Heat Transfer and Specific Heat Heat Transfer and Specific Heat Energy Changes in Chemical Reactions Energy Changes in Chemical Reactions Calculating ∆H

Definition The energy transferred between samples of matter due to differences in their temperature. Standard SI UnitJoule (J) Other Common Unit Calorie (cal) A calorie is the quantity of heat that raises the temperature of 1 g of pure water 1°C. 1 calorie = 4.18 J

Heat transfer – the transfer of energy, in the form of heat, from material at a higher temperature to a material at a lower temperature. The three methods of transferring heat are conduction, convection and radiation.

Conduction is the transfer of heat by the direct contact of particles of matter. Conduction can occur in all three states of matter.

1.Where two particles are in contact there are lots of collisions between the particles of each. 2.The particles with greater kinetic energy transfer some of that energy to the particles with less kinetic energy. 3.As energy is transferred, the temperature of the warmer object decreases and the temperature of the cooler object increases.

Convection is the transfer of heat by the actual motion of a fluid (liquid or gas) in the form of currents. Convection does not occur in solids. Convection currents are responsible for our weather. Warm air is less dense than cold air so warm air rises.

Radiation is heat transfer by electromagnetic waves.

Specific Heat is the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius.

The equation can be rearranged algebraically to solve for the amount of energy released or absorbed. The specific heat of water is 4.18 J/g°C or 1.00 cal/g°C. C p = specific heat q = energy released or absorbed Δ T= change in temperature

Aluminum has a specific heat of J/g°C. Lead has a specific heat of 0.13 J/g°C. If equal masses of both metals are heated to 100°C, which metal will absorb the most energy? The aluminum will absorb the most energy because it has a higher specific heat.

Calorimetry is used to determine the amount of heat released or absorbed during a chemical or physical change.

One indication that a chemical reaction has occurred is a change in energy. This change in energy may be in the form of heat, light or sound.

A thermochemical equation is an equation that includes the heat change. C 2 H 5 OH (l) + 3O 2 (g) → 2CO 2 (g) + 3H 2 O (l) kJ

The heat of reaction ( Δ H) (also called the enthalpy of reaction) is the quantity of energy absorbed or released as heat during a chemical reaction.

If Δ H is negative, the reaction is exothermic and energy is being released. However, when heat is shown as a “product” in an equation, it is expressed as an absolute value. Exothermic reactions are more common in nature. CaO(s) + H 2 O(l) → Ca(OH) 2 (s) Δ H = kJ CaO(s) + H 2 O(l) → Ca(OH) 2 (s) kJ

If Δ H is positive, the reaction is endothermic and energy is being absorbed. 2NaHCO 3 (s) → 2Na 2 CO 3 (s) + H 2 O (g) + CO 2 (g) Δ H = 129 kJ 2NaHCO 3 (s) kJ → 2Na 2 CO 3 (s) + H 2 O (g) + CO 2 (g)

C(s) + O 2 (g) → CO 2 (g) kJ CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O (l) kJ CaCO 3 (s) kJ → CaO(s) + CO 2 (g) Δ H = kJ; exothermic Δ H = kJ; exothermic Δ H = +176 kJ; endothermic

Potential Energy of the Reactants Activation Energy of the Forward Reaction Potential Energy of the Activated Complex Heat of Reaction (∆H) Potential Energy of the Products Activation Energy of Reverse Reaction A B C+ E C D E CC-E or B-D

The heat of combustion ( Δ H c ) is the energy released by the complete combustion of one mole of a substance. C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O (l) Δ H c = kJ/mol Note: In this reaction the heat of combustion and the heat of reaction would be the same since only one mole of C 3 H 8 is being combusted.

The heat of formation ( Δ H f ) is the energy absorbed or released in the formation of one mole a compound from its elements in their standard state. H 2 (g) + ½ O 2 (g) → H 2 O(l) Δ H f = kJ/mol What would be the heat of reaction for the reverse reaction? kJ/mol What would be the heat of reaction for the following reaction? 2H 2 (g) + O 2 (g) → 2H 2 O(l) 2( kJ) = kJ

Stoichiometrical Calculations 1.Use the equation below to calculate the kilojoules of heat required to decompose 2.24 mol NaHCO 3. 2NaHCO 3 (s) kJ → 2Na 2 CO 3 (s) + H 2 O(g) +CO 2 (g)

Stoichiometrical Calculations 2.Use the equation below to determine the amount of heat released when g of calcium oxide reacts with excess water. CaO(s) + H 2 O(l) → Ca(OH) 2 (s) kJ

Stoichiometrical Calculations 3.The heat of combustion for ethane (C 2 H 4 ) is kJ/mol. Calculate the amount of heat produced when 4.79 g C 2 H 4 reacts with excess oxygen. C 2 H 4 (g) + 3O 2 (g) → 2CO 2 (g) + 2H 2 O(l) kJ

Hess’s Law There are several different ways in which the enthalpy change can be calculated from a reaction. The first method we are going to look at is called Hess’s Law. Hess’s Law states that the overall enthalpy change in a reaction is equal to the sum of the enthalpy changes for the individual steps in the process.

General Principles for Combining Thermochemical Equations 1.If a reaction is reversed, the sign of ∆ H is also reversed. 2.If the coefficients in a balanced equation are multiplied by an integer, the value of ∆H is also multiplied by the same integer.