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YR2 Chemistry 24-25 > M3 - Enthalpy changes > Flashcards

M3 - Enthalpy changes Flashcards

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1
Q

What is the temperature change in an endothermic reaction?

A

A decrease in temperature

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2
Q

what’s the enthalpy change equation using enthalpy change of combustion values ?

A

= sum of reactants - sum of products

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3
Q

How do you find enthalpy changes using enthalpy change of formation values?

A

= sum of products - sum of reactants

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4
Q

how do you calculate the enthalpy changes from average bond enthalpies?

A

ΔH= sum of bonds broken - sum of bonds formed

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5
Q

What is Hess’ law?

A

If a reaction can take place by more than one route and the initial and final concentrations are the same, the total energy change is the same for each route.

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6
Q

What kind of enthalpy change is bond formation?

A

Exothermic as it releases energy

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7
Q

What are some properties of bond enthalpies?

A

Energy is always required to break bonds
Bond enthalpies are always endothermic
Bond enthalpies always have a positive enthalpy value

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8
Q

Define average bond enthalpy

A

The energy required to break one mole of covalent bonds in a gaseous molecule.

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9
Q

How can heat loss be accounted for using a graph of temperature against time?

A

Extrapolate the cooling curve back to when it was added.

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10
Q

What are the causes for less energy being transferred than expected when working out ΔHc?

A

Heat loss to the surroundings
Incomplete combustion
Evaporation
Non-standard conditions

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11
Q

what’s the enthalpy change equation using ‘average bond enthalpies’ ?

A

ΔrH=Σ(bond enthalpies of reactants) - Σ (bond enthalpies products)

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12
Q

Define standard enthalpy change of neutralisation

A

The enthalpy 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.

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13
Q

How is enthalpy change worked out from the energy change and change in temperature?

A

Q = mcΔT and then
ΔH = -Q/n

ΔH - enthalpy change (J/mol)
Q - energy change with surroundings (J)
n - number of moles (mol)

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14
Q

What is the equation used to measure an energy change?

A

Q=mcΔT

Q - energy change with surroundings (J)
m - mass (g)
c - specific heat capacity (J/g/K)
ΔT - change in temperature (K)

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15
Q

Define standard enthalpy change of combustion

A

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.

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16
Q

Define standard enthalpy change of formation

A

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.

17
Q

Define standard enthalpy change of reaction

A

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.

18
Q

What are the standard conditions?

A

Standard pressure - 101kPa / 1atm
Standard temperature - 298K / 25 degrees celsius
Standard concentration - 1mol/dm^3
Standard state - The physical state of a substance under standard conditions

19
Q

Define activation energy

A

The minimum energy required for a reaction to take place by breaking chemical bonds.

20
Q

Define an endothermic reaction

A

A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat beaing taken out of the surroundings.

21
Q

Define an exothermic reaction

A

A reaction in which the enthalpy of the products is lower than the enthalpy of the reactants, resulting in heat loss to the surroundings.

22
Q

Define enthalpy change

A

The heat exchanged with the surroundings during a chemical reaction.
The difference between the enthalpy of the products and the reactants.

23
Q

Define enthalpy

A

The heat content that is stored in a chemical system.

24
Q

What is the law of the conservation of energy?

A

Energy cannot be created or destroyed, just converted from one form to another.

25
Q

Define ‘standard enthalpy change of formation’, ΔfHθ (1)

A

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (1)

26
Q

What is Hess’ Law?

A

The enthalpy change of a reaction depends only on the initial and final states and is independent of the route taken

27
Q

What are the units of enthalpy change?

A

Units = kJ mol-1

28
Q
A

B

29
Q

What are the standard conditions for temperature, pressure and concentration?

A

Temperature = 298K or 25 degrees celsius
Pressure = 1atm or 101kPa
Concentration = 1 moldm-3

30
Q
A

B

31
Q

Define ‘standard enthalpy change of neutralisation’, ΔneutH (1)

A

The enthalpy change that accompanies the formation of one mole of H2O(l) from neutralisation, under standard conditions (1)

32
Q

What is meant by ‘activation energy’? (1)

A

Energy: The minimum energy required to start a reaction by breaking bonds (1)

33
Q

Define ‘standard enthalpy change of combustion’, ΔcHθ (1)

A

The enthalpy change for complete combustion of one mole of a substance under standard conditions, all reactants and products being in their standard states (1)

34
Q
A

B

35
Q

Define ‘Standard enthalpy change of reaction’, ΔrHθ

A

The enthalpy change that accompanies a reaction in molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states

36
Q

Define average bond enthalpy, ΔEHθ

A

The average enthalpy change for the breaking of 1 mole of bonds in gaseous
molecules (by homolytic fission) (1)

37
Q

Define Enthalpy change, ΔH

A

The amount of heat released (or absorbed) by a chemical reaction, carried out at constant pressure

YR2 Chemistry 24-25 (15 decks)

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