energetics

Question 1

what is an exothermic process

Answer 1

energy is released from the chemical to the surroundings during the process
the temperature of the surroundings increase
exo-energy exits

Question 2

what is an endothermic process

Answer 2

energy is taken in by the chemical from the surroundings during the process
the temp of the surroundings decrease
en-energy enters

Question 3

definitions for enthalpy change

Answer 3

enthalpy change is the change in heat energy at constant pressure

Question 4

for exothermic processes the sign for deltaH is

Answer 4

negative

Question 5

for endothermic processes the sign for deltaH is

Answer 5

positive

Question 6

what are the standard conditions

Answer 6

100 KPa ( or 1 atmosphere, 1 atm) pressure
298 (25c) temp
1 mol dm-3 concentration for all solutions

Question 7

bend and mex

Answer 7

breaking bonds - endothermic
making bonds - exothermic

Question 8

definition of activation energy

Answer 8

the minimum energy needed to start a reaction

Question 9

mean bond enthalpy

Answer 9

a mean bond enthalpy is the energy required to break 1 mol of a covalent bond into gaseous atoms averaged over a range of different compounds

Question 10

mean bond enthalpies are ..

Answer 10

endothermic process and have a positive deltaH sign

Question 11

standard enthalpy of formation

Answer 11

the enthalpy change when 1 mol of substance is formed
from its constituent elements under standard conditions
with all reactants and products being in their standard states

Question 12

what does standard enthalpy of formation need to show

Answer 12

formation of 1 mol

Question 13

the standard enthalpy of formation of all elements in their standard states is …

Answer 13

zero

Question 14

standard enthalpy of combustion

Answer 14

the enthalpy change when 1 mol of a substance is burnt completely in excess oxygen
under standard conditions and all reactants and products in their standard states

Question 15

substances that cant be combusted

Answer 15

have 0 enthalpy of combustion

Question 16

why may the enthalpy of combustion be difficult to measure

Answer 16

incomplete combustion may occur

Question 17

hess law

Answer 17

hess’ law states that the enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products

Question 18

bond enthalpy hess cycle

Answer 18

reactants -> products
both arrows face down to gaseous atoms

BERP-deltaH=sum of reactants - sum of products

Question 19

formation enthalpies hess cycle

Answer 19

reactants -> products
both arrow face up from gaseous atoms

deltaH = sum of products - sum of reactants

Question 20

combustion enthalpies hess cycles

Answer 20

reactants -> products
both arrows face down to gaseous atoms

CRP=deltaH = sum of reactants - sum of products

Question 21

Q = mcdeltaT

Answer 21

Q=energy(J)
m=mass of water (surroundings) g
c = specific heat capacity ( usually 4.81)
delta t = change in temp Celsius

Question 22

Q to enthalpy change ( deltaH)

Answer 22

deltaH=Q/mols
Q needs to be divided by 1000 from J to KJ

Question 23

Assumption made in calorimetry

Answer 23

The energy transferred to the water from the reacting chemicals (or vice versa for endothermic reactions) is equal to the energy released (or taken in) by the reaction.

Question 24

sources of error in combustion calorimetry

Answer 24

• Heat loss to the surroundings
• Incomplete combustion of the fuel
• Heat energy transferred to the metal calorimeter
• Some fuel evaporates

Question 25

sources of error in solution calorimetry

Answer 25

heat loss to surroundings

Question 26

improvements to minimise sources of error in combustion calorimetry

Answer 26

• Add a lid – reduces heat loss
• Insulate sides of calorimeter – reduces heat loss
• Reduce distance between flame and beaker – reduces heat loss
• Put sleeve around flame to protect it from draughts

Question 27

improvements to minimise sources of error in solution calorimetry

Answer 27

• Add a lid – reduces heat loss
• Insulate calorimeter – reduces heat loss

Question 28

Steps to Measure an Enthalpy Change Using a Cooling Curve

Answer 28

• Record the temperature for a suitable time (3 minutes) before adding reactants together
• To establish an accurate initial temperature
• Mix reactants then record temperature every minute until a trend is seen
• Plot a graph of temperature against time
• Extrapolate the cooling curve back to the point of addition
• To establish a theoretical temperature change accounting for heat loss