Enthalpy and FuelsUsing and Controlling Reactions

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Energy

FuelsIndustrial P rocess

Heat Light and Sound

Galvanic cells E lectrolytic cells

E lectrical K inetic Potential

E nerg y fromC hem ica l R eactions

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The capacity for doing work

Energy is inter-convertible between different forms

Chemical Energy

The energy from reactions comes from the breaking and forming of bonds between atoms.

When chemical bonds are formed energy is released.

When chemical bonds are broken energy is required.

This energy can be heat, light or electrical.

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Heat Energy

Measurement of the heat change in chemical reactions is called calorimetry.

The heat energy involved in a chemical reaction can be observed from the temperature changes resulting from that reaction.

A calorimeter is used to measure these heat changes in chemical reactions.

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Exothermic reactions

Exo: Out Thermic: Heat Releases heat energy to the

surroundings. Temperature of the surroundings

increases. Energy released when forming

bonds > Energy used to break bonds

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Exothermic reactions

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Heat Energy

Time

Reactants

Products

Change in Enthalpy

Exothermic

H < 0

Endothermic reactions

Endo: To take in Heat energy is taken from the

surroundings. Temperature of the surroundings

decreases. Energy required to break bonds >

Energy released when forming bonds.

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Endothermic reactions

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Time

Heat Energy

Reactants

Products

Change in Enthalpy

Endothermic

H > 0

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Enthalpy

The quantity of heat released or absorbed when specific amounts of substances react is called the Heat of Reaction.

The difference in heat energy between products and reactants at constant pressure is called the Enthalpy Change.

H = Hproducts – Hreactants12

Enthalpy

The Molar Enthalpy Change for a reaction is the quantity of heat energy released or absorbed when 1.00 mole of a substance reacts in a chemical reaction under constant pressure.

Measured in kJmol-1 or kJ g-1

The quantity of heat released or absorbed is directly proportional to the number of moles of reactants and products.

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Enthalpy

The quantity of heat energy released or absorbed can be calculated by

Heat = n x H EXAMPLE: Calculate the enthalpy

change for the neutralisation of 112.2g of KOH (H = – 56.0kJmol-1)

n = m/M = 112.2/56.1 = 2.00 moles Energy released = 2 x 56.0 = 112 kJ

released

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Thermochemical Equations

An equation showing the enthalpy change is worth 4 marks.

Correct formulae Must be BALANCED The physical states of matter MUST

be shown. The sign MUST be included with the

H value.

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6CO2(g) + 6H2O(l) --> C6H12O6(s) + 6O2(g) ∆H = +2800 kJ

Thermochemical Equations If the coefficients are doubled the H

value must be doubled. CH4(g)+2O2(g) → CO2(g)+2H2O(l) H = -890

kJmol-1 2CH4(g)+4O2(g) →2CO2(g)+4H2O(l) H = -1780 kJ

If the reaction is reversed then the H value is equal but opposite in sign

N2(g) + 3H2(g) → 2NH3(g) H = - 92.4 kJmol-1

2NH3(g) → N2(g) + 3H2(g) H = + 92.4 kJmol-116

Enthalpy of Combustion

The molar enthalpy of combustion of a substance is the quantity of heat energy released when 1.00 mole of a pure element or compound is burnt completely in oxygen under constant pressure.

The products are gaseous carbon dioxide and liquid water.

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Enthalpy of Combustion

When a fuel is burnt in air rather than oxygen, the non reactive gases (N2) absorb some of the heat energy released. Lower flame temperature.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH= -890kJ

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Enthalpy of Solution

The molar enthalpy of solution is the quantity of heat energy released or absorbed when 1.00 mole of substance dissolves in sufficient solvent so that further dilution causes no further change in heat energy.

NH4NO3(s) + aq NH4+

(aq)+ NO3–(aq) H = +26kJ

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Enthalpy of Neutralisation

The molar enthalpy of neutralisation is the quantity of heat energy released when 1.00 mole of hydrated hydrogen ions from an acid is neutralised by 1.00 mole of hydrated hydroxide ions from an alkali.

Heat change per mole of water formed when dilute solutions of acids and bases are mixed.

H+(aq) + OH–

(aq) H2O(l) H= –57kJ

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Calculating Enthalpy values

n

TmcH p

1000

Experimental data can be used to calculate the enthalpy change for chemical reactions.

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m = mass of water

cp (specific heat capacity of water) = 4.18 J g-1 oC-1

ΔT = change in temperature

n = number of moles of reactant

Assumptions

All of the heat energy produced or absorbed by the reaction is transferred to or from the known mass of water or aqueous solution in the calorimeter. No exchange of heat occurs with the calorimeter or the surrounding air.

The reaction occurs quickly enough for the max or min temperature of the liquid in the calorimeter to be reached before the liquid begins to return to room temperature.

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Approximations

The specific heat capacity of any aqueous solution is taken to be the same as that for water.(i.e. 4.18 J g-1 oC-1)

The density of any aqueous solution in the calorimeter is taken to be the same as the density of water. 1mL=1g

Always read the question carefully in any test or exam and use the specific heat capacity given.

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Fuels

Useable fuel source provides maximum energy from minimum amount of material

Chemical fuels release energy from combustion.

Carbon based Fuels can be divided into: Fossil Fuels e.g. Coal, Oil,

Natural Gas Biofuels e.g. Ethanol,

Biodiesel, Biogas 25

Advantages of Burning Fossil Fuels

High enthalpy of combustion (High energy density)

Can be burnt at point of use. i.e. Internal combustion engine, home heating etc (80% of fossil fuels are used this way)

Versatile and inexpensive. Provide governments with tax

income.26

Disadvantages of Burning Fossil Fuels

Finite reserves of coal, oil and gas. Oil and gas expected to run out before end of this century.

Burning releases CO2 which contributes to the enhanced greenhouse effect.

Other pollutants are released due to contaminants in the fuel or when incomplete combustion occurs e.g. soot, CO, SO2, NOx

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Disadvantages of Burning Fossil Fuels

Fossil fuels are used as feedstock (raw material) for the chemical industry

Used to make synthetic fibres, solvents, paints, detergents, dyes and plastics

Burning fossil fuels for energy, reduces raw material required for these industries

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Advantages of Burning Biofuels

Renewable, approximately Carbon neutral

Less harmful air pollutants released due to less fuel contaminants

Biodegradable Can be domestically produced

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Disadvantages of Burning Biofuels

Non renewable fossil fuels can be used in the production of ethanol

Slightly decreases fuel economy May solidify at low temperatures Not widely available Many developing countries are

growing plants to produce biofuels rather than food crops leading to increased famine

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Complete Combustion

Occurs with unlimited oxygen Products are Carbon dioxide and

water Maximum quantity of heat energy

produced when a fuel undergoes

complete combustion

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Incomplete Combustion

Occurs if the oxygen supply is limited.

Additional products formed include carbon (Soot) and carbon monoxide.

Less heat energy is produced than complete combustion.

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Carbon Monoxide CO

Persists in the environment for long periods of time.

Strongly bonds to haemoglobin in blood preventing it from carrying oxygen to cells.

Symptoms of CO poisoning range from visual disturbance, headaches, fatigue to unconsciousness and death dependant on concentration of Carbon monoxide.

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Soot C

Particulate pollution. Settles on objects.

Clogs air filters/ inlets lowering efficiency of engines and burners.

Coats plant leaves reducing photosynthesis.

Damages the respiratory system and aggravates asthma, bronchitis and other lung diseases.

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Comparing Fuels

Fuel comparisons can be made per mole, per gram or per litre of fuel.

Molar enthalpy of combustion: Energy released per mole of fuel (kJmol-1)

Energy density: Energy released per gram (kJg-1) or per litre (kJL-1) Per gram: Energy density = H/M Per Litre: Energy density(liquid fuel) =

H/M x (where = the density of the fuel in gL-1)

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