3.1.4 Energetics

Question 1

what is enthalpy change?

Answer 1

the heat energy change measured under conditions of constant pressure

Question 2

what is standard enthalpy change?

Answer 2

the heat energy change measured under conditions conditions.
100kPa 298K

Question 3

what is the symbol for enthapy change

Answer 3

∆H

Question 3

what is an exothermic reaction

Answer 3

energy is transferred from the system to the surroundings

Question 3

in the exothermic energy diagram what has less energy

Answer 3

the products

Question 4

what are common exothermic processes

Answer 4

combustion of fuels
oxidation of carbohydrates

Question 5

is (∆H) positive or negative in an exothermic reaction

Answer 5

negative

Question 6

why do the products in an exothermic reaction have less energy

Answer 6

because the neat energy is being lost

Question 7

what is an endothermic reaction

Answer 7

energy is transferred from the surroundings to the system

Question 8

what has the least amount of energy in an endothermic reaction

Answer 8

the reactant

Question 9

is (∆H) positive or negative in an endothermic reaction

Answer 9

its positive

Question 10

what type of reaction is bond breaking

Answer 10

endothermic process
(cause it requires energy)

Question 10

what type of reaction is bond breaking

Answer 10

exothermic reaction
(energy is released)

Question 10

what is the activation energy

Answer 10

the minimum amount of energy required for a reaction to occur.

Question 11

Define standard enthalpy of formation

Answer 11

enthalpy change when 1 mole of a compound is formed from its constituent elements understandard conditions. (298K and 100kPa)
All reactants and products being in their standarrd states.

Question 12

define standard enthalpy of combustion

Answer 12

the enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions.(298k, 100kPa)
all the reactants and products being in their standard states.

Question 13

what are the errors during the calorimetry experiment

Answer 13

• incomplete combustion of the fuel
• energy loss from calorimeter
• evaporation of fuel afteer weighing

Question 14

what is hess’s law

Answer 14

the total enthalpy change is independant of the route by which the chemical change takes place.

Question 15

what are the units for energy change

Answer 15

J

Question 16

what are the units for enthalpy change

Answer 16

kJ

Question 17

with calorimetry equations what musn’t you forget

Answer 17

• the negative sign
• and converting from J to kJ by dividing by 1000

Question 18

what type of reactions are the calorimetry practical

Answer 18

exothermic reactions

Question 19

how do we know that the calorimetry practical is exothermic

Answer 19

there is a temperature increase

Question 20

remember the temperature change units dont matteer

Question 21

what are the 3 steps that you need to take for calorimetry questions

Answer 21

1. calculate Q
2. calculate the moles
3. divide Q/moles

Question 22

why do the experimental and theeoretical values differ

Answer 22

• heat loss
• incomplete combustion
• fuel is evaporated
• impurities

Question 23

define mean bond enthalpy

Answer 23

the enthalpy needed to break the covalent bonds into gaseous atoms, averaged over different molecules

Question 24

Explain why values from mean bond enthalpy calculations differ from those determined using
Hess’s law.

Answer 24

the mean bond enthalpy values are average values of bond enthalpies from various compounds.

Question 25

what equation is often used for mean bond enthalpy

Answer 25

total bond energies broken - total bond energies made

Question 26

what is the realtionship betweeen the number of carbon atoms and the enthalpy of combustion

Answer 26

as the number of carbon atoms increase within a homologus series
the enthalpy of combustion increases

Question 27

explain why the enthalpy of combustion of hydrocarbon chain increases linearly with the length of the chain

Answer 27

the value of the bond enthalpy for a carbon-carbon bond remains constant with each addition

Question 28

what way does the hesses law profile go from

Answer 28

left to right

Question 29

explain why the standard enthalpy of formation for is zero

Answer 29

it is an element.

Question 30

suggest why its not possible to measure the enthalpy change directly for the following combustion reaction

graphite+ 0.5O2 -> CO(g)

Answer 30

CO is not the only product
further oxidation occurs

Question 31

what is the equation for bond enthalpy

Answer 31

⅀ (bonds broken) – ⅀ (bonds formed) = ΔH