energetics

Question 1

what is enthalpy change

Answer 1

energy heat change in a reaction at a constant pressure + temp

Question 2

what are standard conditions of enthalpy change reactions

Answer 2

100kPa
298K (25 degrees celcius)

Question 3

what is an endothermic reaction

Answer 3

reaction that absorbs energy from surroundings
delta H = positive

Question 4

what is an exothermic reaction

Answer 4

reaction that releases energy to the surroundings
delta H= negative

Question 5

what is ^rH

Answer 5

standard enthalpy change of reaction

change of enthalpy during formation of 1 mole of the substance from its constituent elements under standard conditions

Question 6

what is ^neutH

Answer 6

standard enthalpy change of neutralisation

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

Question 7

what is ^cH

Answer 7

standard enthalpy change of combustion

the enthalpy change when 1 mole of a substance is completely burned in oxygen to make co2 and h2o under standard conditions

Question 8

what is ^fH

Answer 8

standard enthalpy change of formation

the enthalpy change when 1 mole of a compound is formed from its elements in standard states under standard conditions

Question 9

what is bond breaking

Answer 9

energy is absorbed
endothermic process
^H is positive

Question 10

what is bond making

Answer 10

energy is released
exothermic process
^H is negative

Question 11

what is bond enthalpy

Answer 11

amount of energy needed to break 1 mole of a bond type in a molecule in the gaseous state

Question 12

what is Hess’s Law

Answer 12

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

Question 13

how to work out overall enthalpy change

Answer 13

energy to break bonds + energy to make bonds

Question 14

what is calorimetry

Answer 14

an experimental method for finding enthalpy change by measure temp change over time, data can be extrapolated

Question 15

what is specific heat capacity

Answer 15

the energy required to raise 1g of substance by 1K without a change of state

Question 16

what is the equation for energy change

Answer 16

q=MC delta T

Question 17

what is the equation for enthalpy change per mol

Answer 17

delta H = q/moles

Question 18

why is calorimetry prone to errors

Answer 18

heat can be lost from reaction by conduction, convection or inaccuracies in measuring temperatures

specific heat capacity=4.18 however this is of just the water and not the actual solution
the shc of calorimeter is not taken into account

Question 19

how can you reduce heat loss to surroundings

Answer 19

put lid on calorimeter
insulating outsides of calorimeter with polystyrene

Question 20

what is Hess Law

Answer 20

energy in a reaction system must be conserved, as energy cannot be created or destroyed. therefore the overall enthalpy change for a reaction is the same, regardless of the route taken

Question 21

what ways do the arrows go in enthalpy change of formation hess cycle

Answer 21

both pointing upwards

Question 22

what ways do the arrows point in enthalpy change of combustion

Answer 22

both downwards , with products always being H2O and CO2

Question 23

what is the definition of bond enthalpy

Answer 23

the energy required to break one mole of the stated bond in a gaseous state under standard conditions

Question 24

why are calculated bond enthalpies different to databook values

Answer 24

the same covalent bond but in different environments often have slightly different bond enthalpies, and the databook is averaged values