Thermochemistry The study of the changes in heat energy that accompany chemical reactions and physical changes

OBJECTIVES: Explain the relationship between energy and heat.

Thermochemistry Heat a form of energy. Heat - represented by “q”, is energy that transfers from one object to another, because of a temperature difference between them. Heat is transferred from matter at a higher temperature to matter at a lower temperature.

Thermochemistry Units of Heat Joule (SI unit) calorie

Thermochemistry Specific heat capacity, cp the amount of energy required to raise the temperature of one gram of a substance one degree Celsius used in equation q = m x cp x ΔT

Thermochemistry q = m x cp x ΔT note that in the Metric System, Joules are the unit of measure for heat. Energy, or heat (J) Δ temp (°C) mass (g) specific heat (J/g•°C)

Heat capacity, cp

Example Problems A 45.0-gram sample of iron is heated from 25.0°C to 50.0°C. How much energy is required? (cp iron = 0.449 J/g°C) q = ? m = 45.0 g cp = 0.449 J/g°C ΔT = 50.0°C – 25.0°C = 25.0°C q = m cp ΔT q = 45.0g (0.449 J/g°C) (25.0°C) = 505.125 q = 505 J

2. What is the specific heat capacity of an object if a 12 2. What is the specific heat capacity of an object if a 12.5-gram sample is heated from 12.0°C to 28.0°C using 100.0 joules? q = 100.0 J m = 12.5 g cp = ? ΔT = 28.0 – 12.0 = 16.0°C q = m cp ΔT

Problem 1 Determine the specific heat of a material if a 35 g sample absorbed 48 J as it was heated from 293 K to 313 K.

Determine the specific heat of a material if a 35 g sample absorbed 48 J as it was heated from 293 K to 313 K. q = 48.0 J m = 35.0 g cp = ? ΔT = 313 – 293 = 20.0 K q = m cp ΔT

Challenging Problem If 984 kJ of energy are added to 6.2 L of water at 291 K, what will the final temperature of the water be? Density of water = 1 gram/mL Cp for water = 4.18 g/J•K

Challenging Problem q = m x cp x ΔT If 984 kJ of energy are added to 6.2 L of water at 291 K, what will the final temperature of the water be? q = m x cp x ΔT 984000 J = 6200 g x 4.18 x (T2 – 291K) T2 = 329 K

Homework Calorimetry Worksheet

Two Substances

Due to the Law of Conservation of Energy the total heat of the system must be 0. Energy is conserved. q1 + q2 = 0 or q1 = -q2

Suppose a 55.0 gram piece of metal is heated to 373K and then dropped into cool water. The quantity of water is 225 grams and the initial temp. of water is 294K. The final temp. of the metal and water is 296K. What is the specific heat capacity of the metal?

qmetal = m x cp x ΔT qwater = m x cp x ΔT The values of qmetal and qwater are equal but of opposite sign. The metal is negative (loss of heat) and the water is positive (gain of heat)

qwater = -qmetal m x cp x ΔT = m x cp x ΔT Cmetal = 0.47 J/g•K

A 15.5 gram piece of chromium, heated to 100oC (373K) is dropped into 55.5 grams of water at 16.5oC (289.5K) The final temp. for the chromium and water is 18.9oC (291.9K) What is the specific heat capacity of chromium?

Changes of State

When a substance is undergoing a change of physical state (melting or vaporization) the temperature of the system does not change. All the energy added as heat is used to break the intermolecular forces holding the molecules together.

Melting Vaporization Heat of Fusion (J/g) J = (Heat of Fusion)(grams) Heat of Vaporization J = (Heat of Vaporization)(grams)

Calculate the amount of energy needed to convert 500 grams of ice at -50oC to steam at 200oC. The heat of fusion of water is 333 J/g and the heat of vaporization is 2256 J/g. The specific heat capacities of ice, liquid and steam are: 2.06 J/g●K, 4.184 J/g●K and 1.86 J/g●K.

Calculate the amount of energy needed to raise the temperature of 1L of ethanol (d=0.7849 g/mL) from 25oC to its boiling point at 78oC and then to vaporize the liquid (heat capacity-2.44J/g●K, heat of vaporization at 78oC = 38.56 J/mol)