Chemistry Do Now 12-4-17 Directions: Use your notes to calculate the molar mass of each compound.   Au2(SO4)3 Fe(CN)3 C8H18

Chemistry Do Now 12-4-17 Key Directions: Use your notes to calculate the molar mass of each compound.   Au2(SO4)3 – 2(197)+3(32)+12(16) = 682 g/mol Fe(CN)3 – 1(56)+3(12)+3(14) = 134 g/mol C8H18 – 8(12)+18(1) = 114 g/mol

Objective Students will know calculate the calculate change in heat problems and thermochemical problems by taking notes, guided practice and independent practice problem solving. Mastery Level: 70% or better

Engage Apply your notes on Heat Transfer and Exothermic and Endothermic reactions to explain how water melts and freezes. Words to include: exothermic, endothermic, heat gain, heat loss, +ΔH, -ΔH.

Engage – Sample Response Melting ice is an endothermic process because the ice is gaining heat as it melts. Therefore, the process has a positive delta H. The freezing of water into ice is an exothermic process because the liquid water loses heat as it freezes. Therefore, the process has a negative delta H.

Hydrocarbons and Heat Most hydrocarbons are used as fuels. Knowing how much energy a fuel provides, can tell us if it is useful in certain applications. 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.

Exothermic and Endothermic changes 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. Heat is measured in Joules (J) or kiloJoules (kJ). Before we do any heat calculations, you should know several terms …

Specific heat capacity 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 (Joule) 9/14/2018

Heat Calculations To determine the amount of heat a substance produces or absorbs we often use q = mTc 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 g x C gC Specific Heatwater = 4.18 J/gC Note: 1 mL (milliliter) of water has a mass of 1 gram. Ex: 100 mL of water has a mass of 100 grams

What does each variable mean in the heat formula, q = m ΔTc? q = heat lost or gained ΔT = change in temperature (Tfinal- Tinitial) m = mass of object heated/cooled c = specific heat capacity You will be given all but one of the above variables and asked to find the missing variable.

Variations on the formula q = m ΔTc, q = m(Tf – Ti)c Solving for mass (m) m = q÷ΔTc Solving for the change in temperature (ΔT) ΔT = q÷mc Solving for the final temperature (Tf) Tf = (q÷mc) + Ti Solving for the initial temperature (Ti) Ti = Tf – (q÷mc) Solving for the specific heat (c) c = q÷mΔT

Guided Practice Problem #1 A student must use 225 mL of hot water in a lab procedure. Calculate the amount of heat required to raise the temperature of 225 mL of water from 20.0 oC to 100.0 oC.

Guided Practice Problem #2 Calculate the specific heat capacity of a new alloy if a 15.4 g sample absorbs 393 J when it is heated from 0.0 oC to 37.6 oC.

Guided Practice Problem #3 A 40.0 g sample of ethanol releases 2952 J as it cools from 50.0 °C. Calculate the final temperature of the ethanol. Specific Heat of Ethanol: 2.460 J/gC

Guided Practice Problem #4 Calculate the heat change associated with cooling a 350.0 g aluminum bar from 70.0 oC to 25.0 oC. Is the change endothermic or exothermic? Why? Specific heat of Aluminum: 0.900 J/gC

Guided Practice Problem #5 Calculate the specific heat capacity of titanium if a 43.56 g sample absorbs 476 J as its temperature changes from 20.13 oC to 41.06 oC.

Guided Practice Problem #6 A 63.5 g sample of an unidentified metal absorbs 355 J of heat when its temperature changes by 4.56 oC. Calculate the specific heat capacity of the metal.

Guided Practice Problem #7 750.0 g of water that was just boiled (heated to 100.0 oC) loses 78,450 J of heat as it cools. What is the final temperature of the water?

Guided Practice Problem #8 A 175 g piece of iron and a 175 g piece of aluminum are placed in a hot water bath so that they are warmed to 99.7 oC. The metal samples are removed and cooled to 21.5 oC. Which sample undergoes the greater heat change? Specific Heat Values: Fe: 0.444 J/gC Al: 0.900 J/gC