Presentation on theme: "Chapter 17 – Thermochemistry 4-1"— Presentation transcript:
1Chapter 17 – Thermochemistry 4-1 -is the study of energy ( heat) relationships in chemical and physical reactions
2Thermochemistry is concerned with the flow of heat from the system to it’s surroundings, and vice-versa.almost nothing happens in chemistry unless there is an energy advantage; all reactions are an attempt to arrive at a lower energy within the system.
3a) 1st law of Thermodynamics: Law of conservation of energy. Energy is never created nor destroyed , it only changes form.This law states that energy can change form but is never really lost in any closed system.b) 2nd law of Thermodynamics:Heat Energy always travels spontaneously from a warmer body (body with higher temperature) to a colder body.-dropping a hot rock in water-frostbite
4temperature is simply an indirect measure of the average kinetic energy of the particles in a sample of matter.i) Celsius scale sets the zero value at the freezing point of water at kPa and 100 at the boiling point of water at the same pressure.ii) Kelvin scale sets the zero value at absolute zero.Remember: 0K = °CK = t °C
5Energy and Heat Energy - capacity for doing work or supplying heat if within the chemical substances- called chemical potential energy
6Heat - represented by “q”, is energy that transfers from one object to another, because of a temperature difference between them.
7Essentially all chemical reactions, and changes in physical state, involve either: release of heat, (exothermic) orabsorption of heat (endothermic)the system (the people) gain heat from it’s surroundings (the fire)this is endothermic
8heat flowing into a system from its surroundings: q has a positive valuecalled endothermicAbsorbs in the form of heatsystem gains heat as the surroundings cool downDemo: Ba(OH) NH4NO3 mix together to create an endothermic reaction.
9heat flowing out of a system into it’s surroundings: q has a negative valuecalled exothermicReleases in the form of heatsystem loses heat as the surroundings heat up
10Every reaction has an energy change associated with it Exothermic reactions release energy, usually in the form of heat.Endothermic reactions absorb energyEnergy is stored in bonds between atoms
11Heat Capacity and Specific Heat A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1 oC.
12The calorie is related to the joule, the SI unit of heat and energy 4.184 J = 1 calHeat Capacity - the amount of heat needed to increase the temperature of an object exactly 1 oCnamed after James Prescott Joule
13Specific Heat Capacity - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC often called simply “Specific Heat”mass; measured in g or kg. The greater the mass, the more heat required to change the temperature.
14Water is used as a coolant! For water, C = 4.18 J/(g oC),Thus, for water:it takes a long time to heat up, andit takes a long time to cool off!Water is used as a coolant!
15To calculate, use the formula: q = m C T heat abbreviated as “q” m = massT = change in temperature (Tf - Ti)C = Specific HeatUnits are either J/(g oC) or kJ/(kg oC)J/(g K) or kJ/(kg K)
17Ex: Determine the heat required to raise the temperature of 100 Ex: Determine the heat required to raise the temperature of 100.g of water from K to K .Q = m c ΔTQ =100.g (4.184 J/g K)( K –298.0 K)Q = J/K (75 K)Q = JQ = 31.4 kJ
18Molar heat capacityCp0of a substance is the amount of energy required to raise the temperature of one mole of a substance by one Celsius degree.Molar heat capacity= (specific heat) (molar mass)
19Calculate the molar heat capacity of water, given that the specific heat water is 4.184 J/g°C. Cp0 = (specific heat)(molar mass)= (4.184 J/g°C)(18.02g/mol)= J/mol °C