9 Enthalpy

exothermic profile diagram

endothermic profile diagram

equation for energy change

Q = m x c x ΔT

C = 4.18

enthalpy def

Enthalpy, H: measure of heat energy in a chemical system.

• Enthalpy is sometimes thought of as the energy stored within bonds. It can't be measured, but enthalpy changes can.

chemical system def

Chemical system: atoms, molecules, or ions making up chemicals

ΔH formula

ΔH = H(products) - H(reactants)

ΔH can be positive or negative depending on whether the products contain more or less energy than the reactants.

conservation of energy rule

energy can’t be created or destroyed

examples of conservation of energy

Heat energy is transferred between the system and the surroundings in a chemical reaction.

• System: chemicals - the reactants and products

• Surroundings: apparatus (e.g. thermometer), the laboratory, everything that isn't in the system

• The universe is everything, including system and surroundings.

describe the energy transfer in exothermic reactions

Energy transfer from the system to the surroundings - exothermic, negative ΔH

• Chemical system loses energy

describe the energy transfer in endothermic reactions

Energy transfer from the surroundings to the system - endothermic, positive ΔH

• Chemical system gains energy

activation energy Ea

Energy input required to break bonds acts as an energy barrier to the reaction, activation energy E_a

why is Ea needed

• Atoms and ions held together by chemical bonds

• Bonds in reactants broken by input energy

• New bonds in products from to complete the reaction

how does size of Ea affect a reaction

Generally, reactions with small Ea take place very rapidly as energy needed to break bonds is readily available from surroundings.

 

Very large Ea's may mean there is such a large energy barrier the reaction takes place extremely slowly or not at all.

standard conditions

100kPa

298K (25°C)

1moldm^-3 for solutions

standard state

ΔrH

Enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.

ΔfH

• Enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, w all reactants and products in their standard states.

• Forms 1 mole

ΔcH

• Enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, w all reactants and products in their standard states.

• When 1 mole of a substance completely reacts

ΔneutH

• Energy change that accompanies the reaction of an acid by a base to form one mole of H₂O_(l), under standard conditions, with all reactants and products in their standard states.

• 1 mole of H₂O formed

• The value of ΔneutH is the same for all neut reactions.

average bond enthalpy

The energy required to break one mole of a specified type of bond in a gaseous molecule.

• Energy is always required to break bonds

• Bond enthalpies are always endothermic

• Bond enthalpies always have a positive enthalpy value

reasons why experimental values of combustion aren’t accurate

• Heat loss to surroundings other than the water (beaker and air surrounding flame)

• Incomplete combustion of alcohol (CO and C produced instead. C as a black layer of soot on beaker)

• Evaporation of alcohol from wick (burner must be weighed as soon as possible after extinguishing flame)

• Non-standard conditions (conditions are unlikely to be identical to standard conditions)

Therefore ΔcH is less exothermic than expected

how to make experimental value of combustion more accurate

Use of draught screens and an input of oxygen gas could minimise errors from heat loss and incomplete combustion.

cooling curve shape

limitations of average bond enthalpies

• Actual bond enthalpy varies on chemical environment of bond.

• An average bond enthalpy is calculated from the actual bond enthalpies in different chemical environments.

how is energy related to bond breaking/making

• Energy is required to break bonds, endothermic

• Energy is released when bonds form, exothermic

what does the difference between energy required for bond breaking and energy released by bond making determine

The difference between energy required for bond breaking and energy released by bond making determines whether overall reaction is exo or endothermic.

enthalpy profile diagrams for bond breaking and bond making in exo and endo reactions

enthalpy change from average bond enthalpies

Δ_rH = ∑(bond enthalpies in reactants) - ∑(bond enthalpies in products)

what state do all species need to be in when doing calculations w avg bond enthalpies

When doing calculations using avg bond enthalpies all species need to be gaseous molecules.

Therefore calculated Δ_rH is NOT a standard enthalpy change.

what does hess’ law state

if a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route