topic 8

Question 1

what is enthalpy and what is it measured in

Answer 1

heat change in a raction
units = KJmol^-1

Question 2

what are the standard conditions of enthalpy change

Answer 2

100 KPa
298 K

Question 3

draw and endothermic reaction profile

Answer 3

reactants lower than products

Question 4

what is endothermic and example

Answer 4

absorb energy from surroundings
eg. thermal decomposition

Question 5

what is the enthalpy change for an endothermic reaction

Answer 5

positve

Question 6

draw an exothermic reaction profile

Answer 6

reactants higher than products

Question 7

what is an exothermic reaction and example

Answer 7

release energy to surroundings
eg. combustion

Question 8

what is the enthalpy change for an exothermic reaction

Answer 8

negative

Question 9

what is the standard ethalpy chnage of reaction

Answer 9

the enthalpy change of a reaction according to the molar quantities in the equation under standard conditions

Question 10

what is the standard enthalpy change of neutralisation

Answer 10

the enthalpy change when an acid and alkali react to form 1 mole of water under standard conditions

Question 11

what is the standard enthalpy change of formation

Answer 11

the ethalpy change when 1 mole of compound is formed from its elements in their standard conditions

Question 12

what is the standard enthalpy change of combustion

Answer 12

the enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H2O under standard conditions

Question 13

draw a reaction profile for bonds broken

Answer 13

reactants lower than products
see flash card

Question 14

what happens when bonds are broken in terms of energy and enthalpy

Answer 14

energy absorbed
endothermic
positive enthalpy
if more energy is needed to break bonds that energy given out when bonds formed = endothermic

Question 15

draw and bonds made energy reaction profile

Answer 15

reactants higher than products
arrow down
see flashcard

Question 16

what happens when bonds are made in terms of energy and enthalpy

Answer 16

energy released
exothermic
negative enthalpy
if more energy is required when bonds are formed than what is needed to break = exothermic

Question 17

how to work out enthalpy chnage in terms of bonds broken and made
include units

Answer 17

= total energy to break bonds - total energy released forming bonds
units = KJ mol^-1

Question 18

what is calorimetry used for

Answer 18

work out enthalpy change of combustion

Question 19

describe the process of calorimetry

Answer 19

• a fuel is burnt to raise the temp of water to be a specific amount, weigh fuel before and after to work out mass burnt
• energy transfered to water although some is lost to the surroundings
• a lid is placed on top to prevent heat loss and wind sheilds around flame to prevent draught moving flame

Question 20

how to work out enthalpy from calorimetry with units of all parts of equation

Answer 20

q = mcAT
q- energy lost/ gained (J)
m- mass od water (g)
c- specific hear capacity of water ( 4.18Jg^-1K^-1)
T - temp change (K)

enthalpy = q/ moles
moles - fuel mass/Mr

Question 21

what is hess’ law

Answer 21

the total enthalpy change of a reaction is independent to the route taken

Question 22

what would a hess cyle of formation look like

Answer 22

see flash card
arrows up with formation data

Question 23

how do you work out enthalpy change using hess cycles

Answer 23

go with the arrow - keep sign same
go against arrow - change sign

Question 24

draw a hess cycle of combustion

Answer 24

see flash card
arrows down