enthalpy chp 9

Question 1

what is enthalpy

Answer 1

enthalpy (H) is a measure of the heat energy in a chemical system.

Question 2

what is meant by a chemical system

Answer 2

• the chemical system refers to the atoms, molecules, or ions making up the chemicals of the reactants and products

Question 3

can enthalpy be measured

Answer 3

enthalpy cannot be measured but enthalpy changes can

Question 4

what does a negative enthalpy value tell us

Answer 4

• a negative enthalpy change value means the reaction is exothermic because the energy that was in the chemical system has been transferred to the surroundings in the form of heat this causes the surroundings to heat up.
• energy has been lost giving a negative value.

Question 5

what does a positive enthalpy change value tell us

Answer 5

• energy from surroundings drawn into chemical system
• make surroundings colder.
• more energy in system than before so gives positive value

Question 6

what is the law of conservation of energy

Answer 6

the law of conservation of energy states that energy cannot be created or destroyed.

Question 7

what are the features of exothermic reactions

Answer 7

• heat out of the system
• the energy transferred from the system = energy transferred to the surroundings
• enthalpy change is negative
• the temperature of the surroundings increase

Question 8

what does a exothermic enthalpy profile diagram look like

Answer 8

Question 9

What are the features of endothermic reactions

Answer 9

• heat into the system
• energy transferred from the surrounding = energy transferred to the system
• enthalpy change is positive
• the temperature of the surroundings decreases as it lose energy

Question 10

what does a endothermic enthalpy profile diagram look like

Answer 10

Question 11

what is activation energy

Answer 11

• the energy input required to break bonds in reactants
• activation energy is the minimum energy required for a reaction to take place

Question 12

would reactions with lower or higher activation energy occur quicker

Answer 12

• reactions with lower activation energies take place very rapidly, because the energy needed to break bonds is readily available from the surroundings,
• very large activation energies may take place extremely slowly or not at all.

Question 13

what is meant by standard enthalpy changes

Answer 13

the enthalpy change that occurs during a reaction under standard conditions

Question 14

what are the units and values for standard conditions

Answer 14

• standard pressure is 100 kPa ( 1 atmosphere is 101 kPa)
• standard temperature is 298k (25 °C)
• standard concentration is 1 mol dm-3 (only relevant for solutions)
• standard state is the physical state of a substance under standard conditions

Question 15

what is the enthalpy change of reaction

Answer 15

• the enthalpy change that accompanies a reaction in the molar quantities shown in a stated chemical equation under standard conditions

Question 16

what is the enthalpy change of formation

Answer 16

The standard enthalpy change of formation is the enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 17

what is the enthalpy change of combustion

Answer 17

• the enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions

Question 18

what is the enthalpy change of neutralisation

Answer 18

• the energy change that accompanies the reaction of an acid by a base to from 1 mole of H20 under standard conditions

the value of enthalpy change of neutralisation is the same for all neutralisation reactions
- 57 Kj mol^-1

Question 19

What does a 1k rise in temperature relate to in °C

How do you convert from degrees to kelvin

Answer 19

• A 1 k rise in temperature is the same as a 1°C rise in temperature
• °C + 273 = K
• K - 273 = °C

Question 20

what is the formula for calculating energy change

Answer 20

q = energy change
m = mass of substance that is being heated
c = specific heat capacity (4.18 Jg^-1K^-1)
T = temperature

Question 21

what is meant by specific heat capacity

Answer 21

the specific heat capacity is the energy required to raise the temperature of 1g of a substance by 1 k

Question 22

what things could make a enthalpy change of combustion value less accurate

Answer 22

• heat loss to the surroundings other than the water
• incomplete combustion of fuel (lower heat given off than complete combustions)
• evaporation of fuel from the spirit burner wick (effect calculation made at the end)
• non standard conditions, the conditions for the experiment are unlikely to be identical to standard conditions

all but the last of these reasons would result in a less exothermic value than expected

Question 23

what could be done to reduce the effect of the errors on the enthalpy change of combustion value

Answer 23

use of draught screens and an input of oxygen gas could minimise errors from heat loss and incomplete combustion

Question 24

how would you correct for cooling on a cooling curve

Answer 24

extrapolate the cooling curve section of the graph back to the time when the substance was added

Question 25

what is average bond enthalpy ## Footnote what are some important points to remeber about average bond enthapy

Answer 25

average bond enthalpy is the energy required to break 1 mole of a specified type of bond is a gaseous molecule ## Footnote -energy is always required to break bonds -bond enthalpies are always endothermic -bond enthalpies always have a positive enthalpy value

Question 26

what are the limitations of average bond enthalpies

Answer 26

The actual bond enthalpy can vary depending on the chemical environment of the bond

Question 27

is bond breaking endo or exothermic

Answer 27

-energy is required to break bonds and so bond breaking is endothermic -△H is positive

Question 28

is bond forming endo or exothermic

Answer 28

-energy is released when bonds form making bond forming exothermic -△H is negative

Question 29

what determines if a reaction is endo or exothermic

Answer 29

The difference between the energy required for bond breaking and the energy released by bond making determines wether a reaction is exothermic or endothermic

Question 30

how can average bond enthalpy be used to calculate enthalpy change of reaction

Answer 30

Question 31

what does hess's law state

Answer 31

hess's law states that, 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. ## Footnote comes from the idea of conservation of energy

Question 32

what are the formulas for calculating the enthalpy change of reaction from enthalpy change of formation and reaction

Answer 32