Energetics

Enthalpy change (∆H): the change in heat energy at constant pressure

Exothermic: negative enthalpy changes
– eg. combustion, neutralisation

Endothermic: positive enthalpy changes

Measuring energy changes

q=mc∆T

q = energy supplied to water (J)

m = mass of water (g)

c = specific heat capacity of water – 4.2 (J/g/°C)

∆T = change in temp of water (°C)

∆H = -q/n

Types of energy changes

of combustion: the energy change when one mole of a particular fuel is burned fully in oxygen under standard conditions

of neutralisation: the ∆H when one mole of an acidis fully neutralised in solution by an alkali
–> H+ + OH- –––> H2O

of solution: ∆H when one mole of a particular substance is dissolved in water (to give a conc of 1mol/dm3)

Lefty clap-trap

Pollutants from fuels

Sulfur dioxide

Dissolves in water vapour to make sulfurous acid (H2SO3)
–> may later oxidise to form sulfuric acid – H2SO4

Fall as acid rain: damaging trees, crops and stone buildings,
killing plants and animals in lakes and rivers

Forms as a result of burning fuels containing sulfur compound impurities
–> can be removed by bubbling waste gases through an alkaline solution

Nitrogen oxides (NO, NO2)

Form when nitrogen and oxygen in the air react together at the very high temperatures created in a car engine

Dissolve in water vapour to form nitrous acid (HNO2) and nitric acid (HNO3)
–> form acid rain

Can be removed in the catalytic converter:
2CO + 2NO –––> 2CO2 + N2

Hydrocarbons

Unburned hydrocarbons released by car engines when they are unable to burn fuel completely

Can irritate the lungs and cause harmful smogs to form

Can be removed by oxidation in the catalytic converter

Greenhouse effect

Greenhouse gases: absorb infra-red radiation from the earth's warm surface which would otherwise be radiated into space
–> traps heat energy in the atmosphere

Increased greenhouse gases => global warming => climate change
–> alterations in long-term weather patterns
–> melting polar ice caps, rising sea levels, more extreme weather

Hydrogen

Can be used as an alternative to fossil fuels – only produces water vapour when burned

Difficult to store and transport – forms very explosive mixture with oxygen

No deposits of hydrogen available – has to be produced from water – requires energy from another source

Makes a better energy store than energy source

Calculating enthalpy changes

When one mole of a particular bond between two atoms is formed, a certain amount of energy is released as heat

In order to break one mole of the same bond, the same amount of energy must be supplied as heat

Breaking bonds requires energy (endothermic), forming bonds releases energy (exothermic)

Two changes in energy:

Energy supplied to break all of the bonds

Energy released when new bonds are formed

Can be represented in an enthalpy profile diagram

Diagrams:

Exothermic: Up a bit, then down a lot

Endothermic: up a lot, down a bit

Process:

  1. energy put in to break all bonds – increase in enthalpy
  1. energy released as new bonds form to give product – decrease in enthalpy

∆H = change in height between reactant and products

Calculation

  1. write balanced equation
  1. find total bond energy of every bond in reactants, then products
    –> take into account balancing numbers
  1. ∆H = energy of reactants – energy of products

Carbon dioxide

*experimental values different to theoretical values

Heat loss: reaction to air, water to air, heating calorimeter and water

Incomplete combustion: formation of soot and CO – doesn't release as much energy as complete combustion

dissolves sparingly in water – forms slightly acidic solution – carbonic acid
H2O + CO2 <=> H2CO3

Formed from thermal decomp of metal carbonates
eg. CuCO3 ––heat––> CuO + CO2

Prepared in a lab by marble chips (calcium carbonate) and dilute HCl
CaCO3 – 2HCl –––> CaCl2 + H2O + CO2

Uses:

Fire extinguisher – denser than air, stops oxygen reaching combusting material

Carbonated drinks – dissolved under high pressure, slowly bubbles out