3.1.4 Energetics

Question 1

Thermochemistry

Answer 1

Exothermic - energy is given out
Bond breaking - endothermic
Negative energy output

Endothermic - energy has been taken in
Bond making - exothermic
Pos energy output

Question 2

Quantities in endo and exo reactions

Answer 2

Amount of heat given out depends on quantity of reactants
Depends on temperature, pressure or conc of solutions
Usually in Kj mol -1
Need to give the equation
To look at ratios

Question 3

Examples of exothermic and endothermic reactions

Answer 3

Exothermic
Neutralization
Endothermic
Heating copper sulphate heat energy must be put in

Question 4

Cold pack reaction

Answer 4

NH4NO3(s) + aq -> NH4NO3(aq)

Question 5

Enthalpy change

Answer 5

Heat change at a constant pressure
The heat given out to the surroundings while the reaction mixture cools is the enthalpy change
Not over till products back to temp they started at

Question 6

Standard conditions

Answer 6

100kPa
298K

Question 7

Pressures effect in enthalpy conditions

Answer 7

If gas given out energy is required to push away the atmosphere
Greater the atmospheric pressure the more energy required
Meaning less energy available to be given out as heat by reaction

Question 8

Physical states of reactants and products effect on enthalpy change

Answer 8

Heat must be put in to change liquid to gas
And given out when gas is changed to a liquid
Means you have to include state symbols

Question 9

Standard enthalpy of formation

Answer 9

The standard molar enthalpy of formation is the enthalpy change w hen one mole of substance is formed from its constituent elements under standard conditions. all reactants and products being in their standard states.

Question 10

Standard enthalpy of combustion

Answer 10

The standard molar enthalpy of combustion is the enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions, all reactants and products being in their standard states.

Question 11

Relationship between heat and temperature

Answer 11

Temperature is independent if the amount of particles present
Heat is a measure of the total energy of all the particles present in a substance - does matter how many particles present

Question 12

Measuring the enthalpy change of a reaction

Answer 12

You need to know:
Mass of a substance that is being heated up or cooled down
Temperature change
Specific heat capacity of the substance

Question 13

Specific heat capacity

Answer 13

The amount of heat needed to raise the temperature of 1g of substance by 1K
J per g per k
4,18 JgK to raise the temp of 1 g of water by 1 K

Question 14

Enthalpy change equation

Answer 14

Q = mc delta T

Question 15

Calorimeter

Answer 15

You burn the fuel to heat a known mass of water
Then measure the temperature change of the WATER
Assume all heat goes to heating the water
Improve result by reducing heat loss

Question 16

Flame calorimeter

Answer 16

Used for measuring enthalpy change of combustion designed to reduce heat loss even further
Spiral chimney is made of copper , flame is enclosed, fule burns in pure oxygen rather than air

Question 17

Measuring enthalpy change of reactions in solutions

Answer 17

The heat is generated in the solution themself - has to be kept in the colourimeter
Expanded polystyrene beakers are often used in colorimeter - good insulators low heat capacity

Question 18

Measuring the enthalpy change of a Neutralisation reactions

Answer 18

Exothermic give out heat
To measure enthalpy change you used the quantities given out in moles given by the balanced equations
To find the mass add the alkaline and the acids mass together

Question 19

Measuring the enthalpy change of a displacement reaction

Answer 19

When a more reactive metal displaces a less reactive metal
If compound dissolves in water - reaction can be measured in a polystyrene beaker
Metal is the fuel

Question 20

Allowing heat loss

Answer 20

Pot a cooling curve
Best estimate of the temperature immediately after mixing - draw a line of best fit through graph
Extrapolate back to the time of mixing

Question 21

Hesses law

Answer 21

Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
Shown in a thermochemical diagram

Question 22

Thermochemical cycle for the formation

Answer 22

If it’s going to an element on its own - value = 0
arrows go up

Question 23

Thermochemical cycle general structure

Answer 23

Go clockwise from the reactants to the products
If the arrow is going the opposite direction the take it away

Question 24

Thermochemical cycle for combustion

Answer 24

arrows go down

Question 25

Enthalpy of elements

Answer 25

The enthalpies of all elements in their standard states (i.e., the states in which they exist at 298K and IOOkPa) are taken as zero. (298 K and lOOkPa are approximately normal room conditions.)
If element such as pure carbon can exist in a number of states - the most stable allotropes state is used

Question 26

Plot the enthalpy change of combustion against the carbon number

Answer 26

Straight line means enthalpy change of combustion same amount for each extra carbon in the chain

Question 27

Bond enthalpy

Answer 27

A definite amount of energy to a particular bond

Question 28

Bond breaking and making

Answer 28

Bond breaking - endothermic
Bond making - exothermic

Question 29

Bond dissociation enthalpy

Answer 29

Enthalpy change required to break a covalent bond with all species in a gaseous state

Question 30

The mean bond enthalpy

Answer 30

Average amount of energy to make a bond
Slightly different for each molecule
Useful quick and easy to use

Question 31

Comparing the results with that from a thermochemical cycle

Answer 31

Thermochemical cycles give accurate values for bond enthalpies for that molecule
Mean bond enthalpies means you can calculate an approximate means bond enthalpy for a compound never been made

Question 32

In q = mc delta t
To workout the enthalpy of combustion of a fule

Answer 32

It is (mass of WATER) (specific heat capacity if water 4.18)(temperature change)

Divide by 1000 to get kj

Divide by mols of fule to get kj mol -1

Must put a sighn - if water is being heated to a higher temp then heat released by fule so exothermic so neg sighn

Question 33

Reason enthalpy change of combustion is less exothermic than using hess to calculate

Answer 33

Incomplete combustion.
Heat loss

Question 34

Enthalpy change

Answer 34

Heat change at a constant pressure

Question 35

Enthalpy change formualr using bind enthalpies

Answer 35

= bonds broken - bonds made
= reactants - products

Question 36

When bond enthalpy of 0=0 always the same

Answer 36

O2 is the only substance that contains 0=0 bond

Question 37

Table tor ecord the readings needed ot workout the experimental value of combustion

Answer 37

One side
Temp
Initial
Final
Delta T

Other side
Mass g
Burner befor
Burner after
Mass …. Burnt

Question 38

Q = mc delta t or neutralisation

Answer 38

The m is the mass of the acid + base

And to calculate mol dm-3 you have to use the mols of either the acid or base

Question 39

if using (TFinal - Tinital) for delta T rule

Answer 39

then gives you always -q
(stick a neg sigh in front)

Question 40

when doing uncertainty questions on initial and final reading for temperature

Answer 40

you have to do ( 2 x uncertainty/ (final - initial reading ) x 100

Question 41

why heat change CALCULATED FROM A BOMB CALORIMITER EXPERIMENT IS NOT ENTHALPY CHANGE

Answer 41

PRESSURE IS NOT CONSISTNT IN A BOMB CALORIMETER

Question 42

when calculating enthalpy change f combustion of a neutralization what do you have to assume

Answer 42

that they are both starting at the same initial temperature

Question 43

reaction for the enthalpy change of formation equation

Answer 43

the mols of product must be one - so use fractions or what not to fix equation to make sure not multiple mols of product