(Blue) Energetics

Question 1

what is the definition of enthalpy

Answer 1

Enthalpy change for a reaction = the heat change when the reaction is carried out at constant pressure (i.e. in an open vessel). It is given the symbol ΔH

Question 2

how do we annotate that energy has left the compound and is transferd to the surroundings

Answer 2

During an exothermic reaction energy is transferred to the surroundings (i.e. they get hot) and the energy level goes down i.e. a –ve energy change so ΔH is negative

Question 3

how do we annotate that the reaction is endothermic and that energy has been transferd to the compound

Answer 3

· During an endothermic reaction energy is transferred from the surroundings (i.e. they get cold) and the energy level goes up i.e. a +ve energy change so ΔH is positive

Question 4

draw or think of a basic enthalpy LEVEL diagram

Answer 4

https://www.researchgate.net/figure/Traditional-enthalpy-reaction-diagram-depicting-A-an-exothermic-reaction-and-B-an_fig5_305268662

Question 5

what is the definition of spesific heat capasity

Answer 5

Specific heat capacity = the enthalpy in joules/KJ which heats 1g/Kg of the substance by 1oC/K

Question 6

what is the spesific heat capasity of water

Answer 6

4.18Jg-1K-1

Question 7

Write the reaction for the displacement reaction of copper(II) sulpahte and zinc

Answer 7

CuSO4(aq) + Zn(s) —-> ZnSO4(aq) + Cu(s)

Question 8

in standard conditions what is the :
molar quantities of reactants
pressure
tempreature
physical state
concentration of solutions

Answer 8

molar quantities of reactants :
1 molar

pressure :
100 kpa / 1atm

tempreature :
25oC / 298K

physical state :
what ever it is at room tempreature

concentration of solutions :
1 mole per decimeter

Question 9

what are the units of delta H

Answer 9

Kj per mole

Question 10

STATES OF MATTER matter and should always be included because the energy change depends on the state of the substance. Look at the example given below:

2H2(g) + O2(g) —> 2H2O(l) DHO = -572 kJ mol-1

2H2(g) + O2(g) —> 2H2O(g) DHO = -484 kJ mol-

Explain why the enthalpy changes have different values

Answer 10

diffrent bonds are made and broken so they will give diffrent values

Question 11

what is the definition of THE STANDARD ENTHALPY OF COMBUSTION DHoC,298

Answer 11

The heat energy released when one mole of a substance in its standard state is completely burned in excess oxygen under standard conditions (1atm. 298K)

Question 12

how do you calcilate dleta H if you have more or less than one mole

Answer 12

energy / moles

Question 13

Can you be sure complete combustion has taken place? What might have been formed as well as carbon dioxide and water?

Answer 13

No, some soot deposited on the base of the calorimeter

Question 14

List the main sources of heat loss.

Answer 14

· Draughts.

· Heat radiated from calorimeter.

· Heat absorbed by tripod, pipe clay triangle, clamp and stand and spirit burner.

· Incomplete combustion

Question 15

Will heat loss and incomplete combustion make the standard enthalpy value more or less negative?

Answer 15

Less negative e.g. ethanol found to be -1080 should be -1367 kJ mol-1

Question 16

How does the number of carbon atoms in the alcohol affect the enthalpy of combustion?

Can you explain why this is?

Answer 16

Longer molecules with more C result in a more exothermic DHc

More bonds formed as more CO2 and H2O is formed from each mole

Question 17

A tin can is not a very effective calorimeter. To do more accurate experiments chemists use a bomb calorimeter:-

What are the advantages of using this calorimeter over a tin can?

Answer 17

· Insulation – prevents heat radiation.

· Lid – prevents convection losses.

· No draughts.

· Stirrer results in even water temperature..

· Electric ignition, sealed system – all heat energy transferred to the water

Question 18

what is the equation for spesific heat capasity

Answer 18

Energy transferred /J = Mass /g x Specific Heat Capacity / J/g/oC x Change in temp / oC

Question 19

what is the definition of THE STANDARD ENTHALPY OF COMBUSTION DHoC,298

Answer 19

THE STANDARD ENTHALPY OF COMBUSTION DHoC,298 The heat energy released when one mole of a substance in its standard state is completely burned in excess oxygen under standard conditions (1atm. 298K).

Question 20

what is the definition THE STANDARD ENTHALPY CHANGE OF FORMATION DHof,298

Answer 20

Enthalpy change when one mole of a substance in its standard state is formed from its elements in their standard states under standard conditions (1atm. 298K).

NOTE; the enthalpy change of formation of a pure element in its standard state is zero

Question 21

what is the definition of THE STANDARD ENTHALPY CHANGE FOR A REACTION,

Answer 21

THE STANDARD ENTHALPY CHANGE FOR A REACTION,

DHor,298 The enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions (1atm, 298K, 1mol dm-3) You must always give an equation when quoting an enthalpy of reaction – otherwise things can get very muddled

Question 22

what is the definition of THE STANDARD ENTHALPY CHANGE OF

Answer 22

THE STANDARD ENTHALPY CHANGE OF

NEUTRALISATION DHoneut,298 The standard enthalpy change of neutralisation is the enthalpy change when one mole of water is formed by the neutralisation between an acid and an alkali under standard conditions, 298K, 1atm..

Question 23

Write equations for the standard enthalpy of enthalpy of neutralization for:-

1) H2SO4 (sulfuric acid) by sodium hydroxide

Answer 23

½H2SO4(aq) + NaOH(aq) —-> ½Na2SO4(aq) + H2O(l)

Question 24

Write equations for the standard enthalpy of enthalpy of neutralization for:-

2) HCl (hydrochloric acid) by potassium hydroxide

Answer 24

HCl(aq) + KOH(aq) —> KCl(aq) + H2O(l)

Question 25

Write equations for the standard enthalpy of enthalpy of neutralization for:-
3) HNO3( Nitric acid) by calcium carbonate

Answer 25

HNO3(aq) + ½CaCO3(aq) —–> ½Ca(NO3)2(aq) + H2O(l) + ½CO2(g)

Question 26

what does hess law state

Answer 26

HESS’S LAW states that:

The total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place.

Question 27

info card read and recite

Answer 27

In the example above, Hess’s law tells us that the enthalpy change for route 1 is equal to the enthalpy change for route 2:-

DH1 = DH2 + DH3 + DH4

This is just another way of saying that energy cannot be created or destroyed, but may be converted from one form to another. (First Law of thermodynamics)

These energy cycles enable us to measure indirectly enthalpy changes which cannot be made directly.

For example, the reaction C(graphite) + 2H2(g) à CH4(g) does not occur under normal conditions.

How can the value of ΔH°f be measured? It has to be done indirectly using energy cycles.

This figure shows what we mean by an energy cycle. There is both a direct and an indirect way to turn graphite (C) and hydrogen (H2) to methane (CH4). We can’t measure the enthalpy change for the direct route. The indirect route goes via CO2 and H2O, and involves two enthalpy changes, which we can measure.

Question 28

Explain why the enthalpy change of formation of carbon dioxide is comparatively easy to measure experimentally, but it is impossible to obtain an accurate experimental value for carbon monoxide.

Answer 28

DHOf [CO2(g)] = DHOc [C(graphite)], excess O2, standard conditions

DHOf [CO(g)] represents incomplete combustion of carbon but this results in some CO2,

some CO and some C, as CO is thermodynamically unstable with respect to CO2.

Question 29

is bond making exo or end thermic and is bond breaking endo or exo

Answer 29

· Bond breaking requires energy, it is endothermic

· Bond making releases energy, it is exothermic

Question 30

what is the definition of bond dissociation enthalpy

Answer 30

Definition of bond dissociation enthalpy (also called ‘bond enthalpy’): DHB DHd E(X-Y) The energy required to break one mole of bonds in a substance when it is the gaseous state

Question 31

The bond dissociation enthalpy of the single H—H covalent bond is +436 kJ mol-1. This can be written as

H2(g) à 2H (g) DH = +436 kJ mol-1

Note the sign of this enthalpy change. Since energy is needed to break covalent bonds, bond breaking is always endothermic, the sign is always?

Answer 31

positive

Question 32

The nitrogen molecule is very unreactive. Why is this so?

Answer 32

N≡N bond is very strong, 945 kJmol-1, needs a lot of energy to overcome it.

Question 33

info card read and recite

Answer 33

When investigating bond breaking, chemists distinguish between:-

• Bond dissociation enthalpies - precise values for particular bonds
• Mean bond enthalpies - average values that are a useful approximate guide. In molecules with two or more bonds between similar atoms, the energies needed to break successive bonds are not the same. In water, for example, the energy needed to break the first OH bond in H-O-H (g) is 498 kJ mol-1 but the energy needed to break the second OH bond in O-H(g) is 428 kJ mol-1.

The mean values of bond enthalpies take into account the facts that:

· The successive bond dissociation enthalpies are not the same in compounds such as water or methane

· The bond dissociation enthalpy for a specific covalent bond varies slightly from one molecule to another,

For most of our calculations we will be using average bond enthalpies.

A selection of average bond enthalpies and bond lengths are given below.

The symbol E(X—Y) stands for the average bond energy of a covalent bond.