Heat Transfer Reaction Energy Slides 1 Take out your homework

Understanding Heat is a form of energy. Temperature is a measure of the average kinetic energy of the particles. Heat is a form of kinetic energy. The absolute temperature of any object is directly proportional to the particles average kinetic energy. KE = ½ mv 2 T α average KE

Temperature and Heat Temperature Not related to the quantity present Measures the average kinetic energy (molecular motion) of particles present. Units: °C, °F, K Heat Is related to the quantity present. Measures the total amount of kinetic energy of particles present. Units: Joule, Calorie 4.18J = 1 cal & 4.18 KJ = 1kcal

Aplications/Skills Calculation of the heat change when a temperature of a pure substance is changed using q = mc  T Guidance: The specific heat capacity of water is provide in the IB data booklet in section 2. A calorimetry experiment for enthalpy of reaction should be covered and the results evaluated.

Heat When a hotter object is in contact with a cooler object, energy in the form of heat flows between them. Flows from higher temp to a system at lower temp until they reach the same temperature (equilibrium)  = Change  T = Change in temp (T f -T i )

Specific heat Specific heat capacity (specific heat, Cp): The amount of energy required to raise the temperature of one gram of the substance by one °C or 1 K. SubstanceSpecific heat (J/g °C) Water (l)4.18 Water (s)2.06 Water (g)1.87 Ethanol (l)2.44 Aluminum (s)0.897 Carbon, graphite (s)0.709 Iron (s)0.449 Copper (s)0.385 Lead (S)0.129

Calculating heat transfer q= heat energy lost or gained - = lost + = gained q= c p x m x  T

Example Problem Example Problem: A 4.0 g sample of glass was heated from 1 °C to 41 °C and was found to have absorbed 32 J of energy as heat. a.What is the specific heat of this type of glass? b.How much energy will the same glass sample gained when it is heated from 41 °C to 71 °C? q= c p x m x  T32 = c p 32 J = c p x 4g x (41-1) °C 4 x = c p x 4 x J/g °C =c p q= c p x m x  T q = 0.20 x 4g x (71-41) °C q = 0.20 x 4 x 30 q= 24 J

Understanding Total energy is conserved in chemical reactions

Understanding Chemical reactions that involve transfer of heat between the system and the surroundings are described as endothermic or exothermic. Exothermic Process (Energy is released from the system) Endothermic Process (Energy is put into the system)

Understanding The enthalpy change (  H) for chemical reactions is indicated in kJ mol -1.  H values are usually expressed under standard conditions, known as  H Θ, including standard states.

Enthalpy change The Enthalpy change is the amount of energy absorbed or lost by a system as heat during a process at constant pressure in terms of moles. In this case the energy is not written as a reactant or a product, but is off to the side. 2H 2(g) + O 2(g)  2H 2 O (g)  H= kJ/mol

Reaction Diagrams  H = H products - H reactants

Sometimes you need to put a little energy in to start the reaction but then it releases a whole lot of excess energy (ex: burning magnesium or the whoosh bottle)

You burn a small gram piece of carbon under a beaker containing mL of water. The water temperature goes from 20.0 °C to a maximum of 59.2 °C. C (s) + O 2(g)  CO 2(g) How much heat energy appears to be produced by this combustion reaction? How many moles of C are being consumed? What is the enthalpy or molar heat of reaction, ∆H in terms of KJ/mole carbon?

A calorimetry experiment for enthalpy of reaction should be covered and the results evaluated. Guidance: Consider reactions in aqueous solution and combustion reactions Standard state refers to the normal, most pure stable state of a substance measured at 100 kPa. Temperature is not a part of the definition of standard state, but 295 K is commonly given as the temperature of interest. Students can a assume the density and specific heat capacities of aqueous solutions are equal to those of water, but should be aware of this limitation. Heat losses to the environment and the heat capacity of the calorimeter in experiments should be considered, but the use of a bomb calorimeter is not required.