Energy, work and heat

1. Energy, work and heat All processes in nature involve energy

2. Learning objectives • Define heat and energy and differentiate among different types of energy • Describe main uses of energy nationally and globally • Apply units of energy to simple calculations of energy changes in chemical processes • Define heat capacity and use in calculations of energy consumption • Describe exothermic and endothermic reactions • Describe principle of entropy and the “heat tax” and the limitations of heat engines • Describe the main sources of energy • Describe basic principles of the Greenhouse effect and global warming

3. Heat and chemical reactions • The burning candle is a chemical reaction • It releases energy which heats up the air molecules • Heat is the flow of energy due to a temperature difference

4. Energy from chemical reactions performs work • In what way is a human like a car? • It’s unreliable • Both burn fuel to run • Humans burn glucose • C6H12O6 + O2 = CO2 + H2O + energy • Cars burn petrol • C8H18 + O2 = CO2 + H2O + energy

5. Where would we be without it?

6. We like to use it

7. Energy is capacity to do work • Work is done in different ways • Kinetic energy is energy due to motion • Potential energy is energy due to position or state • Height • Chemical • Electrical

8. Energy is mercurial • Processes convert energy from one form to another • Falling down stairs • (potential → kinetic → pain) • Chemical reaction • (potential → heat/light) • Battery • (potential → electrical) • Engine • (potential →heat→ kinetic)

9. 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 Thermodynamics • System and Surroundings • The process under study is the system • Everything else is the surroundings

10. The First Law says that “perpetual motion” can’t be • If a friend asks you to invest in his new free energy machine – don’t • Visit the museum of unworkable machines for a history of conmen and futile energy ideas

11. Heat and work • The industrial revolution was founded on conversion of heat into mechanical motion • Joule (1843) was first to recognize connection • Heat: energy in transit – molecular motion • Calorie is energy required to raise temperature of 1 g water by 1ºC • Work: force applied over a distance • Joule is a force of 1 Newton applied for 1 meter 1 cal = 4.18 J

12. Heat engines • Newcomen steam engine invented 1712 • Watt improves design 1760 • Carnot (1820) described the operation of heat engines in abstract terms – the Carnot cycle: the foundation of modern thermodynamics • All engines based on burning fuels are heat engines

13. Rules of the road • First law: energy is conserved • Is that all there is to it? • Heat engines convert heat into mechanical work • Question: Can all heat be converted into work with 100 % efficiency? • First Law says yes • Second Law says No!

14. Limitations on heat engines: the Carnot cycle and entropy • Limit on efficiency of a heat engine is ruled by temperature difference • Entropy and energy dispersal • The second law of thermodynamics Entropy of the universe is always increasing • Processes only occur spontaneously when energy becomes more dispersed – spread out

15. Various spontaneities: dispersal • Matter disperses – gas fills a container, two liquids mix • Heat disperses – hot object cools on cold surface • Motion disperses – a ball stops bouncing • These processes never reverse spontaneously

16. Socks and spontaneity • Would you be stunned if the tumble dryer matched the socks? • Okay, you never match the socks anyway • Chaos in the sock drawer is natural • The same principles apply to chemical change (sort of)

17. Consequences for efficiency • All processes use some of the energy in dispersal • More energy is lost due to inefficiency – friction, wind resistance etc.

18. 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

19. Measurements of energy use

20. 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)

21. Common energy conversions • 10 g of octane is burnt to produce 8500 J. How much is that in calories? • 1 cal = 4.18 J

22. 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?

23. Energy: in or out? • Do chemical processes always create heat? • If you think yes then how does a cold pack work? • Answer: some processes absorb heat from the surroundings

24. Exo-thermic and endo-thermic • H2 + O2 gives out heat – exothermic • N2 + O2 absorbs heat - endothermic

25. Enthalpy and chemical reactions • Enthalpy of reaction (ΔH) measures heat of the reaction CH4 + 2O2 = CO2 + 2H2O ΔH = -11.8 kcal/g

26. 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

27. Energy sources • 85 % comes from fossil fuels • Fossil fuels are hydrocarbons • Petroleum • Coal • Gas • 15 % is everything else • Nuclear (8) • Hydro (3) • Renewables (3)

28. Electrical generation • More than 70 % comes from fossil fuels • Heat of combustion boils water • Steam turns a turbine • Turbine generates electricity

29. Finitude • Gambling on the “Other”: how to turn 3 % into 85 % without hurting anybody