Energetics

Question 1

What is enthalpy change?

Answer 1

the change in heat energy, during a reaction at constant pressure - AH = enthalpy change for a reaction

Question 2

What is AH0

Answer 2

change in enthalpy under standard conditions

Question 3

What are the standard conditions to measure enthalpy change?

Answer 3

pressure - 100KPa temperature - 298K all reactants and products are in their standard states

Question 4

What is an exothermic reaction and its AH value and give examples

Answer 4

a reaction that releases heat energy AH = negative combustion, neutralisation, freezing, condensation

Question 5

What is an endothermic reaction and its AH value and give examples

Answer 5

a reaction that absorbs heat energy AH =positive thermal decomposition, cracking, melting, evaporation

Question 6

What is the definition of The standard enthalpy of combustion?

Answer 6

the enthalpy change when 1 mole of a substance is burned completely in oxygen under standard conditions with all reactants + products in their standard states

Question 7

Why can an equation not be written to represent the standard enthalpy of combustion of petrol?

Answer 7

petrol is a mixture of similar sized alkanes

Question 8

What is the unit of enthalpy change?

Answer 8

KJ/mole

Question 9

What is the definition of The standard enthalpy of formation?

Answer 9

the enthapy change to form 1 mole of a compound from its elements under standard conditions with all reactants and products in their standard state

Question 10

What are the methods of ddetermining unknown enthalpy changes?

Answer 10

1. Calorimetry 2. Bond enthalpy calculations 3. Hess’s law

Question 11

What is calorimetry?

Answer 11

a method of determining AH for a reaction by measuring temperature change

Question 12

What is the equation for heat energy transferred by the reaction (q)

Answer 12

q = m x c x AT where: q is heat energy ….(J) m is mass of substance heated, (g) AT is temperature change (K)

Question 13

State one condition necessary for enthalpies of formation to be quoted as standard values at a specified temperature of 298 K.

Answer 13

pressure is 100KPa

Question 14

Why might the AH of formation and AH of combustion be the same value?

Answer 14

These two enthalpy changes are for the same reaction

Question 15

State the meaning of the term catalyst.

Answer 15

speeds up the rate of reaction AND is chemically unchanged at the end

Question 16

What is the definition of mean bond enthalpy?

Answer 16

1. the energy needed in breaking covalent bonds 2. averaged over different compounds for that bond

Question 17

What is the equation for mean bond enthalpy

Answer 17

ΣB(reactants) – Σ B(products) = ΔH

Question 18

What is the equation fo the standard enthalpies of formation?

Answer 18

ΔHr = ΣΔHf (products) - ΣΔHf (reactants)

Question 19

What is theh definition for bond dissociation enthalpy

Answer 19

the enthalpy change to break 1 mole of covalent bonds in 1 mole of a gasouse molecule

Question 20

Explain why mean bond enthalpies are used

Answer 20

the ehtnalpy to break a bond can vary from one compound to the next

Question 21

Explain the main inaccuarc

Answer 21

The enthalpy to break a covalent bond depends on the chemical enironment. Mean bond enthalpy uses an average from a wide range of compounds