topic 5/15- energetics Flashcards
symbol and description of standard conditions for enthalpy change
- pressure of 100 kPa
- concentration of 1 mol dm-3 for all solutions
- temperature is usually 298K
- each substance involved in the reaction is in its standard state (solid, gas or liquid)
ΔH°
what is heat?
a form of energy, sometimes referred to as thermal energy
what is temperature?
a measure of the average kinetic energy of the particles
total energy is —- in chemical reactions. Why?
conserved; the first law of thermodynamics states that energy cannot be created/destroyed
describe an endothermic reaction
- heat is taken into the system from the surroundings
- the temperature of the surroundings decreases
- the enthalpy change of the system is positive
describe an exothermic reaction
- heat is given out from the system to the surroundings
- the temperature of the surroundings increases
- the enthalpy change of the system is negative.
describe enthalpy
the heat stored by a substance
what three things make up a system?
- the chemical reaction
- the reactants
- the products
what two things make up the surroundings?
the air or solvent around the reactant and product molecules
give the equation for heat change
q=mcΔT
the density and specific heat capacities of aqueous solutions are assumed to be…
equal to those of water
what is ΔH meaured in?
kJ/mol
define standard enthalpy of formation
the enthalpy change when 1 mole of a substance is formed from its elements in their standard states, under standard conditions,
look at how to form standard enthalpy of formation/combustion equations
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define standard enthalpy of combustion
the enthalpy change when 1 mole of a substance combusts completely in oxygen, under standard conditions
what does standard state refer to?
the normal, most pure stable state of a substance measured at 100kPa
describe how temperature fits into the definition of standard state
temperature is not a part of the definition of standard state, but 298K is commonly given as the temperature of interest
what is one of the main limitations of calculations of specific heat capacity?
we assume that the density and specific heat capacities of aqueous solutions are equal to those of water
what is the temperature of interest usually given as part of the definition of standard state?
298K
give 2 limitations of enthalpy change experiments
- heat losses to the environment
- heat capacity of the calorimeter
the enthalpy change of a reaction that is carried out in a series of steps is equal to
the sum of the enthalpy changes for the individual steps
look over how to construct a hess cycle/solve those types of equations
-
hess’ law
the enthalpy change of a reaction is independent of the route between the initial and final states
the standard enthalpy change of formation of any element in its standard state is
zero
give the equation for the use of hess’ law to calculate the enthalpy change of a reaction
ΔH=Σ(ΔHf products)-Σ(ΔHf reactants)
