1.4 Energetics

Question 1

What is enthalpy change?

Answer 1

Heat energy change at a constant pressure

Question 2

How does a thermometer in a hot substance?

Answer 2

It absorbs heat energy from the liquid so the particles gain more kinetic energy and move apart
The space between the particles increases and the substance therefore becomes less dense
The particles will therefore rise showing a temperature increases

Question 3

How does a thermometer work in a cool substance?

Answer 3

Little to no heat supplied means the particles lose kinetic energy and move closer together
The energy moves out of the thermometer and into the substance
The space decreases between the particles and become more dense
The particles will then sink showing a decrease in temperature

Question 4

What are the two systems in a chemical reaction?

Answer 4

Chemical system - reactants and products only

Surrounding system - everything! Hand, beaker, atmosphere, thermometer etc…

Question 5

What does a thermometer record?

Answer 5

The temperature of the surroundings

Question 6

What is the endothermic part of a reaction?

Answer 6

Where bonds are broken

The activation energy

Question 7

What is an endothermic reaction?

Examples?

Answer 7

Reactions which take in heat energy from the surroundings as the reaction takes place

Results in a positive delta H ie increase in enthalpy change

Photosynthesis

Question 8

What does an endothermic reaction result in?

Answer 8

A decrease in temperature surroundings

Question 11

What is an exothermic reaction? Examples?

Answer 11

Reactions in which heat energy is released to the surroundings as the reaction takes place

Results in a negative delta H ie decrease in enthalpy change

Respiration and Combustion

Question 12

What are the standard enthalpy change symbols?

Answer 12

H - enthalpy
Δ - change
θ - standard

ΔH ^θ

Question 13

What are standard conditions?

Answer 13

Temperature - 298k

Pressure - 100kPa

Question 14

What is standard enthalpy change? ΔH ^θ

Answer 14

Heat energy change at a constant pressure and under standard conditions

Question 15

What is standard enthalpy of reaction? ΔH ^θr

Answer 15

The enthalpy change for a chemical reaction under standard conditions and all substances in their standard states

Question 16

What is standard molar enthalpy of combustion? ΔH ^θc

Answer 16

The enthalpy change that takes place when 1 mole of a substance completely reacts with excess oxygen under standard conditions and all reactants and products being in their standard state

Question 17

What is standard molar enthalpy of formation? ΔH ^θf

Answer 17

The enthalpy change that takes place when 1 mole of a substance is formed from its constituent elements under standard conditions with all reactants and products in their standard states

Question 18

What does an exothermic reaction result in?

Answer 18

An increase in temperature surroundings

Question 19

What is the exothermic part of a reaction?

Answer 19

Bond making - to produce products

Question 20

What are mean bond enthalpies?

Answer 20

The average energy change required to break 1 mole of a particular covalent bond with all species in the gaseous state averaged over many different molecules

Question 21

What are bond enthalpies?

Answer 21

The energy change required to break 1 mole of a particular covalent bond with all species in the gaseous state

Question 22

What is the equation to work out (mean) bond enthalpies?

Answer 22

ΔH = ∑ (mean) bond enthalpies of reactants - ∑ (mean) bond enthalpies of products

Question 23

What is Hess’ law?

Answer 23

The enthalpy change for a reaction is independent of the reaction pathway taken as long as the starting and ending reaction conditions are the same

Question 24

Give a general summary of a Hess cycle?

Answer 24

The enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products

ΔH1 = ΔH2 + ΔH3 (as ΔH1 may be too difficult to work out)

Question 25

When working out enthalpy changes what must we do?

Answer 25

Look at the DATA provided!!!
To determine the equation

(Not the balanced equation)

Question 26

What equation is used for working out enthalpy of formation?

Using COMBUSTION DATA?

Answer 26

ΔH^θf =

∑ ΔH^θc reactants - ∑ ΔH^θc products

Question 27

What equation is used for working out enthalpy of combustion?

Using FORMATION DATA?

Answer 27

ΔH^θc =

∑ ΔH^θf products - ∑ ΔH^θf reactants

Question 28

Points about constructing Hess cycles?

Answer 28

• All arrows go away from elements
• If elements are reactants ‘magical oxygen’ is used to make compounds
• If reactants are compounds it will go back to its elements for ‘underneath’ reaction
• if large product on right arrow will go to many products underneath

Question 29

What to remember in combustion reactions?

Answer 29

Oxygen is a reactant

Question 30

What are the parts of an enthalpy level diagram?

Answer 30

Enthalpy in KJ up the side
Time in seconds (s) across the bottom
Activation energy
Overall enthalpy change KJ mol^-1 (+ or -)

Question 31

What is the calorimetry calculation?

Answer 31

q = mc ΔT

q = heat enthalpy (J)
m = mass of water (ml)
c = specific heat capacity 
ΔT = temperature change (degrees C)

Question 32

What is important to remember about q (heat enthalpy)

Answer 32

You have to put (+ or -) depending on if it was exothermic or endothermic

Look at the ΔT to work out

Question 33

What is standard enthalpy of neutralisation?

Answer 33

The enthalpy change that accompanies the formation of one mole of water from an acid-base/alkaline reaction with all reactants and products in their standard states and under standard conditions

Question 34

What are some issues with ‘coffee cup’ calorimetry?

Answer 34

Heat loss
Incomplete reaction
Not in standard conditions

Question 35

What are some advantages of a bomb calorimeter over a combustion calorimeter?

Answer 35

Well insulated
Pure oxygen = complete combustion

Leads to very accurate combustion values fr food packets

Question 36

How to calculate ΔT from a graph?

Answer 36

Draw a vertical line up where the reaction started
Draw a line of best fit from the high values to your vertical line (no anomalies) (can be straight or curved)
Draw a line of best fit from the low values to your vertical line

Where your lines of best fit hit the vertical line - ΔT is the difference between those values

Question 37

If you have to draw a graph to determine ΔT, what features would it have?

Answer 37

Temperature up the side
Time across the bottom
Often broken axis as temp doesn’t need to start at zero

Vertical line where the reaction starts

Question 38

What do you have to in molar enthalpy of neutralisation calculations?

Answer 38

q = mc ΔT

Then work out the moles (usually from concentration and volume)
Ratio to get the Joules to 1 mole

Convert to KJmol^-1