AP Chem You will have a quest (so not a big test) for this unit before Thanksgiving break Today: Intro to Thermochemistry

Thermodynamics The study of energy changes in physical and chemical processes.

Definitions System includes the molecules we want to study (here, the hydrogen and oxygen molecules). Surroundings everything else (here, the cylinder and piston).

Energy Energy the ability to do work or transfer heat Types of Energy: 1. Potential energy energy an object possesses by virtue of its position or chemical composition. 2. Kinetic energy energy an object possesses by virtue of its motion.

Definitions Temperature=measure of the average kinetic energy of a sample of a substance. Heat=is measured as the energy that is transferred from one object to another because of a difference in temperature. The direction of heat flow is always from the hotter object to the colder object. Specific Heat Capacity: The amount of energy needed to raise the temperature of 1 gram of a substance by 1°C

Answer practice questions Heat Capacity Questions

Answer questions 1-6 based on the “Heating Curve”

The heat energy is used to separate the molecules (complete the phase change) instead of raising the temperature. If there is no change in temperature, there’s no change in kinetic energy. However, since it’s a phase change, there’s a change in potential energy.

Heat = Energy transferred due to a difference in temperatures Heat = Energy transferred due to a difference in temperatures. The amount of heat absorbed or released in a physical or chemical reaction can be calculated using the equation q = mCΔT

Example How much heat is required to raise the temperature of 5 grams of water from 20°C to 100°C? The specific heat of water is 4.18 J/g°C.

Answer practice questions

Why can’t we use this formula when there is a phase change Why can’t we use this formula when there is a phase change? Consider the problem “how many joules are required to melt 100 grams of ice at 0°C?” If we actually tried to use q=mCΔT to solve this problem… q = 100 g x 4.18 J/g°C x 0°C q = 0 Joules…. But we know that you need heat energy to melt something…!

Heat of Fusion: amount of heat energy required to change a substance from a solid to a liquid; Heat energy required to melt a substance: q=mHf Note: The amount of energy released when a substance changes from liquid to solid is equal to the Heat of Fusion q=mHf q=-mHf

Heat of Vaporization: amount of heat energy required to change a substance from a liquid to a gas; heat energy required to boil a substance: q=mHv Note: the amount of energy released when a substance changes from a gas to a liquid is equal to the Heat of Vaporization q=mHv q=-mHv

q=mHf and q=mHv Problems: 1. How many joules are required to melt 100 grams of ice at 0°C? The heat of fusion for water is 334 J/g q= m= Hf = q = 100 g x 334 J/g 100 g q = 33,400 J 334 J/g

How many joules are absorbed by a 50 gram sample of H2O that is boiling? The heat of vaporization of water is 2260 J/g q= m= Hv = q = 50 g x 2260 J/g 50 g 2260 J/g q = 113,000 J

Enthalpy A measurement for the heat flow in a system H > 0 = endothermic H < 0 = exothermic

Examples Determine the sign of the enthalpy change in each of the following scenarios: a. An ice cube melts b. 1g of butane is combusted in sufficient oxygen to give complete combustion to CO2 and H2O

Enthalpy of Reaction The change in enthalpy, H, also called the heat of reaction Enthalpy is an extensive property. Ex: CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) ΔH = -890kJ The energy given off by reacting 1mol CH4 with 2mol O2 is -890kJ The energy given off by reacting 2mol CH4 with 4mol O2 is 2(-890kJ) = -1780kJ © 2009, Prentice-Hall, Inc. 19

Enthalpy H for a reaction in the forward direction is equal in size, but opposite in sign, to H for the reverse reaction. Ex. CH4 +2O2 CO2 + 2H2O H =-890kJ CO2 + 2H2O  CH4 +2O2 H = +890kJ

Examples How much heat is released when 4.50g of methane gas is burned in a constant pressure system? CH4(g) + 2O2(g)  H2O(l) + CO2(g) H = -890kJ 4.50 g CH4 × 𝟏 𝒎𝒐𝒍 𝑪𝑯 𝟒 𝟏𝟔.𝟎𝟓 𝒈 𝑪𝑯 𝟒 × −𝟖𝟗𝟎 𝒌𝑱 𝟏 𝒎𝒐𝒍 𝑪𝑯 𝟒 = -250 kJ