3.1.4- Energetics (PAPER 1+2)

Question 1

What is enthalpy? What are its units?

Answer 1

A measure of the amount of energy that is stored in a system- this cannot be measure directly.

Measured in KJmol-1

Question 2

What letter is enthalpy represented by and what is enthalpy change represented by?

Answer 2

Enthalpy= H
Enthalpy change= ΔH

Question 3

What happens due to the enthalpy of the reactants and products not being the same?

Answer 3

Energy is either given out (exothermic) or taken in (endothermic) during the reaction. This is the enthalpy change.

Question 4

Overall enthalpy change equation?

Answer 4

energy to make bonds + energy to break bonds

Question 5

Definition of enthalpy change?

Answer 5

The change in heat energy of a substance at constant pressure and temperature.

Question 6

What is the system, the surroundings and the universe regarding the conservation of energy?

Answer 6

The system- the reactants and products.

The surroundings- the equipment and the immediate surrounding space.

The universe- both the system and surroundings.

Question 7

What is a positive enthalpy change? Draw the energy level diagram.

Answer 7

+ enhalpy change value= endothermic.

Energy is transferred from the surroundings to the system (energy taken in)

The temperature decreases.

Diagram- products have higher enthalpy than reactants- Ea is from reactants to peak of products.

Question 8

What is a negative enthalpy change? Draw the energy level diagram.

Answer 8

• enthalpy change= exothermic.

Energy is transferred from the system to the surroundings (energy given out)

Temperature increases.

Diagram- products have lower enthalpy than reactants- Ae is from peak of reactants to bottom of reactants.

Question 9

Where are bonds broken/ made?

Answer 9

Bonds broken in reactants- energy taken in.

Bonds made in products- energy released.

Question 10

When is a reaction exothermic?

Answer 10

When the energy released from the bonds being made (products) is greater than the energy taken in from bonds being broken (reactants).

Question 11

When is a reaction endothermic?

Answer 11

When the energy to break the bonds (reactants) is greater than the energy released when the bonds are made (products).

Question 12

What is activation energy - Ea?

Answer 12

The minimum energy required for a reaction to take place.

Certain energy needs to be achieved to break the bonds of the reaction.

Question 13

What is a standard enthalpy change?

Answer 13

If the enthalpy change is measured under standard conditions of temperature and pressure.

ΔH°

Question 14

What are the standard conditions?

Answer 14

Standard pressure= 100 kPa
Temperature= 298K
Standard concentration= 1moldm-3

Question 15

What is the standard enthalpy of formation?

Answer 15

The enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.

MUST BE ONE MOLE AT THE END OF THE EQUATION!!!

Question 16

Write the equation for enthalpy of formation of CH4 and enthalpy of formation of NH3.

Answer 16

C(s) + 2H2 (g)—–> CH4(g)

1/2 N2(g) + 3/2 H2 (g) —–> NH3 (g)

Question 17

What is standard enthalpy of combustion?

Answer 17

The enthalpy change when 1 mole of substance is completely burnt in excess oxygen under standard conditions, all reactants and products in their standard states.

Question 18

Write the equation for enthalpy of combustion of C2H6 and enthalpy of combustion of C6H14.

Answer 18

C2H6 (g) + 7/2 O2(g) ——-> 2CO2 (g) + 3H2O (l)

C6H14(g) + 19/2 O2(g) ——–> 6CO2(g) + 7H2O (l)

Question 19

What is standard enthalpy of atomisation?

Answer 19

The enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state, under standard conditions.

Question 20

What is standard enthalpy of neutralisation?

Answer 20

The enthalpy change when 1 mole of water is formed when solutions of an acid and an alkali react together under standard conditions.

This is always - as heat is released.

Question 21

Write the equation for enthalpy of neutralisation of H2SO4 (aq) and NaOH (aq)

Answer 21

1/2 H2SO4 + NaOH (aq) —–> 1/2 Na2SO4 (aq) + H2O (l)

Question 22

Describe a method to find enthalpy change of combustion of a fuel?

Answer 22

1- Measure 100cm3/100g of water into a calorimeter.
2- Weigh a spirit burner containing the liquid/fuel to be burnt.
3- Measure initial temp. of the water using a thermometer.
4- Use spirit burner to heat the water.
5- Stop heating after reasonable temp. change.
6- Reweigh spirit burner then use weigh by difference.
7-Calculate temp change of reaction and mass of fuel burnt (difference between spirit burner masses)
8-Use q=mcΔt, calculate moles of fuel by mass/mr
9- Change J into kJ and then divide by moles

Question 23

Why is the value for enthalpy of combustion lower than in data books?

Answer 23

-Heat losses to the surroundings from the spirit burner, wick and calorimeter.
-Incomplete combustion of the fuel.
-Reaction did not occur in standard conditions.
-Loss of water by evaporation.

Question 24

How to improve simple calorimetry experiment?

Answer 24

-Use lid on calorimeter to reduce heat loss to the surroundings.
-Minimise the distance between the flame and the calorimeter.
-Insulate calorimeter and spirit burner to reduce heat loss.
-Burn in a supply of pure oxygen to reduce incomplete combustion.

Question 25

How to calculate enthalpy change?

Answer 25

q=mcΔt energy change (J)= mass (g) x SHC x temperature change (K)

Question 26

How to find ΔH solution?

Answer 26

This can be measured using insulated plastic cups as a calorimeter.

Question 27

Describe a method to find enthalpy change of neutralisation?

Answer 27

1- Place polystyrene cup in glass beaker for support. 2- Rinse measuring cylinder with 1moldm-3 HCl then measure 25cm3 and transfer into the cup. 3-Stir acid with thermometer and measure temp. 4- Rinse second measuring cylinder with 1moldm-3 NaOH (aq), measure 25cm3. 5- Add NaOH to acid, stir, record highest temp. reached. Highest temp-lowest temp. 6- Add the solution volumes together= mass (50cm3), sub into q=mcΔt, divide by 1000 7- If moles are the same, divide by the moles. 8- If moles are different, use the limiting reagent: moles= mass/mr or volume (dm3) x conc. (moldm-3)

Question 28

Describe a method to find enthalpy change for a displacement reaction?

Answer 28

1- Weigh out roughly 2g zinc (in excess) 2- Pipette 25cm3 0.500moldm-3 copper sulphate solution into dry polystyrene cup in a beaker. 3- Place lid on cup, put under tripod and put thermometer through hole. Stir + record temp every minute. 4- At 4 minutes, add zinc and continue stirring and recording temp. until 15mins. 5- Extrapolate graph of temperature against time back to 4 minutes to find maximum rise. 6- If you do not know which reactant is the limiting one, calculate moles for both. kJ/smallest moles

Question 29

Write the equation for the displacement reaction between copper sulphate solution and zinc?

Answer 29

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

Question 30

What is the definition of Hess' law?

Answer 30

The overall enthalpy change for a reaction is independent of the route the reaction takes at constant pressure.

Question 31

Draw the formation cycle using Hess' law

Answer 31

Reactants-----> Products elements elements have 2 arrows facing up towards reactants and products

Question 32

Draw the combustion cycle using Hess' law

Answer 32

Reactants------> Products combustion products (CO2 + H2O) combustion products have 2 arrows going down from reactants and products

Question 33

Calculate the enthalpy change using formation enthalpies of SO2 + 2H2S------> 3S + 2H20 SO2= -297, H2S= -20.6, H20= -286

Answer 33

can use products-reactants -572--338.2= -233.8 OR hess' cycle CHANGE LEFT ARROW FROM GOING UP TO GOING DOWN sign of -338.2 changes to +338.2 -572+338.2 =-233.8

Question 34

Calculate the enthalpy change using combustion enthalpies of C+H2 + O2--------------->C2H5OH C2H5OH= -1367, C= -394, H2= -286

Answer 34

balance equation 2C+ 3H2+ 1/2 O2-----> C2H5OH can use reactants-products -1646--1367= -279 OR hess' cycle CHANGE RIGHT ARROW FROM GOING DOWN TO GOING UP sign of -1367 changes to +1367 -1646 + 1367= -279

Question 35

What is the equation for enthalpy change of formation?

Answer 35

(sum of ΔfH products) - (sum of ΔfH reactants) products-reactants

Question 36

What is the equation for enthalpy change of combustion?

Answer 36

(sum of ΔcH reactants) - (sum of ΔcH products) reactants-products

Question 37

What is the definition of mean bond enthalpy?

Answer 37

The average (mean) bond dissociation enthalpy for a particular bond in a range of different compounds.

Question 38

Why may experimental enthalpy values differ from calculated mean bond enthalpies?

Answer 38

The mean bond energies are averages and not compound-specific values.

Question 39

What is the equation for ΔH involving bond energies? ALL REACTANTS AND PRODUCTS MUST BE GASES

Answer 39

(sum of ΔH for breaking bonds) - (sum of ΔH for making bonds) bond breaking= endothermic (+) occurs in reactants bond making = exothermic (-) occurs in products REACTANT BONDS- PRODUCT BONDS

Question 40

Calculate enthalpy of combustion for CH4 +2O2 -----> CO2 +2H20 O=O is 496 C=O is 743 C-H is 412 O-H is 463

Answer 40

break-make 2640-1852= -698 kJmol-1

Question 41

Definition of enthalpy change as applied to a chemical reaction?

Answer 41

The amount of heat energy transferred during a chemical reaction at constant pressure.

Question 42

If it asks you to find enthalpy of combustion using standard enthalpies of formation what should you use?

Answer 42

products- reactants as you are using the enthalpies of formation

Question 43

What is total heat capacity in q=mc ∆t?

Answer 43

Total heat capacity is both m and c, the mass and specific heat capacity combined.

Question 44

If it asks you to find enthalpy of formation using combustion data what should you use?

Answer 44

reactants- products as you have combustion data