Chemical Thermodynamics Practice Questions with complete Solutions Graded A+

Chemical Thermodynamics Practice Questions with complete Solutions Graded A+ Reactions for this lab - Answers H2 (g) + 1/2 O2 → H2O (l) (heat of formation of water) (a) Mg (s) + 2 HCl(aq) →MgCl2 (aq) + H2 (g) (b) Ha= -68.3 kcal/ mol MgO(s) + 2HCl (aq) - MgCl2 (aq) + H2O (l) (c) Mg(s) + 1/2 O2 (g) -- MgO(s) (d) Hd= Hf (MgO) = Hb - Hc + Ha = Hb - Hc + (68.3 kcal/mol) Why is it very difficult to measure the heat of formation of MgO? - Answers Because without a pure oxygen environment, it is not possible to prevent the formation of Mg3N2 when Mg(s) burns in air (79% of air is N2) Enthalpy of formation (Hf) - Answers the heat given off when one mole of a substance is prepared from its constituent elements in their standard states Why do you clean the Mg ribbon with the steel wool? - Answers To prevent the formation of MgO that could screw up results Heat capacity - Answers ability to absorb heat How will you find the heat capacity of the calorimeter? - Answers since almost all the heat given off in the reaction is absorbed by the water in the calorimeter, you will use the heat capacity of water. How do you determine the amount of heat given off? - Answers heat= heat capacity x mass x temperature change What are the units of heat= heat capacity x mass x temperature change? - Answers cal = cal/ g °C x g x °C What is the heat capacity of water? - Answers 1.0 cal/ g °C -water has one of the highest heat capacities known (and specific heats) What does a heat capacity of 1.0 cal/ g °C mean? - Answers It takes 1.0 cal of heat to raise the temperature of 1.0 g of water by 1.0 °C How do you determine the heat given off in the experiment? - Answers heat = (1.0 cal/ g C)(100g) (temperature change) What is the heat capacity of Mg metal? - Answers .24 Why is the heat capacity of of water a mixed blessing - Answers Because it takes a large amount of energy to change the average temp. of a body of water (good for living ecosystems). One the other hand, once it's raised it takes a long time to cool down. What is an alloy? - Answers a mixture of two metals to form a solid solution -magnesium is used in a lot of alloys Why can't you use water to put out burning magnesium? - Answers -Magnesium burns with intense heat once it is started and a magnesium fire is incredibly exothermic -Special techniques are required for putting out fires of active metals like magnesium -It's because magnesium eats water for breakfast and makes the fire stronger. Seriously, there is a reaction Mg (s) + 2H2O -- Mg(OH)2 + H2 that produces hydrogen gas. This hydrogen then burns by combining with oxygen in the air and makes the fire hotter. Specific heat of a metal calculation - Answers heat lost by the metal = heat gained by calorimeter and its contents heat lost = sp. h. x # grams x change in T for the metal Heat gained = no. grams x 1 cal / °g x Change in T for calorimeter and its contents What is an intensive property? Example. - Answers A characteristic property that can be used to identify a metal, just like melting point or density. There is a rough correlation between specific heat of a metal and its atomic mass, such that specific heat DIVIDED INTO 6.4 is the approximate atomic mass of the metal. I REPEAT : SPECIFIC HEAT DIVIDED INTO 6.4 IS THE APPROXIMATE ATOMIC MASS OF THE METAL. Law of Dulong and Pettit (1816) - Answers -specific heat DIVIDED INTO 6.4 is the approximate atomic mass of the metal -used to establish the first consistent table of atomic weights Hess's law - Answers if a reaction is carried out in a series of steps, the (delta) H will equal the sum of the enthalpy changes for the individual steps State functions - Answers dependent on initial and final states, not how you got there (independent of path taken)