9 x 104 J
constant-pressure calorimetry - heat gained by solution same as heat lost by reaction
- qsoln = specific heat of solution x grams of solution x DT = -qrxn
Find enthalpy change for reaction if temperature of solution in calorimeter changed from 21.0°C to 27.5°C when 50mL of 1.0M HCl was mixed with 50mL of 1.0M NaOH
- Given:
- total volume of solution = 100 mL
- density of solution = 1.0g/mL
- specific heat of solution = 4.18 J/g-K
- mass of solution = 100mL x 1.0g/mL = 100g
- temperature change = 27.5 - 21.0 = 6.5°C = 6.5K
- qrxn = -(4.18 J/g-K)(100g)(6.5K) = -2.7 x 103 J = -2.7 kJ
- 50mL x 1M = (0.050L)(1.0 mol/L) = 0.050 mol HCl or NaOH in solution
- enthalpy change per mol = -2.7 kJ / 0.050 mol = -54 kJ/mol
bomb calorimetry (constant-volume calorimetry) - studies combustion reactions
- compound reacts w/ excess oxygen
- combustion started by electrical spark
- qrxn = -Ccal x DT
- Ccal = heat capacity of calorimeter
Find heat of reaction for comubstion of 4.00g of CH6N2 if its combustion in a calorimeter w/ heat capacity of 7.794 kJ/ °C changes temperature from 25.00°C to 39.50°C
- qrxn = -Ccal x DT
- DT = 39.50 - 25.00 = 14.50°C
- Ccal = 7.794 kJ/°C
- qrxn = -(7.794 kJ/°C)(14.50°C) = -113.0 kJ
- heat of reaction per mol = -113.0 kJ / 4.00g CH6N2 x 46.1g CH6N2 / 1 mol CH6N2 = -1.30 x 103 kJ/mol CH6N2
fuel value - energy released when 1g of material is combusted
- proteins/carbohydrates produce about 17kJ/g
- fats produce 38kJ/g
- fossil fuels - formed from decomposition of plants/animals
- natural gas - gaseous hydrocarbons
- petroleum - liquid containing mostly hydrocarbons
- coal - solid w/ hydrocarbons of high molecular weight; most abundant fossil fuel
- syngas - "synthesis gas"; coal undergoes coal gasification to become mixture of methane, hydrogen, carbon dioxide gas
- nuclear energy - energy released through splitting/fusion of atom nuclei
- renewable energy - essentially inexaustible energy
- solar energy from sun w/ solar cells (photovoltaic devices)
- wind energy from windmills
- geothermal energy from heat in Earth's mass
- hydroelectric energy from flowing rivers
- biomass energy from crops, biological waste
