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Enthalpy > physical chemistry > Flashcards

physical chemistry Flashcards

1
Q

What is chemical energy?

A

a special form of potential energy that lies within chemical bonds

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2
Q

What are chemical bonds?

A

The force of attraction that bind atoms together in compounds.

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3
Q

During a reaction, bonds break and new bonds form. This process changes the what?

A

chemical energy of the atoms

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4
Q

What is H?

A

enthalpy- the heat content that is stored in a chemical system

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5
Q

Explain the law of conservation of energy

A

If heat is released, the amount of energy that leaves a chemical system is exactly the same as the amount that goes into the surroundings so no heat energy is lost.

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6
Q

what does a heat loss in a chemical system mean?

A

temperature increase to the surroundings

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7
Q

explain what ΔH means

A

heat exchange with the surroundings during a chemical reaction, at a constant pressure

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8
Q

What is an exothermic reaction?

A

The enthalpy of the products is smaller than the enthalpy of the reactants. There is less heat loss from the chemical system to the surroundings, ΔH has a negative sign because heat is lost.

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9
Q

What is an endothermic reaction?

A

the enthalpy of the products is greater than the enthalpy of the reactants. There is heat gain to the chemical system from the surroundings. ΔH is positive..

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10
Q

Name 2 important exothermic reactions and why they are exothermic

A

oxidation of fuels- products have less enthalpy than the reactants so the excess energy is released as heat. and respiration- sugars, such as glucose are oxidised to carbon dioxide and water

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11
Q

equation of respiration

A

C6H12O6 + 60^2 = 6CO^2 + 6H20

ΔH= -2801 KJ mol-1

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12
Q

equation of oxidation of fuels

A

CH4 + 2CO2 = CO2 + 2H20 ΔH= -890 KJmol-1

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13
Q

name two processes of endothermic reactions

A

Thermal decomposition of limestone and photosynthesis

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14
Q

explain the thermal decomposition of limestone

A

limestone contains calcium carbonate. the decomposition of calcium carbonate by heat is used to make calcium oxide.
CaCO3 = CaO + CO2 ΔH= +178 KJ mol-1

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15
Q

equation of photosynthesis

A

6CO2 + 6H20 = C6H12O6 + 6O2 ΔH= + 2801 KJ mol-1

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16
Q

what is the activation energy?

A

the minimum energy required for reaction to take place

17
Q

how is the activation energy always calculated?

A

the input energy from reactants to the top of the energy barrier

18
Q

what are standard enthalpy conditions?

A

pressure of 100Kpa, temp of 298K, concentration of 1moldm-3, physical state of a substance under standard conditions

19
Q

what Is standard enthalpy change shown by?

A

ΔH°

20
Q

give an equation for standard enthalpy change of reaction

A

H2 + 0.5O2 = H2O ΔH°= -286 kjmol-1

21
Q

what is the standard enthalpy change

A

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

22
Q

What is the standard enthalpy change of combustion?

A

The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in standard units.
C2H6 + 3.5O2 = 2CO2 + 3H20 ΔcH°= -1560 JKmol-1

23
Q

What is the standard enthalpy change of formation?

A

The enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.
H2 + 0.5O2 = H20 ΔcH°= 286 KJmol-1

24
Q

What is the standard enthalpy change of neutralisation?

A

The energy change that accompanies the reaction of an acid by a base to form 1 mole of H2O under standard conditions, all reactants and products being in standard units.

25
Q

example of Δneut0

A

H+ + OH- = H20 Δneut0= -57 KJmol-1

26
Q

steps of determining enthalpy change indirectly

A
  1. ) Q=mcΔT

2. ) ΔH = q/ 1000 / n

27
Q

what is combustion?

A

a reaction with oxygen to form oxides

28
Q

why is there a slight difference between the experimental value and enthalpy change ?

A

incomplete combustion, heat loss of surroundings, evaporation from the wick-use cover to reduce this and non-standard conditions

29
Q

what can be used when measuring enthalpy change of combustion to increase reliability?

A

a bomb calorimeter to allow accurate measurements of enthalpy changes

30
Q

what is the simplest calorimeter

A

a polystyrene cup. it insulates he solution inside the cup.

31
Q

explain exothermic and endothermic reactions in direct determination of enthalpy change

A

exothermic: heat produced is trapped in the calorimeter increasing the temp of the solution.
endothermic: heat required for the reaction is removed from the solution, decreasing the temp of the solution

Enthalpy (3 decks)

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