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Hydrocarbons and Heat

Hydrocarbons and Heat. Most hydrocarbons are used as fuels. Knowing how much energy a fuel provides, can tell us if it is useful for a certain application. For example, the amount of energy a food releases when burned, can tell us about it’s caloric content (fats release lots of energy).

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Hydrocarbons and Heat

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  1. Hydrocarbons and Heat • Most hydrocarbons are used as fuels. • Knowing how much energy a fuel provides, can tell us if it is useful for a certain application. • For example, the amount of energy a food releases when burned, can tell us about it’s caloric content (fats release lots of energy). • Heat energy released during combustion can be measured with a calorimeter. • A “bomb calorimeter” is shown. It includes water in a heavily insulated container, a stirrer, valve, bomb chamber, ignition wires, & a thermometer .

  2. Exothermic and Endothermic changes • An alternative to the bomb calorimeter is a “coffee cup” calorimeter, where two nested polystyrene cups take the place of the container • In either case, the change in heat of the water tells us about the reaction of the chemicals. • An increase in water temperature indicates that the chemicals released energy when they reacted. This is called an “exothermic” reaction. • In an “endothermic” reaction, water temperature decreases as the chemicals absorb energy. • We will see that heat is measured in Joules (J) or kiloJoules (kJ). Before we do any heat calculations, you should know several terms …

  3. Specific heat capacityballoon demo The heat needed to  the temperature of 1 g of a substance by 1 C. Symbol: c, units: J/(gC). Heat capacity The heat needed to  the temperature of an object by 1 C. Symbol: C (=c x m), units: J/C Heat of reaction The heat released during a chemical reaction. Symbol: none, units: J. Specificheat (of reaction) The heat released during a chemical reaction per gram of reactant. Symbol: h, units: J/g. Molar heat of reaction The heat released during a chemical reaction per mole of reactant. Symbol: H, units: J/mol.

  4. Heat Calculations • To determine the amount of heat a substance produces or absorbs we often use q = cmT • q: heat in J, c: specific heat capacity in J/(gC), m: mass in g, T: temperature change in C, • This equation makes sense if you consider units J J = x C x g gC Sample problem: (must know water = 4.18 J/gC) When 12 g of a food was burned in a calorimeter, the 100 mL of water in the calorimeter changed from 20C to 33C. Calculate the heat released. q=cmT = 4.18 J/(gC) x 13C = 5.4 kJ x 100 g

  5. More practice q=cmT = 0.38 J/(gC) x 60 C = 114 J x 5 g • 5 g of copper was heated from 20C to 80C. How much energy was used to heat the Cu? 2. If a 3.1 g ring is heated using 10.0 J, it’s temp. rises by 17.9C. Calculate the specific heat capacity of the ring. Is the ring pure gold? q=cmT 10.0 J c = q/mT= = 0.18 J/(g°C) 3.1 g x 17.9C The ring is not pure. Gold is 0.13 J/(gC)

  6. Heat Capacity Calculations • Recall that heat capacity (J/C) is different from specific heat capacity (J/gC). • Heat capacity is sometimes a more useful value • For example, because a calorimeter includes wires, the stirrer, thermometer, etc. some heat will be transferred to these other materials. • Rather than having to calculate q for each material a J/C value is used. Sample problem: A calorimeter has a heat capacity of 2.05 kJ/C. How much heat is released if the temperature change in the calorimeter is 11.6C? q=cmT q = cm T q= 2.05 kJ/C x 11.6 C = 23.8kJ

  7. Thermochemical Equations Thermochemical equations are chemical equations with an added heat term. • KBrO3(s) + 42 kJ  KBrO3(aq) This is endothermic (heat is absorbed/used) • 2 Mg(s) + O2(g)  2 MgO(s) + 1200 kJ This is exothermic (heat is produced/released) Sample problem: 3.00 g of octane was burned in a calorimeter with excess oxygen, the 1000 mL of water in the calorimeter rose from 23.0C to 57.6C. Write the thermochemical equation for octane, representing the molar heat of combustion.

  8. Units • Specific refers to mass in grams. • Specific heat of reaction: J/g or kJ/g, etc. Molar heat of reaction: J/mol or kJ/mol, etc. • J/mol = J/g x g/mol (molar mass is used) • Exothermic. E.g. CH4 + 2O2 CO2 + 2H2O + x kJ Other examples include propane, octane, Mg 49.90 kJ 26.04 g = 1299 kJ/mol x g mol C2H2 + 2.5O2 2CO2 + H2O + 1299 kJ or 2C2H2 + 5O2 4CO2 + 2H2O + 2598 kJ

  9. Sample: First, calculate the heat released by the combustion reaction via q=cmT … J 48.2 kJ 114.26 g x g g mol = 48209 J/g = 48.2 kJ/g J mol = 4.18 J/(gC) x 34.6 C = 144 628 J x1000 g Octane (C8H18) has a molar mass of 114.26 g/mol We can determine the molar heat of reaction 1) via the specific heat of reaction or 2) directly 144 628 J h = H = = 3.00 g = 5508 kJ/mol or 144 628 J H = = 5508 kJ/mol = 0.0263 mol C8H18 + 12.5O2 8CO2 + 9H2O + 5508 kJ For more lessons, visit www.chalkbored.com

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