3.2.1 Enthalpy Changes

Question 1

enthalpy

Answer 1

measure of heat energy in chemical system

Question 2

enthalpy change equation

Answer 2

enthalpy change of bonds broken - enthalpy change of bonds formed

Question 3

where is heat energy transferred to when enthalpy change occurs

Answer 3

between the system and the surroundings

Question 4

what is the system

Answer 4

the chemicals - reactants and products

Question 5

what is the surroundings

Answer 5

the apparatus, laboratory
everything that’s not the chemicals

Question 6

universe =

Answer 6

system + surroundings

Question 7

exothermic enthalpy change

Answer 7

from system to surroundings
enthalpy change is negative
surroundings heat up as they gain energy

Question 8

endothermic enthalpy change

Answer 8

from surroundings to system
enthalpy change is positive
temperature of surroundings decrease as they lose energy

Question 9

activation energy

Answer 9

minimum energy required for reaction to take place

Question 10

standard conditions

Answer 10

100kPa
298K/ 25 degrees
1 mol/dm3

Question 11

standard enthalpy change of a reaction definition

Answer 11

enthalpy change that accompanies a reaction in the molar quantities show in chemical equation under standard conditions with all reactants and products in there standard states and under standard conditions

Question 12

enthalpy change of formation

Answer 12

enthalpy change when one mole of a compound is formed from its elements under standard conditions with all reactants and products in there standard states and under standard conditions

Question 13

enthalpy change of combustion

Answer 13

enthalpy change when one mole of substance reacts completely with oxygen under standard conditions with all reactants and products in there standard states and under standard conditions

Question 14

enthalpy change of neutralisation

Answer 14

energy change that accompanies reaction of an acid by a base to form one mole of water under standard conditions with all reactants and products in there standard states and under standard conditions

Question 15

what 3 quantities measure energy change of surroundings

Answer 15

mass, specific heat capacity and temperature change

Question 16

specific heat capacity definition

Answer 16

energy required to raise the temperature of 1g of substance by 1K

Question 17

calculating energy change

Answer 17

q = mc∆T

Question 18

what does c equal for water

Answer 18

4.18 J g-1 K-1

Question 19

average bond enthalpy

Answer 19

energy required to break one mole of a specific type of bond in a gaseous molecule

Question 20

are bond enthalpies exo or endothermic

Answer 20

always endothermic so ∆H is always positive

Question 21

is bond breaking exo or endo

Answer 21

endo

Question 22

is bond making exo or endo

Answer 22

exo

Question 23

enthalpy change of reaction formulae

Answer 23

sum of bond enthalpies in reactants - sum of bond enthalpies of products

Question 24

what does Hess’ law state

Answer 24

if a reaction can take place by 2 routes and the starting and finishing conditions are the same the total enthalpy change is the same for each route

Question 25

enthalpy change using Hess cycles

Answer 25

construct enthalpy cycle
add ∆H values
calculate unknow
clockwise= anticlockwise

Question 26

∆H

Answer 26

q(in kj) / n( moles of limiting reagent

Question 27

hess’ law definition

Answer 27

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

Question 28

how do you know if standard conditions are used

Answer 28

correct state at room temperature

Question 29

Q- energy in J

Answer 29

m (in grams of solution that changes temp) x c x ∆T

Question 30

∆H for hess law

Answer 30

(sum of all ∆fH products) - (sum of all ∆fH reactants)

Question 31

how to work out bond enthalpies

Answer 31

enthalpy of reactants add enthalpy of products
if negative more energy is released so exothermic if positive its endothermic