1.4 energetics

Question 1

what is enthalpy change?

Answer 1

heat energy transferred in a reaction at constant pressure.
overall effect of bond making/ breaking

Question 2

units of enthalpy change

Answer 2

KJmol-1

Question 3

what are standard conditions?

Answer 3

100KP (1 atm) pressure and 298K/25c

Question 4

exothermic reactions

Answer 4

give energy to surroundings
temp up
products less energy than reactant
- enthalpy change

Question 5

exo reaction examples

Answer 5

oxidation, respiration, neutralisation

Question 6

endothermic reactions

Answer 6

take in energy from surroundings
temp down
products more energy than reactant
+ enthalpy change

Question 7

endo reaction examples

Answer 7

photosynthesis, decomposition, sodium carbonate+ ethanoic acid

Question 8

what are bond enthalpies?

Answer 8

energy needed to break a bond
atoms in molecules held by covalent bonds so take energy to break and release to make

Question 9

is bond breaking exo or endo?

Answer 9

endo

Question 10

is bond making exo or endo?

Answer 10

exo

Question 11

what is a mean bond enthalpy?

Answer 11

average energy needed to break certain type of bond over range of compounds

Question 12

why do we have MEAN bond enthalpies?

Answer 12

because energy needed depends on where bond is
eg c-c bond needs diff amount of energy in methane than ethane

Question 13

How do you calculate enthalpy change?

Answer 13

total energy absorbed-total energy released
use average bond enthalpies to work these out

Question 14

what are the different types of enthalpy change?

Answer 14

standard enthalpy change of formation
standard enthalpy change of combustion
standard enthalpy change of reaction

Question 15

what is standard enthalpy change of formation?

Answer 15

1 mol of a compound formed from elements in standard states and conditions

Question 16

what is standard enthalpy change of combustion?

Answer 16

1 mol of substances completely burned in oxygen under standard conditions and products and reactants in standard states

Question 17

what is standard enthalpy change of reaction?

Answer 17

when a reaction occurs in molar quantities shown in chemical equation under standard conditions with reactants and products in standard states

Question 18

what do you need to know to work out enthalpy change?

Answer 18

moles of reactant
temperature change
mass of stuff youre heating

Question 19

what is a calorimetry experiment?

Answer 19

experiment measuring heat given out

Question 20

calorimetry and combustion reactions

Answer 20

measure temp change as reaction burns to see energy given out
to find enthalpy of combustion of liquid, burn in calorimeter
fuel burns which heats water. can find energy absorbed if know mass, temp change, specific heat cap of water

Question 21

why may calorimetry of combustion reactions be inaccurate?

Answer 21

not all energy is absorbed by water- some heats surroundings
some fuel may evaporate
combustion may be incomplete (less energy given off)

Question 22

how can you calculate an accurate temperature change?

Answer 22

1. record temp at reg intervals starting couple mins before start of reaction
2. plot graph
3. draw 2 lines of best fit- 1 before reaction, 1 during
4. extend the lines- distance between them at start of reaction is temp change.

Question 23

how do you calculate standard enthalpy change of combustion?

Answer 23

1. q=m x c x temp change
2. convert q from J to KJ
3. calculate mol of fuel from mass reacted
4. enthalpy change=q/n

Question 24

Hess’ law

Answer 24

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

Question 25

When is Hess' law useful?

Answer 25

working out enthalpy change you cant find directly by doing experiment eg if you know enthalpy change for route 2, you can calculate enthalpy change for route 1

Question 26

Why is the value for the enthalpy change of formation of elements 0?

Answer 26

elements are being formed from the element so there is no change in enthalpy alreadyin its standard state

Question 27

steps for using enthalpies of formation

Answer 27

write balanced equation write elements balanced and in standard states underneath draw arrows from elements to equation work out enthalpy change of formation for reactants and products. reverse the arrow going to reactants and change sign then find total route 2 = route 1

Question 28

steps for using enthalpies of combustion

Answer 28

write balanced equation write combustion products underneath draw arrows pointing down and work out total enthalpies for reactants and products reverse second arrow and sign of the value then find total route 2 = route 1

Question 29

why do arrows point upwards when using enthalpies of formation?

Answer 29

enthalpy of formation tells you enthalpy change from elements to compounds

Question 30

Why can we say that the volume of liquid used in calorimetry is equal to the mass?

Answer 30

we can assume that the density is 0

Question 31

practicals in this topic

Answer 31

measuring enthalpy change