Topic 8 As Chem

Front 1

What are standard conditions?

Back 1

100 kPa

A specified temp, usually 298K

Front 2

What is 100 kPa in atm?

Back 2

100 kPa = about 1 atmospheric pressure

Front 3

What is 298K in °C?

Back 3

298K = 25°C

Front 4

What is absolute zero (0K) in °C?

Back 4

0K = -273°C

Front 5

What is 0°C in K?

Back 5

0°C = 273K

Front 6

What is 100°C in K?

Back 6

100°C = 373K

Front 7

What is an exothermic reaction in terms of energy? Egs of exo reactions?

Back 7

· Energy released to surroundings

· Combustion, eg. CH4 + 2O2 (g) -> CO2 (g) + 2H2O, hand warmer, burning, respiration

Front 8

What's an endothermic reaction in terms of energy? Egs?

Back 8

· En absorbed from surroundings

· Ammonium chloride dissolving in water: NH4Cl (s) --> NH4Cl (aq), ice pack, melting, vaporisation, photosynthesis

Front 9

Is bond breaking endothermic or exothermic

Back 9

Endothermic

Front 10

Is bond forming endothermic or exothermic

Back 10

Exothermic

Front 11

What makes reactions overall exo/ endothermic?

Back 11

The bonds that need to be broken and formed during the reaction

Front 12

Describe overall exothermic reactions in terms of energy

Back 12

More energy is released when bonds made than energy absorbed to break bonds

Front 13

Describe overall endothermic reactions in terms of energy

Back 13

More energy absorbed to break bonds than is released when bonds made

Front 14

What's the standard conc used for reactions involving solutions?

Back 14

1 mol/dm^3

Front 15

What state must all reactants and products be in?

Back 15

Standard states

Front 16

What is the enthalpy change?

Back 16

The heat energy change measured at constant pressure

Front 17

Is activation energy shown in enthalpy level diagrams or reaction profile diagrams

Back 17

Reaction profile diagrams

Front 18

What is the label for the x axis of an energy profile diagram?

Back 18

Course/ progress of the reaction

Front 19

What is the label for the y axis of an energy profile diagram?

Back 19

E

Front 20

Is the energy level of the reactants or the products higher for an exothermic reaction

Back 20

Reactants

Front 21

Is the energy level of the reactants or the products higher for an endothermic reaction

Back 21

Products

Front 22

Define activation energy

Back 22

Minimum energy required for a reaction to occur

Front 23

What's the sign for enthalpy change? What is it measured in?

Back 23

ΔH

kJ/mol

Front 24

Draw an energy profile for the exothermic reaction of CH4 + 2O2 -> CO2 + 2H2O

Back 24

Front 25

Draw an energy profile for an endothermic reaction

Back 25

Front 26

Is the enthalpy change positive or negative for exothermic reactions?

Back 26

Negative

Front 27

Is the enthalpy change positive or negative for endothermic reactions?

Back 27

Positive

Front 28

Define standard enthalpy change of combustion

Back 28

Enthalpy change when

· 1 mol of substance fully reacted w/ excess O2

· under standard conditions 298K & 100kPa

· All substances in their standard state

Front 29

How'd you write standard enthalpy change of combustion of methane in short hand & write an equation to represent this

Back 29

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)

Front 30

Define standard enthalpy change of formation

Back 30

Enthalpy change when

· 1 mol of a compound is formed from its component elements

· under std conditions 298K & 100kPa

· All substances in their standard state

Front 31

Shorthand for standard enthalpy change if formation?

Back 31

Front 32

Define standard enthalpy change of reaction

Back 32

Enthalpy change which occurs when

· equation quantities of materials react

· under standard conditions 298K & 100kPa

· All substances in their standard state

Front 33

Shorthand for standard enthalpy change of reaction?

Back 33

Front 34

Define enthalpy change of neutralisation

Back 34

Enthalpy change when

· 1 mole of water is formed in a neutralisation reaction between an acid and alkali

· under standard conditions 298K & 100kPa

· All substances in their standard state

Front 35

Equation to work out Q, energy released?

Back 35

Q = m × θ × ΔT

Energy released = mass of water × specific heat capacity (4.18J/g/K for water) × temp change

Front 36

How to calc enthalpy change, ΔH

Back 36

Energy change/ mole (remember + or - for endo or exo)

Front 37

Write equation to represent enthalpy of combustion of butane (C4H10)

Back 37

C4H10(g) + 61/2O2(g) -> 4CO2(g) + 5H2O(l)

Front 38

Write equation to represent enthalpy of formation of ammonia

Back 38

1/2N2(g) + 3/2H2(g) -> NH3(g)

Front 39

Define temperature

Back 39

· degree of hotness of substance on some arbitrary scale

· measure of KE of molecules/ particles in substance

· independent of amount of substance present

Front 40

Define heat

Back 40

· Measure of tot en in substance

· depends on amount of substance present

E.g. bucket of water @ 40°C same temp as small beaker of water @ 40°C but bucket contain more heat en

Front 41

Is there a device for measuring the heat in a body directly? What is it?

Back 41

No but you can measure temp change.

Heat transferred = mass * specific heat capacity * temp change

Front 42

Define specific heat capacity

Back 42

Energy needed to raise temp of 1g of a substance by 1K

Units: J/g/K

Front 43

What do you conduct enthalpy change reactions in? What is it?

Back 43

· In a calorimeter

· Piece of apparatus designed to insulate reaction system thermally from its surroundings

Front 44

What is the main source of error in reactions being studied which are slow? How allow for this?

Back 44

· significant heat loss to surroundings

· so temp rise 👀n in calorimeter never as great as should be

· allowance can be made by plotting temp-time graph

Front 45

What does a graph of temp against time look like for an exothermic reaction?

Back 45

Front 46

How to calc % error for an experiment

Back 46

experimental - accepted X 100 __________________________

Expected (data book) value for ΔH

Front 47

What are the sources of error of experiments to determine enthalpy change? And how to minimise them?

Back 47

· heat loss - add lid to polystyrene cup & lag the beaker in which it stands with cotton wool

· hot spots in solution so thermometer doesn't record true temp - stir

· chemicals may be impure or concs inaccurate

Front 48

What apparatus may be used to measure heat given out from a liquid fuel, eg ethanol? What assumptions are made?

Back 48

· assume that all heat produced from burning fuel heats up water

· assume complete combustion occurs

Front 49

What errors occur in experiment to measure heat given out from a liquid fuel? Effect of these errors?

Back 49

· proportion of heat en heats metal can & surrounding air

· flame is affected by draughts

· incomplete combustion can occur, leaving soot on bottom of can

· these errors reduce heat transferred to water and result in a value lower than it should be

Front 50

How does a bomb calorimeter work

Back 50

· substance burned in the "bomb" in excess oxygen to ensure complete combustion

· temp change recorded with thermometer

· then electrical heater used to create exactly same temp change in calorimeter

· so electrical en needed to bring same temp change can be calculated

· this tells us actual en change more accurately as measured en change replicates any heat losses from calorimeter that occurred during the combustion reaction

Front 51

What does a bomb calorimeter look like

Back 51

Front 52

What are main sources of error for experiment to measure enthalpy change of endo reaction

Back 52

Recorded temp not as low as should be as:

thermal en gained from surroundings

Lumps of the solid added may form, meaning reaction rate will decrease

Chemicals may be impure or concs inaccurate

Front 53

Is enthalpy change of a reaction dependent on rate at which chem change occurs?

Back 53

No

Front 54

What's Hess' law

Back 54

Enthalpy change for any chem reaction is indep of intermediate stages, provided initial and final conditions are same for each route

(ie overall en change of a reaction is same regardless of route you take)

Front 55

Is activation energy shown in an enthalpy level diagram or reaction profile diagram?

Back 55

Activation energy is shown in reaction profile diagrams

Front 56

What can be measured that makes it possible to calculate enthalpy changes

Back 56

· energy given out or taken in during chem reaction can be measured

Front 57

What is a calorie

Back 57

The energy needed to raise the temp of 1g of water by 1°C

4.18J

Front 58

Why is a bomb calorimeter more accurate

Back 58

Sample burns completely as excess oxygen

Energy loss is avoided as bomb is surrounded by water, which absorbs the heat from the reaction

(Also stirrer so no heat spots)

Front 59

How can enthalpy changes for reactions in solution be measured?

Back 59

Insulated plastic containers, eg polystyrene (excellent insulator and negligible specific heat capacity) cups as calorimeters

Front 60

Define bond enthalpy

Back 60

Energy required to break one mole of gaseous bonds

Front 61

Define mean bond enthalpy

Back 61

Mean value of the bond enthalpy values for a certain bond averaged across a wide range of compounds

Front 62

Difference between bond enthalpy and mean bond enthalpy?

Back 62

· bond enthalpies are precise values for specific bonds in compounds

· mean bond enthalpies are average values for one kind of bond in diff compounds

Front 63

What's the most important use of mean bond enthalpies

Back 63

· Estimating enthalpy changes in chem reactions involving molecular substances with covalent bonds

· Particularly helpful when experimental measurements can't be made

Front 64

When do significant differences between values of enthalpy change estimated from bond enthalpies and obtained from experiment occur?

Back 64

· if there are variations in strength of 1 kind of bond in diff molecules (mean bond enthalpies shouldn't be used)

· one reactant or product not in gaseous state

Front 65

Which is more accurate: change in enthalpy calcd with:

a) bond enthalpies

b) reaction enthalpies

Back 65

Reaction enthalpies as bond enthalpies are an average value

Front 66

Why isn't it possible to measure the enthalpy change for the decomposition of potassium hydrogencarbonate directly?

Back 66

· heat en must be supplied

· so, temp change isn't solely due to the decomposition

Front 67

Explain why reactions are conducted in a polystyrene cup rather than a glass beaker

Back 67

· better insulator than glass

· less en is lost to/ gained from surroundings

· so temp changes are more accurate