Topic 8 energetics

definition of enthalpy change

heat energy change measured at constant pressure

standard conditions

100Kpa and 298k

c to k is +273

what's an exothermic reaction and how how does enthalpy level diagram look like?

-give out heat energy

-negative sign

-temp often goes up

whats an endothermic reaction and what does the enthalpy level diagram look like?

-absorb energy

-positive sign

-temp often falls

the less enthalpy a substance has , is it more or less stable?

more

reaction profile diagram for exothermic reaction and endothermic reaction.

definition of Standard enthalpy change of reaction

Enthalpy change when reaction occurs in the molar quantities shown in the chemical equation under standard conditions.

definition of standard enthalpy change of neutralisation

Enthalpy change when an acid and alkali react together under standard conditions to form 1 mole of water

- always exothermic

definition of Standard enthalpy of combustion

Enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.

definition of Standard enthalpy change of formation

Enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

what is calorimetry used for

to find enthalpy changes of combustion

describe apparatus of calorimetry

what are possible errors in measuring enthalpies of combustion using a calorimetry?

• Energy losses from calorimeter

• Incomplete combustion of fuel

• Incomplete transfer of energy

• Evaporation of fuel after weighing

• Heat capacity of calorimeter not included

• The experiment was not carried out under standard conditions because water formed as H₂O(g) rather than H₂O(l), which is the standard state in the data booklet. This means the measured enthalpy change is less exothermic than the data-booklet value

what are the possible errors in measuring enthalpy change for a reaction experimentally using a calorimeter?

• energy transfer from surroundings (usually loss)

• approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water.

• neglecting the specific heat capacity of the calorimeter- we ignore any energy absorbed by the apparatus.

• reaction or dissolving may be incomplete or slow.

• density of solution is taken to be the same as water.

equation for enthalpy change of combustion

q = mc∆T

q is KJ

J to KJ is divide by 1000

whats hess's law and why is it used?

The total enthalpy change of a reaction is always the same, no matter which route is taken

-to find enthalpy changes that cant be worked out experimentally

enthalpy change of formation for elements is

zero

where does arrows point in enthalpy change of formation

upwards

it always products minus reactants for the route

where does arrows point in enthalpy change of combustion

downwards

what happens when bonds break in reactants?

• energy is absorbed

• endothermic process so delta H is positive

what happens when bonds are formed in products?

• energy is released

• exothermic process so delta H is neg

define bond enthalpy

is the amount of energy required to break one mole of a type of bond in a molecule in the gas phase

why would the data booklet values for mean bond enthalpy be different to actual ones?

mean bond enthalpy is the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds.

write the formula for enthalpy change of reaction involving bond enthalpies

sum of bond enthalpies of reactants - sum of bond enthalpies of products