06 - enthalpy changes

Question 1

exothermic reaction?

Answer 1

heat energy is released during a reaction, change in H = - ive

Question 2

surroundings include:

Answer 2

. the solvent
. air around test tube
. test tube
. anything dipping in the test tube

Question 3

examples of exothermic reactions:

Answer 3

. combustion of fuels
. oxidation of carbs in plant and animals bodies
. reaction of water with calcium oxide

Question 4

endothermic reaction?

Answer 4

heat is absorbed during a reaction, value of change in H is +ive

Question 5

examples of endothermic reactions?

Answer 5

. thermal decomposition reactions
. photosynthesis
. dissolving certain ammonium salts in water

Question 6

what is enthalpy change?

Answer 6

the heat energy transferred during a chemical reaction

Question 7

what is enthalpy change?

Answer 7

the heat energy transferred during a chemical reaction

unit = kJ mol-1
symbol = <> (delta)

Question 8

what are reaction pathway diagrams?

Answer 8

shows the relative enthalpies of the reactants and products and rhe enthalpy change as an arrow, may include activation energy

x = reaction pathway
y = enthalpy change

Question 9

what is activation energy?

Answer 9

the minimum energy that colliding particles must possess to break bonds to start a chemical reaction

always positive value of <>H

Question 10

enthalpy change in reaction pathway diagrams

Answer 10

exo - arrow goes down
endo - arrow goes up

FROM REACTANTS TO PRODUCTS

Question 11

activation energy in reaction pathways diagram

Answer 11

exo - arrow goes up
endo - arrow goes up

FROM REACTANTS TO PRODUCTS

Question 12

what are standard conditions?

Answer 12

. 101 kPa pressure
. 298K temperature
. each substance involved in reaction is in its normal physical state (solid, liquid, gas)

symbol = circle with horizontal line between

Question 13

what are the types of enthalpy changes?

Answer 13

. enthalpy change of formation
. enthalpy change of combustion
. enthalpy change of neutralisation
. enthalpy change of reaction

Question 14

what is enthalpy change of reaction

Answer 14

the enthalpy change when the amount of reactants shown in the stoichiometric equation react to give products under standard conditions

EXO OR ENDO

Question 15

what is enthalpy change of formation

Answer 15

the enthalpy change when one mole of compound is formed from its elements under standard conditions

EXO OR ENDO

Question 16

enthalpy change of formation or any element in its standard state is

Answer 16

0

Question 17

what is enthalpy change of combustion?

Answer 17

the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions

ALWYAS EXO

Question 18

what is enthalpy change of neutralisation?

Answer 18

the enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard condition

Question 19

when writing enthalpy changes, use most stable form of atoms

Question 20

what is the technique used to measure enthalpy chnage of some reaction

Answer 20

calorimetry

Question 21

what is the apparatus used in calorimetry?

Answer 21

calorimeter

Question 22

what is the apparatus used in calorimetry?

Answer 22

calorimeter

Question 23

what is specific heat capacity?

Answer 23

the energy needed to raise the temperature of 1g of a substance by 1K

Question 24

q = mc<>T

Answer 24

q - heat transferred, J
m - mass of water, g
c - specific heat capacity, J g-1 °C - 1
T - temperature change in °C

Question 25

<>H = - mc<>T

Question 26

finding the enthalpy change of combustion

Answer 26

. weigh spirit burner . pour 100cm3 water into calorimeter . stir water and record temperature . light spirit burner and remove the cap . stir water with thermometer until a temperature rise of 10°C .remove spirit burner and reweigh

Question 27

whta is hess's law?

Answer 27

the enthalpy change ina chemicla reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions and states of reactants and products are the same for route

Question 28

law of conservation of energy/ forst law of thermodynamics

Answer 28

energy cannot be created or destroyed

Question 29

direct route = indirect route

Answer 29

<>H2 = <>H1 + <>Hr

Question 30

enthalpy change reaction from formation

Answer 30

. elements at bottom . arrows up

Question 31

enthalpy change formation from combustion

Answer 31

. combustion products at bottom . arrows down

Question 32

bond break energy > bond make

Answer 32

endo

Question 33

bond break energy < bond make

Answer 33

exo

Question 34

what is exact bond energy

Answer 34

energy needed to break a specific covalent bond in a named molecule in the gaseous state AKA. bond dissociation energy, bond enthalpy

Question 35

what is average bond energy?

Answer 35

the average energy needed to break a specific covalent bond averaged from a variety of molecules in the gaseous state

Question 36

what is standard enthalpy change of atomisation?

Answer 36

when one mole of gaseous atoms is formed from its element under its standard conditions