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Chemistry A Level > Energetics - Enthalpy > Flashcards

Energetics - Enthalpy Flashcards

1
Q

Describe an enthalpy diagram for an Exothermic change

A
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2
Q

Describe an enthalpy diagram for an Endothermic change

A
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3
Q

What are key features to remember for an exothermic reaction?

A
  • Heat energy loss to environment
  • Reacting chemicals lose energy
  • Increase in temperature
  • Bond making
  • Enthalpy = -kJmol-1
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4
Q

What is important to remember about the enthalpy of an exothermic reaction?

A

The enthalpy of the products is less than the enthalpy of the reactants so energy has been released to the surroundings

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5
Q

Examples of exothermic reactions

A
  • Combustion
  • Neutralisation
  • Respiration
  • Displacement
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6
Q

What are key features to remember for an endothermic reaction?

A
  • Heat energy gained from the environment
  • Reacting chemicals absorb energy
  • Decrease in temperature
  • Bond breaking
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7
Q

What is important to remember about the enthalpy of an endothermic reaction?

A

The enthalpy of the products is greater than the reactants so less energy has been absorbed from the surroundings

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8
Q

Examples of endothermic reactions

A
  • Thermal decomposition
  • Photosynthesis
  • Dehydration of hydrated copper sulfate
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9
Q

What is activation energy?

A

The minimum amount of energy which the reactants must have in order to react

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10
Q

What is enthalpy?

A

Enthalpy relates to the energy of the bonds broken and made during a chemical reaction.

Endo = bond breaking

Exo = bond making

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11
Q

What are the Standard conditions / states?

A

Standard pressure : 100kPa (1atm)

Standard temperature : 298K

Standard concentration : 1.00mol dm-3

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12
Q

What is Mean Bond Enthalpy?

A

The energy required to break 1 mol of a gaseous covalent bond averaged across many compounds containing the bond.

Units : kJmol-1

H = Σ MBE bonds broken (reactants) - Σ MBE bonds made (products)

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13
Q

What is the Standard enthalpy of reaction?

A

This is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction.

E.g. CaO(s) + H20 → Ca(OH)2(s) ChangerH° = -63.7kJ mol-1

This means that when 1 mole of calcium oxide reacts with 1 mole of water to form 1 mole of calcium hydroxide, 63.7kJ of heat would be released.

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14
Q

What are calorimetery experiments?

A

Experiments that measure the heat given out by reactions. These reactions take place in solutions, they can be placed into a polystyrene container and the temperature change recorded with a thermometer.

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15
Q

Why are polystyrene cups used?

A

They are good insulators and have a low heat capacity. This reduces heat loss through the sides of the container.

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16
Q

What are the issues involving caloirmetry?

A
  • Some heat will be absorbed by the container, rather than going towards heating up the water.
  • Some heat is always lost to the surroundings during the experiment (depending on how well the container is insulated)
17
Q

What calculations are used for direct enthalpy change?

A

Enthalpy change = -q (J) / n

q = mcAT

Density = mass/volume

18
Q

What is the density of water?

A

1 g/cm3

It can be used in the calculation to replace the mass in q = mcAT

This can be used for example when using the mass as the volume of water making up a solution in the reaction.

19
Q

What is the enthalpy change of formation?

A

This is the enthalpy change when one mole of compound is formed from its elements under standard conditions, all reactants and products being in their standard states.

20
Q

What is the Enthalpy change for all elements and why?

A

Zero, because there is no change in enthalpy

21
Q

What are the equations for the element formations of methane, water, sulfur dioxide, hydrogen sulfide and nitrogen monoxide?

A

C(s) + 2H2(g) → CH4(g)

H2(g) + 1/2O2(g) → H2O(l)

S(s) + O2(g) → SO2(g) - Sulfur is a yellow solid

H2(g) + S(s) → H2S

1/2N2 + 1/2O2(g) → NO(g)

22
Q

What is Hess’s Law?

A

Hess’s Law states that the enthalpy change for a chemical reaction is the same, what ever route is taken from reactants to products.

23
Q

What is the diagram for Hess’s Law, which is used to calculate the enthalpy for a reaction?

24
Q

What can this diagram be arranged to?

A

ArH° = AfH° products - AfH° reactants

25
Q

What is the standard enthalpy change of combustion?

A

AcH° is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states

26
Q

What is the formula for calculating the enthalpy of formation using the enthalpy of combustion?

A

AfH° = AcH° reactants - AcH° products

27
Q

Define enthalpy change (DELTA H)

A

Change in heat energy (of a reactant) at a constant pressure

28
Q

What are the units for enthalpy change?

29
Q

Name the standard conditions

A
  • 1 atm (100kPA)
  • 298 K

Chemistry A Level (25 decks)

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