Introducing Enthalpy and Enthalpy Change Flashcards
what does the first law of thermodynamics state
- that during a reaction energy cannot be created or destroyed
- however, it can be converted from one form of energy to another
what are the two most important forms of energy for a chemist
chemical and heat energy
what are the two components chemical energy is made up of
kinetic and potential energy
what is kinetic and potential energy
- kinetic energy is the measure of the motion of particles in a substance
- potential energy is the measure of how strongly these particles interact with one another
what is heat energy
- the portion of the kinetic and potential energy of substance
- that is responsible for the temperature of the substance
what is the heat energy of a substance directly proportional to
- its absolute temperature
- the temperature measured in kelvin
what is enthalpy
the measure of the total energy of a system
if we cant directly measure the enthalpy of a system, what do we measure in stead
the enthalpy change that takes place during a physical or chemical change
what is the symbol for enthalpy change
delta H
what is the enthalpy change of a process
- the heat energy that is transferred between the system and the surroundings
- at a constant temperature
if enthalpy is the total measurement of the energy in a system, what formula can be derived from the relationship between enthalpy and internal energy and why
- enthalpy = internal energy + (pressure x volume)
- H = U + PV
- because enthalpy includes the internal energy
what is internal energy and why are pressure and volume included in the formula
- internal energy is the energy required to create a system
- but enthalpy also includes the amount of energy required to make room for it
- by displacing its environment and establishing its volume and temperature
- so enthalpy would be the sum of the internal energy and the product of the pressure and volume
what is an exothermic reaction
where heat energy is transferred from the system to its surroundings
what is an endothermic reaction
where heat energy is transferred from the surroundings into the system
are processes such as freezing water and the combustion of petrol exo or endo and why
- they are exothermic
- because the energy level of the products is less than that of the reactants
- meaning heat energy has been lost from the system and has been transferred to the surroundings
are process such as melting ice an photosynthesis exo or endo and why
- they are endothermic
- because the energy level of the product(s) is higher than the reactants
- meaning heat energy was absorbed from the surroundings and transferred into the system
the reaction HCl + NaOH = NaCl + H20 is exothermic and has an energy change with a magnitude of 57.1 kJmol-1. how would you write this to accurately show the enthalpy change and why
- the delta H would = -57.1kJmol-1
- because if it is exothermic, heat energy has been lost to the surroundings
- meaning the whole system has actually lost 57.1kJmol-1 of energy
- so a minus sign is used to show this
what sign would you then use to show the enthalpy change of an endothermic reaction and is it even necessary
- delta H = +x kJmol-1
- the plus sign isnt really needed as the number would be positive anyway
how can you know a reaction is exothermic through observtion
- exothermic reactions result in an immediate increase in temperature
- so the temperature of the reaction mixture increases
- which can be felt or measured with a thermometer
how can you know a reaction is endothermic through observation
- endothermic reactions result in a decrease of temperature of the reaction mixture
- which can be felt or measured with a thermometer
what is any reaction that has to be heated in order for it to take place called
an endothermic reaction
what is an example of a type of reaction that is usually endothermic with a real example
- thermal decomposition
- like CaCO3 = CaO + CO2
- which has a delta H of 178kJmol-1
what are the standard conditions that enthalpy changes should usually be quoted in
- a pressure of 100kPa
- with a temperature of 298K
what is the enthalpy change measured under these conditions called and what is its symbol
- the standard enthalpy change
- given the symbol deltaH- with a subscript of 298K
- or just deltaH-
- the - has a circle drawn around it
what is the symbol for the standard enthalpy change of reaction
delta rH-
what do you need to remember about what the standard enthalpy change for reaction is determined by
- the reaction as written, not necessarily the reaction with whole number ratios
- so a reaction can be written as N2 + 3H2 = 2NH3
- giving a delta rH- of -92kJmol-1
- but for the reaction when written it is 1/2N2 + 3/2H2 = NH3
- giving you a delta rH- of -46kJmol-1
what does the ‘per mole’ in the enthalpy change mean in each case
- it refers to one mole of the equation
- and not to one mole of any reactant or product
