A-Level Chemistry AQA - Energetics

define the term ‘enthalpy’

the thermodynamic property of a system linked to it’s internal energy

define the term ‘enthalpy change’

heat energy change at constant pressure

define the term ‘endothermic’

takes in heat energy from surroundings

define the term ‘exothermic’

gives out heat energy to the surroundings

what are standard conditions?

• 100kPa

• 298K

define the term ‘enthalpy of formation’

enthalpy change when one mole of a substance is produced from its elements under standard conditions

define the term ‘enthalpy of combustion’

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

what does the term endothermic refer to in terms of bond breaking and bond making?

more energy taken in breaking bonds than is given out making bonds

what does the term exothermic refer to in terms of bond breaking and bond making?

more energy given out making bonds than is taken in breaking bonds

what is the difference between a reaction profile and a reaction pathway diagram?

• reaction profile only shows enthalpy change

• reaction pathway shows enthalpy change and activation energy

what does an endothermic reaction profile/reaction pathway diagram look like?

what does an exothermic reaction profile diagram look like?

define the term ‘activation energy’

the minimum energy required for a reaction to take place

what is calorimetry?

experimental methods used to find enthalpy change by measuring temperature change over time

what is the formula for energy change?

Q (J) = m (g) x c x T^c (K)

what is the formula for enthalpy change?

• enthalpy change = -Q (KJ) / n (mol)

• use moles of limiting reactant

describe how you would carry out the practical for measuring an enthalpy change (not combustion)?

• measure out 25cm³ of NaOH

• add to polystyrene cup in a beaker

• measure out 25cm³ of HCl

• record the temperature of NaOH at minute intervals for 3 minutes

• at the 4th minute, add HCl and don’t record the temperature

• continue recording the temperature every minute after that for 10 further minutes

• plot a graph of temperature against time

• draw two lines of best fit for before + after addition

• extrapolate the two lines

• find the maximum temperature at the point of addition

• use this for calculations

what should a graph of temp against time for measuring enthalpy change look like?

describe how you would carry out the enthalpy of combustion practical

• set up apparatus as shown

• measure temperature of 50cm³ of water (already added to copper cup) and record this

• weigh the spirit burner with fuel in, record the mass

• light the spirit burner and allow the water to increase in temperature until this temperature increase reaches 50C

• extinguish the burner and reweigh it, recording the final mass of it

• calculate energy and enthalpy changes

why might the enthalpy change value obtained experimentally be inaccurate?

• heat energy transfer to or from surroundings

• specific heat capacity of water might not be that of a solution (if used)

• any heat absorbed by apparatus is ignored

• reaction may be incomplete/incomplete combustion

• density of solution (if used) is taken as the same as water

• room temperature could have changed/not under standard condition

• some water/fuel may have evaporated

how would you improve the accuracy of an enthalpy change experiment?

• use an electronic temperature sensor and data logging software

• bomb calorimeter used

what is the principle of conservation of energy?

energy cannot be created or destroyed

what is Hess’s law?

states that: overall enthalpy change for a reaction is independent of the route taken

what does a Hess’s law diagram look like using enthalpies of formation?

• arrows point out from elements

what is the overall equation for enthalpy change using enthalpies of formation?

enthalpy change of reaction = total enthalpy of products - total enthalpy of reactants

what does a Hess’s cycle using enthalpies of combustion look like?

• arrows point to products of combustion

what is the overall equation for enthalpy change using enthalpies of combustion?

enthalpy change of reaction = enthalpy of reactants - enthalpy of products

what is the enthalpy of formation of an element?

0

what is an enthalpy of vaporisation?

• the energy required to turn something into a gas

• this is required as for enthalpy changes, enthalpies have to be measured in the gaseous state

define the term ‘average bond enthalpy’

energy required to break 1 mole of covalent bonds measured in the gaseous state in kj/mol under standard conditions

why are bond enthalpy values means?

it is an average of the energy required to break the same bond across different compounds (energy required will vary slightly)

what does a shorter covalent bond mean?

it is stronger

what is the increasing order of shortness of covalent bonds?

• triple - shortest

• double

• single - longest

how do you calculate enthalpy change using bond enthalpies?

• enthalpy change = total bond enthalpies of bond broken - total bond enthalpies of bonds made (reactants - products)

• may have to include enthalpies of vaporisation