enthalpy

what is enthalpy change

When chemical bonds break / form during a reaction, energy is often released

This heat energy change, which occurs at constant pressure is enthalpy (ΔH)

Measured in kJ mol^-1

Exothermic reactions:

Release heat energy to surroundings

Negative enthalpy change

Temperature of surrounding increase

E.g combustion of methane

Endothermic reactions:

Absorb heat energy from surroundings

Have positive enthalpy change

Temperature of surroundings decrease

E.g thermal decomposition of calcium carbonate

Draw enthalpy profile diagrams of exo and endothermic reactions + key features of them

Activation energy is the minimum energy needed for a reaction to occur

Exo - reactants have higher energy than products

chants have lower energy than products

What standard conditions are used for enthalpy change

298K temperature

101kPa pressure

⦵ Used after ΔH to denote that enthalpy change refers to standard conditions

Standard enthalpy change of reaction ΔH^⦵_r

Enthalpy change when a reaction occurs in molar quantities shown in the chemical equation under standard conditions

Standard enthalpy change of formation ΔH^⦵_f

Enthalpy change when 1 mole of a compound forms from elements under standard conditions

Standard enthalpy change of combustion:

Enthalpy change when 1 mole of a substance combusts fully in oxygen under standard conditions

Standard enthalpy changes of neutralisation ΔH^⦵_neut

Enthalpy change when an acid and alkali neutralise under standard conditions to form 1mole of water

What are the two types of calorimetry?

We measure energy transferred in a reaction as heat using apparatus called a calorimeter, and we can deter,Ive enthalpy changes

Combustion calorimetry = used to find enthalpy change when a fuel is burned e.g combustion

Solution calorimetry = used to find enthalpy change for reactions occurring in aqueous solutions, neutralisation

Describe process of combustion calorimetry:

1, weighted fuel sample is burnt underneath a metal canister (calorimeter) containing a known mass of water

As fuel combusts, heat transferred to water, thermometer measures temperature change,

Temp change with mass and heat capacity of water used to calculate heat energy

Common sources of error in calorimetry:

Heat loss = heat escaping to surrounding mean less is absorbed by water, lower enthalpy change

Fuel evaporation = if fuel evaporated before burning, reduces amount of heat generated, lower measured enthalpy change

Solution calorimetry process

Enthalpy changes for reactions that occur in aqueous solutions = neutralisation, dissolution, displacement

1. Measured volume of reactant to polystyrene cup calorimeter and record initial temp

2. Add measured volume of second reactant and seal container w lid

3. Stir moisture with thermometer and recor temp at intervals as heat is released / absorbed

why are polystyrene cups used as calorimeter for solution calorimetry?

Because it is a better insulation, lightweight, waterpolo

Reduce heat loss to surrounding

How to create accurate temperature time graph

Equation to calculate enthalpy change:

First calculate heat energy change, then use that to calculate enthalpy change per mole

Heat energy change: q=mcΔT

Q = heat energy (j)

M = mass of water (g)

C = specific heat capacity of water

ΔT = temperature change

Use value of q:

ΔH= -q/n

ΔH = enthalpy change kJ mol^-1

q = heat energy (kJ)

n = number of moles of the limiting reactant (mol)

What is the overall enthalpy change of a reaction?

Combined effect of these two processes:

Breaking bonds in the reactant molecule = endothermic process that requires energy, breaking stronger binds requires more energy

Forming bonds to create the product molecules = an exothermic process that releases energy, formation of stronger bind releases more energy

when does a reaction have a positive / negative enthalpy change?

Bond breaking requires more energy than released, has positive enthalpy change e

If bond breaking requires less energy than is released during bond formation p, reaction has a negative enthalpy change

What is average bind enthalpy?

Average energy needed to break one mole a type of covalent bond in gaseous molecules

They are always positive (endo) because breaking bonds required energy

Represent average value across different molecule environments

How to calculate reaction enthalpy changes ΔH_r?

ΔH_r = Σ(bond enthalpies of bonds broken)  Σ(bond enthalpies of bonds formed)

where Σ = sum of

or

ΔH_r = Σ(bond enthalpies in reactants) Σ(bond enthalpies in products)

Why is Hess cycle more accurate than ΔH_{r values?}

Calculating reaction enthalpy change with mean bond enthalpy is less accurate than Hess cycle because average bond enthalpjes don’t account for unique molecule environment of each bond

Hess is more precise

What Hess law states

Overal standard enthalpy change of a reaction. Is same, regardless of whether the reaction takes place in one stop or several steps

Allows you to break a reaction into component steps and add up their enthalpy changes to find total enthalpy change for reaction

How can standard enthalipes of formation be used with Hess law to determine unknown ΔH^⦵ Values

These values are known for many compounds

standard enthalpy of formation for elements in their standard state is define as 0kj mol

To calculate = ΔH^⦵_r = ΣΔH^⦵_f(products) - ΣΔH^⦵_f(reactants)

where Σ = sum of

Using enthalpy of combustion to find enthalpy of reaction values

ΔH^⦵_r = ΣΔH^⦵_c(reactants) ΣΔH^⦵_c(products)

where Σ = sum of

How to form the Hess cycle for enthalpy of formation

Reacts → products

Elements at bottom

Arrow point up from elements

How to form Hess cycle for enthalpy of combustion

Reactant → products

Combustion products at bottom

Point towards combustion products