OCR A CHEMISTRY MODULE 3

Question 1

Enthalpy and Standard Conditions

Answer 1

change of heat in a reaction at CONSTANT TEMP & PRESSURE ,
Standard Conditions:
100 KPa
298K

Question 2

Endothermic & Exothermic Reactions

Answer 2

Endothermic: ABSORB energy from Surroundings , POSITIVE enthalpy change , Reactants LOWER than Products

Exothermic: RELEASE energy to surroundings , NEGATIVE enthalpy change , Reactants HIGHER than Products

Question 3

Bond Making & Breaking

Answer 3

Bonds broken :in REACTANTS= Endothermic reaction , if there is more energy to BREAK bonds than energy RELEASED when FORMING BONDS = Endothermic

Bonds MADE: when PRODUCTS , are Produced = Exothermic , if more energy is RELEASED when bonds are FORMED then needed to BREAK them = Exothermic

Question 4

types of enthalpy changes

Answer 4

Standard enthalpy change of reaction: number of MOLES in BALANCED equation REACT together in STANDARD conditions

Standard enthalpy change of neutralisation : ACID + ALKALI REACT forming 1 mole of WATER in STANDARD conditions

Question 5

types of enthalpy changes

Answer 5

Standard enthalpy change of combustion : 1 MOLE of a SUBSTANCE is completely BURNT in OXYGEN forming CO2 + H2O under STANDARD conditions

Standard enthalpy change of formation: 1 MOLE of a COMPOUND is formed from its ELEMENTS in STANDARD conditions

Question 6

Mean Bond enthalpy

Answer 6

MEAN bond enthalpy : energy REQUIRED to BREAK 1 MOLE of SPECIFIED type of bond in ‘GASEOUS’ molecule

change in enthalpy = TOTAL energy needed to BREAK bonds ( ENDothermic) - TOTAL energy needed to MAKE bonds (EXothermic)

Question 7

Determining of enthalpy change of combustion via experiment

Answer 7

• Measure EXACT volume of WATER , POUR in BEAKER, measure INITIAL TEMP ,
• add FUEL to spirit BURNER under BEAKER & LIGHT & STIR with THERMOMETER
• EXTINGUISH FLAME after ‘3 mins ‘ IMMEDIATELY RECORD TEMP , REWEIGH spirit BURNERReasons for experimental errors:
• LESS EXothermic than expected
• INcomplete combustion
• NON standard conditions

Question 8

Calculating Enthalpy changes

Answer 8

Q=  M x  C  x CHANGE IN TEMP
Q= energy 
M= Mass
C= specific heat capacity ( 4.18j/kg)
Enthalpy = Q/Moles  NO UNIT

Question 9

Determining Enthalpy change of reaction via experiment

Answer 9

• Carry out reaction in Polystyrene cup ( good insulator) with THERMOMETER clamped

Cooling curves to CORRECT heat LOSS in enthalpy experiment:

• add 1 reactant to polystyrene cup
• take temp every 30 secs until temp is CONSTANT
• add OTHER reactant STIR, measure temp every 30 secs until temp DECREASES
• plot graph temp against time

For enthalpy change in neutralisation : use TOTAL volume of BOTH reactants & MOLES of reactant NOT in EXCESS

Question 10

Hess’ Law (Formation cycle)

Answer 10

TOTAL energy of a reaction is INDEPENDENT of ROUTE taken
formation cycle:

Reactants Products - SUBstitute formation SYMBOL for NUMBERS
- MULTIPLY by number of MOLES IN EQUATION
Elements in standard
conditions - going AGAINST arrow = change sign to NEGATIVE

Question 11

Hess’ Law (Combustion cycle)

Answer 11

Reactants Products. - SUBstitute combustion SYMBOL for NUMBERS
- MULTIPLY by number of MOLES IN EQUATION
combustion
products - going AGAINST arrow = change sign to NEGATIVE