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Energy, work and heat All processes in nature involve energy Learning objectives • Apply units of energy to simple calculations of energy changes in chemical processes • Describe the main sources of energy Energy is capacity to do work • • • • Work is applying force over distance Energy comes in different forms Kinetic energy is energy due to motion Potential energy is energy due to position or state – Height – Chemical – Electrical Energy changes form • Processes convert energy from one form to another – Falling down stairs • (potential → kinetic → pain) – Chemical reaction • (potential → heat/light) – Battery • (potential → electrical) – Car engine • (potential →heat→ kinetic) But it never goes away • Energy is conserved in any process – None is lost – None is gained – But it goes from one place to another • Law of Conservation of Energy: Energy is neither created nor destroyed in a chemical reaction • Also known as the First Law of Thermodynamic Measuring energy: calories are case sensitive • calorie is the energy required to raise temperature of 1 g of water 1 degree C • Calorie is the food version = 1,000 cal – Raises temperature of 1 pint of water 3.8ºF • Joule is SI unit derived from mechanical work: the work done when a force of 1 newton is applied for 1 meter 1 cal = 4.18 J What’s watt? • Watts measure the rate of delivery of energy or power 1 W = 1 J/s How many Mars Bars to power a 100 W bulb for one minute? • – – 1 min = 60 s x 100 W = 6 kJ 6 kJ = 1.4 kcal = 1.4 Cal (just a nibble) Measurements of energy use Common energy conversions • Octane is burnt to produce 8500 J. How much is that in calories? • 1 cal = 4.18 J 1cal cals 8500Jx 2030cal 4.18J Power consumption • An air conditioner is rated at 1,500 W. How many kWh are used per month if it operates 6 h per day? • What is cost at $0.15 per kWh? 1kW 30days 6h kWh 1500Wx x 270kWh 1000W 1mo 1day $0.15 cos t 270kWhx $40.5 1kWh Enthalpy and chemical reactions • Enthalpy of reaction (ΔH) measures heat of the reaction CH4 + 2O2 = CO2 + 2H2O ΔH = -11.8 kcal/g Enthalpy of combustion example What is energy obtained from burning 10 kg of coal. Coal has energy density of -28kJ/g 1000g 28kJ joules 10kgx x 280000kJ 1kg g The sun as our energy source: directly and indirectly • Indirect: – Solar radiation provided energy for fossil fuels – Heats the air (wind power) – Evaporation of water (hydro power) • Direct: – Solar panels – Photovoltaic cells • Nonsolar: – Nuclear – Geothermal Energy sources • 85 % comes from fossil fuels • Fossil fuels are hydrocarbons – Petroleum – Coal – Gas • 15 % is everything else – Nuclear (8) – Hydro (3) – Renewables (3) Electrical generation • More than 70 % comes from fossil fuels • Heat of combustion boils water • Steam turns a turbine • Turbine generates electricity Finitude • Gambling on the “Other”: how to turn 3 % into 85 % without hurting anybody