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Thermochemistry: The heat energy of chemical reactions

Thermochemistry: The heat energy of chemical reactions. • Enthalpy is the amount of ________ transferred during a reaction. The symbol for the change in enthalpy is ∆H.

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Thermochemistry: The heat energy of chemical reactions

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  1. Thermochemistry:The heat energy of chemical reactions • Enthalpyis the amount of ________ transferred during a reaction. The symbol for the change in enthalpy is ∆H. An endothermic reaction is one that ___________ heat from the surroundings. (___ ∆ H) An endothermic reaction feels ______. Example--an “instant” ice pack • An exothermic process is one that _____________ heat to the surroundings. (___ ∆ H) An exothermic reaction feels _____. Example--burning paper heat gains + cold loses – hot

  2. Thermochemistry:How to measure heat (Energy) changes calorie (cal) kilocalorie 1000 Joule (J) 1 cal = 4.184 J • A ____________ is the amount of energy (heat) required to raise the temperature of one gram of water by one degree Celsius. • The “calorie” written on food is actually not one calorie in chemistry. It is actually 1 __________ (or ____calories) and is written with a capital C (Calorie) to keep the two separate. • A ____________ is the SI unit for measuring the amount of energy or heat transferred in chemistry. • Write down this conversion factor:

  3. McDonalds McChicken

  4. McDonalds McDouble

  5. Crunchy Cheetos

  6. So what do we do with all these Calories? • Your body will use these Calories as energy to do everyday activities but what if you don’t use all the calories you consume? • Your body will either use the energy or it will store it as fat! • So people who eat more food than their body can use exercise as a way of releasing the extra energy. But how much exercise do you really need to do in order to burn off those extra Calories?

  7. Exercise! • Running and walking are not the same!! Even though you can run OR walk a mile, there is a difference. According to David Swain, a Ph.D. in exercise physiology, “When you perform a continuous exercise, you burn five Calories for every liter of oxygen you consume and running in general consumes a lot more oxygen than walking.” • Running burns approximately 100 Calories per mile. • SO WHAT DOES THIS MEAN?

  8. I LOVE FOOD! But I hate running!  To run off the food previously mentioned, this is approximately how long you would have to RUN! 1 McChicken: 1.38 MILES 1 McDouble: 3.10 MILES 1 bag of Cheetos: 3.30 MILES

  9. Thermochemical Reactions A thermochemical reaction is written as follows: 2S + 3O2 2SO3 + 791.4 kJ This equation represents an ___________ reaction since the heat is a ________. H2 + Br2 + 72.80 kJ  2HBr This equation represents an ___________ reaction since the heat is a __________. exothermic product endothermic reactant

  10. Thermochemistry Problems Problem 1: How much heat will be released when 6.5 moles of sulfur reacts with excess oxygen according to the following equation? Also, tell whether it will be exothermic or endothermic! 2S + 3O2 2SO3 ∆H = -791.4 kJ -791.4 kJ 6.5 mol S _______________ X = -2572 kJ 2 mol S Exothermic ~ • ∆H means heat • is a product!

  11. Phase Changes & Energy Endothermic: melting, evaporating/boiling & sublimation Exothermic: freezing, condensation, & deposition

  12. “Reaction Profiles” Endothermic Exothermic

  13. Calculations in Thermodynamics heat mass specific heat capacity • In order to calculate how much heat is transferred by a thermochemical reaction the equation we use is q = mc∆T • q = the ______ lost or gained in the process • m = the _____ of the substance • c = the ________ _____ ________ The specific heat of water is 4.186 Joules/gram °C • ∆T = Change in temperature

  14. Calculations Practice q = mc∆T q = 250g (2.108 J/g °C)( 15 °C) q = 7905 Joules = 7.9 kJ Example 1: How many Joules would it take to raise the temperature of 250 g of ice from -20 °C to -5 °C? (The specific heat of ice is 2.108 Joule/gram °C)

  15. More Practice q = mc∆T q = 100 g (0.58 cal/g °C) (20 °C) q = 1160 cal Example 2: How many calories would it take to raise the temperature of 100 grams of Ethyl Alcohol from 30 °C to 50 °C? (The specific heat of Ethyl alcohol is 0.58 cal/g °C)

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