Enthalpy Changes

Question 1

what is enthalpy?

Answer 1

it measures the heat energy in a chemical system

Question 2

what are the units for standard enthalpy change?

Answer 2

kJmol-1

Question 3

how to calculate enthalpy change?

Answer 3

H(products) - H(reactants)

Question 4

what type of reaction has a negative enthalpy change?

Answer 4

exothermic

Question 5

what type of reaction is a positive enthalpy change?

Answer 5

endothermic

Question 6

what is an exothermic reaction?

Answer 6

a chemical system that releases heat energy to the surroundings

Question 7

what is an endothermic reaction?

Answer 7

a chemical system that takes in heat energy from its surroundings

Question 8

what is activation energy?

Answer 8

minimum energy required for a reaction to take place

Question 9

what happens to the temperature of the surroundings in an exothermic reaction?

Answer 9

increases

Question 10

what is on the x axis of an energy profile?

Answer 10

progress of reaction

Question 11

what is on the y axis of an energy profile diagram?

Answer 11

enthalpy, H

Question 12

for an exothermic reaction, what has a higher enthalpy?

Answer 12

reactants

Question 13

for an endothermic reaction, what has a higher enthalpy?

Answer 13

products

Question 14

what would the speed of a reaction be if it has a small activation energy?

Answer 14

• takes place quickly
• because the energy needed to break bonds is readily available from the surroundings

Question 15

what would the speed of a reaction be if it has a large activation energy?

Answer 15

the reaction would be slow or don’t occur at all

Question 16

what is standard pressure?

Answer 16

100kPa

Question 17

what is standard temperature?

Answer 17

298K (25 degrees celsius)

Question 18

what is standard concentration?

Answer 18

1moldm-3

Question 19

what is standard state?

Answer 19

physical state of a substance under standard conditions (100kPa and 298K)

Question 20

what is enthalpy change of reaction?

Answer 20

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

Question 21

what is enthalpy change of formation?

Answer 21

the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Question 22

what is enthalpy change of combustion?

Answer 22

the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states

Question 23

what is enthalpy change of neutralisation?

Answer 23

the energy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions with all reactants and products in their standard states

Question 24

what are the 3 methods to calculate enthalpy changes?

Answer 24

• calorimetry
• bond enthalpies
• Hess’ law

Question 25

what is the equation for calorimetry?

Answer 25

q=mc^T

Question 26

what is the value of c?

Answer 26

4.18

Question 27

how do you work out enthalpy change using q=mc^t?

Answer 27

enthalpy change= q/ moles

Question 28

what mass and moles do you use when calculating enthalpy change of reaction?

Answer 28

mass: total mass moles: limiting reagent

Question 29

what mass and moles do you use when calculating enthalpy change of neutralisation?

Answer 29

mass: total volume of acid and base moles: moles of water

Question 30

what type of reaction has a rise in temperature?

Answer 30

exothermic

Question 31

what type of reaction has a fall in temperature?

Answer 31

endothermic

Question 32

in terms of bond breaking and bond forming, why are some reactions endothermic?

Answer 32

more energy is required for breaking bonds compared to the energy released when forming bonds

Question 33

in terms of bond breaking and bond forming, why are some reactions exothermic?

Answer 33

more energy is released when bond forming compared to the energy required when breaking bonds

Question 34

what affects the accuracy of the enthalpy change?

Answer 34

- heat loss to surroundings - incomplete combustion - evaporation of alcohol from the wick - non-standard conditions were used

Question 35

what is bond enthalpy?

Answer 35

when one mole of a specified type of bond in gaseous bonds is broken

Question 36

what is a limitation of bond enthalpy?

Answer 36

- in different environments, the actual energy involved in breaking and forming individual bonds may differ slightly

Question 37

what does Hess' law state?

Answer 37

- route 1 = route 2 - sum of clockwise arrows = sum of anticlockwise arrows

Question 38

how do you draw Hess' cycle for an enthalpy change of formation?

Answer 38

reactants --> products elements in standard states - arrows point upwards from elements in standard states to the reactants and products

Question 39

what is the equation for enthalpy change of formation(Hess's law)?

Answer 39

^H = sum of ^H products - sum of ^H reactants

Question 40

what is the enthalpy change of elements?

Answer 40

- 0kJmol-1

Question 41

how do you draw Hess' cycle for an enthalpy change of combustion?

Answer 41

reactants --> products oxides - arrows point downwards from reactants and products towards the oxides

Question 42

what is the equation for enthalpy change of formation(Hess's law)?

Answer 42

^H = sum of ^H products - sum of ^H reactants

Question 43

what is the acronym for remembering endothermic and exothermic for bond breaking and bond forming?

Answer 43

- bendo - mexo

Question 44

what is average bond enthalpy?

Answer 44

the breaking of 1 mol of bonds in gaseous molecules