5.1 Measuring energy changes
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Heat
Transfer of energy between objects of different temperatures
2nd law of thermodynamics
Heat will spontaneously flow from an object of higher temperature to an object of lower temperature
Thermal equilibrium
When no energy is transferred as objects reach the same temperature
Enthalpy
Heat released or absorbed by a system at constant pressure
Can total enthalpy be measured?
No but the changes in enthalpy can
What are the standard conditions?
100kPa and 25C/298K
Temperature
Average kinetic energy of particles in a substance
Absolute zero
Temperature at which particles have least amount of kinetic energy 0K or -273C
Difference between open and closed systems
Matter and energy move freely in open systems however only energy can move in closed
Exothermic and endothermic
Exo is negative while endo is positive. Heat is released in exothermic while heat is absorbed in endothermic
Example of exothermic reactions
Combustion and neutralization reactions
Describe energy profile of exothermic reactions
Products release more energy then products so delta H is negative
Example of endothermic reactions
Thermal decomposition
Is exothermic and endothermic bond breaking or bond forming?
Bond forming is exothermic while bond breaking is endothermic
Equation for enthalpy change
q= mcT where: q--> energy absorbed or released (J) m--> mass (g) c--> Specific heat capacity (J/gC) T--> Change in temp. (C)
Specific heat capacity
Amount of heat required to raise temp. of one g of a substance by 1C or 1K
Do metals have high or low specific heat capacity?
They have low SHC sp they heat up and lose heat quickly
Enthalpy change of combustion
Change of energy when one mole of a substance is completely burned in the presence of oxygen under standard conditions
Enthalpy change of formation
Energy change upon formation of 1 mole of a substance from its constituent elements in standard states
Enthalpy change of formation of an element
0kJ/mol as they can’t be formed from anything
Standard enthalpy change of a reaction
Energy change that takes place when molar amounts of reactants as shown in stoichiometric equations react together to form products under standard conditions
What are the two ways to calculate standard enthalpy change of a reaction?
• Bond energy values: Heat absorbed - Heat released 2. Sum of enthalpy of products - Sum of enthalpy of reactants
Enthalpy change of neutralization
Energy change when an acid and base react together to form one mole of water
What apparatus can be used to find molar enthalpy of combustion and how is it used?
Calorimeters and spirit burners. Known mass of alcohol is burned and temperature increase of water in calorimeter is measured for a certain period. Calculate molar EOC of alcohol by looking at mass of alcohol burned and temp change of water