enthalpy

Not in spec , what is enthalpy

Measure of the heat energy stored in a chemical system , in reactions there is a difference between the enthalpy of reactants and the enthalpy of products, this difference- enthalpy change

Not in spec what is enthalpy change

The heat energy change measured under conditions

Some chemical reactions

Are accompanied by enthalpy changes that are exothermic: the products have less enthalpy than reactants , chemical energy so changed into thermal energy , chemicals lose energy

The energy lost by chemicals

Is gained by surroundings which increase in temp . As a result the enthalpy change for an exothermic reaction is negative - chemical system loses energy and surroundings gain energy and increase in temp

Other chemical reactions

Are accompanied by enthalpy changes that are endothermic , products have more enthalpy than reactants , thermal energy is changed into chemical energy and chemicals gain energy

The energy gained by chemicals

Is lost by the surroundings which decrease in temp as a result the enthalpy change of an endothermic reaction is positive - chemical system gains energy surroundings lose energy and temp decreases

construction of enthalpy profile diagrams to show the difference in the enthalpy of reactants compared with products

qualitative explanation of the term activation energy, including use of enthalpy profile diagrams

Explanation and use of term: standard conditions and standard states (physical states under standard conditions):

enthalpy changes depend on temp and pressure, standard enthalpy changes are measured using the same standard conditions 

standard conditions

Any gases must have a pressure of 100kPa , stated temp must be used normally 298K 

enthalpy change of reaction (enthalpy change associated with a stated equation, ∆rH)

the enthalpy change that accompanies a reduction in the molar quantities shown in a chemical equation under standard conditions with all reactants and products in their standard states

enthalpy change of formation (formation of 1 mol of a compound from its elements, ∆fH)

enthalpy change when one mole of a compound is formed from its elements in their standard stated under standard conditions 

For an element

enthalpy change of formation of an element in its standard state is zero because there is no chemical or physical change 

enthalpy change of combustion (complete combustion of 1 mol of a substance, ∆cH)

the enthalpy change when one mole of a compound is completely burnt in oxygen under standard conditions

enthalpy change of neutralisation (formation of 1 mol of water from neutralisation, ∆neutH)

enthalpy change that accompanies the reaction of an acid by a base to form one mole of water under standard conditions with all reactants and products in their standard states

determination of enthalpy changes directly from appropriate experimental results, including use of the relationship: q = mc∆T

in experiments we measure the heat energy change in the surroundings , the heat energy change (q) in joules is given by the equation q= mc delta T

q= mc delta T

• m is the mass of the surroundings (g)

• c is the specific heat capacity of the surroundings (J g^-1 K^-1) 

• delta T is the temp change (final temp - initial temp) 

Steps for calculating enthalpy change of reaction from experimental results:

1)calculate the energy (q) transferred with the surroundings in kJ , q=mc delta T

2)calculate the amount in moles 

3)calculate the energy transfers for the moles in the equation (q/n)

4)decide on sign for delta H (the enthalpy change) whether it is positive or negative

How to decide on sign for enthalpy change (delta H)

 if the surroundings increase in temp and the reaction is exothermic then the enthalpy change has a negative sign

if the surroundings decrease in temp the reaction is endothermic then the enthalpy change has a positive sign 

explanation of the term average bond enthalpy

mean amount of energy required for the breaking of 1 mol of bonds in gaseous moelcules

explanation of exothermic and endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds

energy has to be put in to break bonds (endothermic) while energy is released when bonds are formed (exothermic) 

During chemical reactions there is usually a difference between the energy involved in bond breaking and bonds making

In exothermic reactions

more energy is released in forming new bonds than is needed to break bonds, the

overall energy change is exothermic (delta H or enthalpy change is negative) 

In endothermic reactions

more energy is needed to break bonds than is released when new bonds are formed

the overall energy change is endothermic (delta H or enthalpy change is positive) 

use of average bond enthalpies to calculate enthalpy changes and related quantities

EQ

Value of enthalpy change of reaction for the equation rH

given the f H (enthalpy change of formation) of the reactants and products

Use Hess Law, vector and directions

What is the bond enthlapy in kJ mol^-1

{ Sum of bonds broken (kJ/mol) } - {Sum of bonds made (kJ/mol) } = rH (enthalpy change of reaction)

Draw out the compounds

Suggest a modification to this experiment using the same apparatus which would reduce the percentage errors in experiments

Use a greater mass of the reactant

Use larger change in temp reduces % uncertainty in temp change

Sign on bond enthalpies

Always positive +, they are endothermic

Student finds that the experimental enthalpy chance of combustion of propane is much less exothermic (less negative, higher) than the standard enthalpy chance in database give reason why

The experiment had not been carried out under standard conditions

Heat loss to surroundings

Incomplete combustion

Evaporation of water

State the conditions of temp and pressure used for standard enthalpy measurements

Temp: 298K (25°C)

Pressure: 100kPa

Explain in terms of bond breaking and bond forming why a reaction can be exothermic

More energy is released by forming bonds than is required when breaking bonds

Define the standard enthalpy change of combustion include the standard conditions used

Enthalpy change that occurs when / energy required / energy released

when one mole of a substance complete combusts / reacts fully with oxygen

Standard conditions: 298K , 100kPa

What is meant by the term average bond enthalpy

Average enthalpy change when one mole of bonds of gaseous covalent bonds is broken

Explain in terms of bonds breaking and bond making why the enthalpy change of formation is endothermic

More energy is required for bond breaking than is released by bond making

What is meant by the term standard enthalpy change of formation

enthalpy change when / energy required / energy released

When one mole of substance / compound

Is formed from its elements

Reversible reaction: H2 (g) + I2 (g) -<>- 2HI (g) , activation energy for forward reaction is 173 kJ/mol what is the activation energy for the reverse reaction

Draw out the enthalpy profile diagram , take into account the enthalpy change H and whether it is going up or down

Why is the answer the enthalpy change of neutralisation

1 mole of water has been formed

Explain why this equation represent the standard enthalpy change of combustion of butane

One mole of butane completely combusts in oxygen

Why do all bond enthalpies have a positive value

Bonds are breaking which is endothermic , energy is required/ needed

What is meant by the term the enthalpy change of reaction

enthalpy change for stated equation

Calculate the rH for this method using temp change of water and all of this, state any assumptions made

density of solution is 1g/cm³ same as water

SHC © of solution is the same as water

ignore the mass and c of solid (zinc)

No heat is lost to surroundings/ escapes system

Mass of solution remains constant

No water is lost / evaporated

Reaction goes to completion

Reactions completed under standard conditions

Suggest improvements for obtaining more accurate value for rH

Use a polystyrene cup

Use a lid

More precise thermometer

Use more precise balance

Measure mass of solution

Use burette to measure volume

Use stands conditions

The enthalpy change of combustion of an isomer of ethanol, methoxymethane is more negative, explain why the values are different in terms of bonds broken and the bonds formed

Less energy so required to break bonds in methoxymethane , bonds in methoxymethane are weaker than in ethanol

Energy released in bond forming is the same , same bonds being formed

What is meant by the term enthalpy change of combustion

Enthalpy change for complete combustion of 1 mol of substance , (reacts completely in oxygen)

Student carries out experiment to determine enthalpy change of combustion of alcohol

apparatus: spirit burner containing alcohol , wick above it held by clamp a is beaker containing water and thermometer

Suggest how the experimental design could be modified to improve accuracy of cH obtained

Burn in plentiful oxygen

Use draft shield

Add life to beaker

Place cap over wick when not burning

Reduce gas between burner and beaker

Use standard conditions

Heat for longer to reduce % uncertainties in temp measurements

Use 3.D.P balance

Student concluded that rH is the neut H , explain why the students conclusion is incorrect and determine the correct value for the neut H

neutralisation forms 1 mol of H2O, rH forms 2 mol of H2O