2.8 - Energetics

Define endothermic

reaction where product enthalpy higher than reactant enthalpy

Define Conservation of Energy

energy can’t be created/destroyed, but change forms

Define exothermic

reaction where product enthalpy lower than reactant enthalpy

Define average bond enthalpy

energy required to break one mole of given bond averaged over many compounds

Define standard enthalpy changes

change in heat energy at constant pressure in standard conditions

Define standard conditions

298K and 100kPa

Define standard enthalpy of neutralisation

enthalpy change when one mole of water produced in neutralisation reaction in standard conditions

Define standard enthalpy of combustion

enthalpy change when one mole of substance completely burned in O2 in standard conditions

Define standard enthalpy of formation

enthalpy change when one mole of compound formed from its elements in standard conditions

Define Hess’s law

enthalpy change for reaction is independent of route taken provided initial/final conditions same

Enthalpy change equation

enthalpy change = Hp - Hr

What are the rules of bond enthalpies?

endothermic values, average values, only compared for bonds in gaseous compounds, apply to one mole of bonds

How to calculate enthalpy change in reaction?

calculate total bond enthalpy for bonds broken/made and broken - made

What changes enthalpy change?

temperature, pressure, physical state

Why are energy changes for weak acids/alkalis different to normal enthalpy of neutralisation?

partially ionised in water, energy used to complete dissociation before reaction between H+ and OH- occurs so less heat energy released

Why are enthalpies of combustion always -ve

combustion releases energy

How does the value of enthalpy of formation affect stability?

more -ve the value, the more stable compound is compared to elements

What container is used for combustion reactions?

copper can

Equation for temp change

q = mc•T

Does exothermic or endothermic have a +ve enthalpy change value?

endothermic

Method for determining enthalpy of neutralisation when alkali reacts with acid

with pipette, add alkali to polystyrene cup, record initial temp, add acid, add lid, record highest temp, calculate temp change

Determining enthalpy of combustion of ethanol

record initial water temp/spirit burner with ethanol mass. Clamp calorimeter with known mass of water above burner, add lid, light and stir until 10C temp rise, add lid, record final water temp/burner mass

What can the equipment in determining enthalpy of combustion be described as?

simple colorimeter

Waters heat capacity value

4.2

How to calculate enthalpy changes from enthalpy change of combustion values

make enthalpy cycle by putting combustion products at bottom with arrows down

How to calculate enthalpy changes from enthalpy change of formation values

construct enthalpy cycle by putting elements at bottom with arrows up