2.Energy changes and fuels

Question 1

What is enthalpy?

Answer 1

the heat content that is stored in a chemical system.

Question 2

Endotheric-

Answer 2

+ve delta H

Question 3

Exothermic-

Answer 3

-ve delta H

Question 4

What are self heating cans?

Answer 4

a can which contains a chamber where an exothermic reaction can happen.

Question 5

What are self cooling beer cans.

Answer 5

water evaporates to use heat energy.

Question 6

Exothermic reaction example?

Answer 6

Combustion respiration.

Question 7

Endotheric reaction example?

Answer 7

thermal decomposition of limestone ( CaCO3) photosynthisis

Question 8

What is a enthalpy profile diagram?

Answer 8

A diagram for a reaction to compare the enthalph of the reactants with the enthalpy of the products. Shows Ea, and Delta H .

Question 9

Define activation energy?

Answer 9

Ae is the minimum energy required to start a reaction by the breaking of bonds.

Question 10

What are standard condions.

Answer 10

25 dec c(298K), 1atm, 1 moldm-3

Question 11

Define the standard enthalpy change of reaction.

Answer 11

The enthalpy change that accompanies a reaction in the molar quantities(the number of moles of reactants as stated), expressed in a chemical equation under standard conditions, with reactants and products in their standard states.

Question 12

Define the standard enthalpy change of combustion.

Answer 12

is the enthalpy change when one mole of a substance is completely reacted with oxygen under standard conditions, and all the products in their standard states

Question 13

Define the standard enthalpy change of formation

Answer 13

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Question 14

Define 1st Ionisation energy

Answer 14

the required for 1 e- to be removed from every atom in 1 mole of gaseous atoms.

Question 15

define mean bond enthalpy.

Answer 15

The average amount of energy needed to break 1 mole of a specific bond in the gaseous state.

Question 16

Why might mean bond enthalpy be incorrect.

Answer 16

as its an average not for that specific molecule.

Question 17

what is specfic heat capacity?

Answer 17

the energy required to raise the temp of 1g of a substance by 1 deg C

Question 18

Q(heat exchange)=

Answer 18

m cp deltaT

Question 19

How is enthalphy change calculated?

Answer 19

-Q(in KJ)/mol

Question 20

What is the mass used in calorometry?

Answer 20

the mass of the thing being heated- usually water

Question 21

How do you find delta Hc

Answer 21

Burn a know mass of substance in air. measure the volume of water being heated. measure the waters temp change.

Question 22

Why might measure enthalpy of combustion be different?

Answer 22

Incomplete combustion? Heat loss to surroundings. heat lost to calorometer. mass of fuel lost as vapour

Question 23

Why might measure enthalpy of formation/reaction be different?

Answer 23

heat lost to surroundings heat lost to calorimeter heat lost to solid reactants.

Question 24

whats the relationship between carbon atoms in a alcohol and enthalph of combustion.

Answer 24

more c atoms = higher enthalpy of combustion.

Question 25

ENthalpy change using bond enthalpies=

Answer 25

Energy released when broken - energy requred to form product bonds.

Question 26

What is hess' law?

Answer 26

if a reaction can take place by more then one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.

Question 27

Why isnt it always possible to directly measure enthalphy change?

Answer 27

More then one reaction takes place a high Ae a very slow rate.

Question 28

Work our enthalpy change of reaction from their combustion energies.

Answer 28

construct a hess cycles/diagram. work out the reaction routes. (energy to combust left- energy to combust right = enthaply of reaction)

Question 29

Work our enthalpy change of reaction from their formation energies.

Answer 29

construct a hess cycles/diagram. work out the reaction routes. (energy to form right- energy to form left right = enthaply of reaction)

Question 30

Two hese cycles for formation and combustion (not andys way)

Answer 30

Question 31

Standarf for for anys hess cycle.

Answer 31

Elecments fuel XCO2+yH2+O2 Xco2+yH20

Question 32

Why are some cyclic thigns more exothermic

Answer 32

the ring structure has been squashed from 109.5 to 60 in a tri, 90 in a but, etc, this requires energy which is released when its burnt.

Question 33

Why does enthalpy of combustionn increase linearly for homologeos series.

Answer 33

a regualr amour more of the same bond is being broken each increase= more energy released.

Question 34

What is a procedural error?

Answer 34

a fault with the method eg incomplete combustion, non standard condtitions, cp of equipment ignored,head lost to environment...