Enthalpy

Question 1

How to calculate enthalpy change of combustion

Answer 1

1) Calculate energy change (q) of water
q=mc^t

2) Calculate amount, in mol, of substance burn
3) Calculate enthalpy change of combustion

energy change
————————
moles

Question 2

How to calculate bond enthalpy

Answer 2

1) Calculate enthalpy of bonds broken
2) Calculate enthalpy of bonds made
3) Subtract bonds broken from bonds made to calculate bond enthalpy

4) Add in sign for value
+ endothermic

   - exothermic

Question 3

What is the standard enthalpy change of a reaction?

Answer 3

Enthalpy change when the amount shown in the chemical equation react under standard conditions with the reactants and products in their standard states

Question 4

What is enthalpy of neutralisation?

Answer 4

Enthalpy change of a reaction when amounts of an acid and an alkali (as shown in the chemical equation) react under standard conditions

Question 5

What is enthalpy of combustion?

Answer 5

Enthalpy change when ONE MOLE of substance completely burns in oxygen under standard conditions with the reactants and products in their standard states

Question 6

What is enthalpy of formation?

Answer 6

Enthalpy change when ONE MOLE of compound forms from its elements under standard conditions with the elements and the compound in their standard states

Question 7

What is bond dissociation enthalpy?

Answer 7

The enthalpy change on breaking one mole of a particular covalent bond in a gaseous molecule

Question 8

Steps for calorimeter experiment for enthalpy change of combustion

Answer 8

1) Measure mass of spirit burner
2) Add set amount of cold water into a copper calorimeter
3) Suspend calorimeter at fixed distance from spirit burner
4) Measure starting temperature
5) Heat water for fixed amount of time
6) Record mass of burner to calculate mass of fuel burnt
7) Record temperature change of water

Question 9

What do you measure the mass of in enthalpy change calculations?

Answer 9

Mass of the substance that changes temperature

NOT REACTANTS

Question 10

Hess Cycle

Answer 10

Write card

Question 11

What signs do you put in front of exo and endothermic reactions?

Answer 11

Exothermic -

Endothermic +

Question 12

What things reduce accuracy of calorimeter experiment?

Answer 12

• Heat loss to surroundings other than water (lid to reduce this)
• Incomplete combustion
• Evaporation of fuel from wick

Question 13

What affect do the factors reducing accuracy have on the energy change value?

Answer 13

Lead to an enthalpy change value which is less exothermic than expected

Question 14

Where does the heat go in exothermic reactions?

Answer 14

Release energy to the surroundings

Question 15

Why do exothermic reactions have a negative enthalpy change?

Answer 15

Because the products have less energy than the reactants

Question 16

Where does the heat go in endothermic reactions?

Answer 16

Take in heat from the surroundings

Question 17

Why do endothermic reactions have a positive enthalpy change?

Answer 17

Because the products have more energy than the reactants

Question 18

What effect does a catalyst have on the value of an enthalpy change?

Answer 18

No effect

Question 19

What is the shorthand version of writing activation energy?

Answer 19

Ea

Question 20

What is activation energy?

Answer 20

The minimum energy required for a reaction to take place

Question 21

What is enthalpy?

Answer 21

The amount of energy that a substance contains

Question 22

What is the enthalpy change value for all neutralisation reactions?

Answer 22

-57kJmol-1

Question 23

What equation can all neutralisation reactions be simplified to?

Answer 23

H+ (aq) + OH- (aq) ——> H2O(l)

Question 24

What is the method of experiment to calculate the enthalpy change of neutralisation?

Answer 24

1. Allow equal volumes and conc. of acid and alkali to reach the same temp in separate containers
2. Measure temperature of acid and alkali
3. Place both solutions in polystyrene cup
4. Measure max temperature reached

Question 25

What are the limitations of enthalpy change of neutralisation practical?

Answer 25

• Heat lost to surroundings
• Experiments carried out under non-standard conditions
• SHC of cup not take in account

Question 26

What is a cooling curve used for?

Answer 26

Used to correct for heat loss by use of a cooling curve correction

Question 27

How to draw a cooling curve?

Answer 27

• Extrapolate cooling curve section of the graph back to the time when the substance was added
• Draw a vertical line from the time the solution was mixed to the extrapolated cooling curve

Question 28

What is Hess’ Law?

Answer 28

If a reaction can take place by more than one route, the total enthalpy change is the same for each route

Question 29

How to calculate percentage error?

Answer 29

Degree of accuracy
——————————– x 100
Measurement taken

Question 30

Why do you sometime times the degree of accuracy by two when calculating percentage error?

Answer 30

If you’ve taken two readings from the piece of equipment

Question 31

What does an energy profile look like for an exothermic reaction?

Answer 31

• Product level below reactant level
• Difference between two lines labelled enthalpy change
• Peak labelled activation energy from reactants line (top one) to top of peak

Question 32

What does an energy profile look like for an endothermic reaction?

Answer 32

• Product level above reactant level
• Difference between two line labelled enthalphy change
• Peak labelled activation energy from reactants line (bottom one) to top of peak

Question 33

What are the standard conditions?

Answer 33

100kPa

298K