Thermochemistry The study of energy changes that accompany physical or chemical changes in matter The study of energy changes that accompany physical or chemical changes in matter All thermodynamics depends on the LAW OF CONSERVATION OF ENERGY: the total energy of a system and its surroundings remains the same All thermodynamics depends on the LAW OF CONSERVATION OF ENERGY: the total energy of a system and its surroundings remains the same Changes that occur in matter may be classified as physical, chemical or nuclear Changes that occur in matter may be classified as physical, chemical or nuclear

Heat and Energy Changes Physical, chemical and nuclear changes are all accompanied by a change in energy –Physical: a change in the form of a substance. No chemical bonds are broken. Ex. hydrogen boils at -252°C H 2 (l) + heat  H 2 (g) –Chemical: a change in the chemical bonds between atoms resulting in new substances Ex. hydrogen is burned as fuel in the space shuttle’s engines. 2H 2 (g) + O 2 (g)  2H 2 O (l) + heat –Nuclear: a change in the protons or neutrons in an atom resulting in a new atom. Ex. hydrogen undergoes nuclear fusion in the Sun, producing helium.

Heat and Energy Changes cont’d Chemical system – the substances undergoing a change Surroundings – the system’s environment Thermal Energy – energy available from a substance as a result of the motion of its molecules Temperature (T) – the average kinetic energy of the molecules in a sample, measured in °C or K Heat (q) – amount of thermal energy transferred between substances (hot to cool), measured in Joules (J) Energy flows between substances because of their difference in temperature.

Heat and Energy Changes cont’d A. Exothermic: releasing thermal energy, heat (q) flows from the system to the surroundings, usually causing an increase in the temperature of the surroundings. q has a negative value q has a negative value B. Endothermic: absorbing thermal energy, heat (q) flows into the system from the surroundings, usually causing a decrease in the temperature of the surroundings. q has a positive value q has a positive value A B A B

Heat Transfer and Enthalpy Change Thermal Energy – total quantity of potential energy and kinetic energy of a substance. Kinetic Energy – –Energy of motion Eg. – –moving electrons in atoms – –vibration, rotation and translation of atoms and molecules Potential Energy Potential Energy – –Energy due to the position or composition of an object Eg: – –nuclear potential energy of protons and neutrons – –bond energy – –intra and intermolecular forces

Endothermic & Exothermic Reactions Revisited… If more energy is released from the formation of new bonds in the products than is required to break bonds in the reactants – heat/energy is released – Exothermic If more energy is released from the formation of new bonds in the products than is required to break bonds in the reactants – heat/energy is released – Exothermic Products of an exothermic reaction have lower potential energy (and usually stronger bonds) than the reactants. Products of an exothermic reaction have lower potential energy (and usually stronger bonds) than the reactants. The temperature of the surroundings must then… The temperature of the surroundings must then…Increase Endothermic reactions – opposite, products have a higher potential energy than reactants and usually weaker bonds. Endothermic reactions – opposite, products have a higher potential energy than reactants and usually weaker bonds.

A fire is started in a fireplace by striking a match and lighting crumpled paper under some logs. Explain all the energy transfers in this scenario using the terms ‘exothermic’, ‘endothermic’, ‘system’, ‘surroundings’, ‘potential energy’, and ‘kinetic energy’. Explain why the water in a swimming pool at 24°C has more thermal energy than a cup of boiling water at 100°C.

More Terms

Measuring Energy Changes Calorimetry: Calorimetry: experimental technique used to measure energy changes/transfers (the quantity of heat (q) in chemical systems different substances vary in their ability to absorb amounts of heat Substances used in a system have a specific heat capacity: amount of energy required to raise the temperature of 1 g of a substance by 1 °C or 1K amount of energy required to raise the temperature of 1 g of a substance by 1 °C or 1K (see Table 1, pg.301)

Equation used to determine the value of q: The amount of heat transferred (q) depends on »measured in grams »measured in J/g°C or J/gK – –measured in °C or K Measuring Energy Changes

Calorimeters

Laboratory calorimeter: –Can provide precise data for changes of state or chemical reactions that do not involve gases. Bomb Calorimeter: Bomb Calorimeter: –Container inside which a fuel is burned –Much more accurate measurements of heat transfer. –Pressure changes depending on the reaction –Often described as ‘isolated’ Calorimeters

Calorimetry cont… Therefore: Therefore: HEAT LOST by the PROCESS = HEAT GAINED by the WATER OR HEAT GAINED by the PROCESS = HEAT LOST by the WATER +q -q

How much energy is required to raise the temperature of 250mL of water in a 500g kettle that is 100% iron from 20°C to 99°C?

Determine the final temperature of an isolated system if a 300g lead rock at 150°C is dropped into 150g of water at 4°C. Determine the final temperature of an isolated system if a 300g lead rock at 150°C is dropped into 150g of water at 4°C.

If mL of 0.50 mol/L HCl at 20.0°C is combined with 250.0mL of 2.50 mol/L NaOH at 35.0°C, the final temperature of the mixture is 36.5°C. Find the heat released in this dilution process. q total = q acid +q base = mcΔT acid +mcΔT base = (350.0g. 4.18J/(g*°C). (36.5°C – 20.0°C) + 250g. 4.18J/G°C. ( °C) = 24, 140J+1568J =25, 708J or 25.7kJ Calorimetry of an Acid/Base Dilution

Heat Transfer and Enthalpy Change ΔH > 0, endothermic reaction (q= negative) ΔH < 0, exothermic reaction (q=positive) This system is exothermic b/c product have lower enthalpy than the reactants so ΔH<0. This example of exothermic change shows that the change in potential energy of the system (ΔH) equals the change in kinetic energy of the surroundings (q) Energy Reaction Progress Changes in Kinetic and Potential Energy high potential energy low potential energylow kinetic energy high kinetic energy ΔHq ΔH system = ± l q surroundings l

Investigation 1: Thermal Energy Transfer in a Coffee-Cup Calorimeter Thermal Energy Transfer in a Coffee-Cup Calorimeter