2.1 Thermochemistry Flashcards
what are the standard conditions?
conc of 1 moldm-3 (for aq solutions)
a temperature of 298K
a pressure of 1atm
define the term enthalpy, H
what is it measured in?
the total energy content of a system held at standard conditions
measured in kJmol-1
Define the term conservation of energy
energy that cannot be created or destroyed only changed from one form to another
what are the typical characteristics of exothermic reactions?
-products have less energy than reactants
-delta H is negative
-heat energy is released to surroundings
-energy gain by surroundings is identified by a temp rise so change in temp is +
what are the typical characteristics of endothermic reactions?
-products have a higher energy than reactants
-delta H is positive
-heat energy is taken in from surroundings
-energy loss from surroundings is identified by a temp drop so change in temp is -
define the term activation energy
the minimum energy which a reacting species must possess before a reaction can occur.
what is the standard enthalpy of reaction? (ΔHθr)
molar quantities of the reactants that are specified by the chemical equation, react to form the products at standard conditions
give an example of a standard enthalpy of reaction and what must always be included in the reaction equation.
H2(g) + 1/2O2(g) -> H2O(g)
always include state symbols
what is the standard enthalpy of combustion? (ΔHθc)
enthalpy which takes place when 1 mole of the substance is completely burned in oxygen under standard conditions
give an example of a standard enthalpy of combustion and what must always be included in the reaction equation.
CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
must always start with one mole
what is the standard enthalpy of formation? (ΔHθf)
enthalpy change when 1 mol of a substance is formed from its constituent elements in their standard states under standard conditions
give an example of a standard enthalpy of formation and what must always be included in the reaction equation.
Mg(s) + 1/2O2(g) -> MgO(s)
must always finish w/1mole
finish this sentence about enthalpy change of formation:
by definition….
by definition, the ΔHθf for any element is 0
what is the equation used in measuring the enthalpy change? what do they all stand for?
what are the units?
ΔH = -m x c x ΔT
ΔH = change in heat constant (J)
-m = mass of solution (g)
c = specific heat capacity (4.18Jg-1k-1)
ΔT = change in temperature (K or *C)
how do you convert your answer in the measuring enthalpy change to kJmol-1?
what’s the equation now?
divide by the number of moles used of the limiting reagent
ΔH=(-mxcxΔT) ÷ n
what are some common errors in enthalpy changes of reaction practicals?
-not letting the temperature reach constant before adding the solid
-not adding all of the solid at once
Why is it important to be able to measure the enthalpy change indirectly?
important to be able to predict enthalpy change eg in a chemical plant so engineers know how much heat will be generated or absorbed
What is Hess’ law?
The total enthalpy change for a reaction is independent of the route taken from the reactants to the products
using the alphabet, make an energy cycle (book)
what is the alternative method for calculating enthalpy changes of combustion? what do you need to remember about this?
ΔH = ΣΔHθc (reactants) - ΣΔHθc (products)
combustion data only
what is the alternative method for calculating enthalpy changes of formation? what do you need to remember about this?
ΔH = ΣΔHθf (products) - ΣΔHθf (reactants)
why is it practically impossible to find the enthalpy change of formation of ethane?
since burning carbon in hydrogen will produce a mixture of hydrocarbons
describe bond breaking and what process it is
energy must be supplied to break a bond between 2 atoms in the reactants, bond breaking is endothermic so requires energy
describe bond making and what process it is
energy must be released when a bond forms in the products, bond making is an exothermic process and releases energy
