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CHEMISTRY TOPIC 8 GENERAL > HEAT ENERGY AND ENTHALPY > Flashcards

HEAT ENERGY AND ENTHALPY Flashcards

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1
Q

What does exothermic mean?

A

When heat is transferred from the system to the surroundings.

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2
Q

What does endothermic mean?

A

When heat energy is transferred from the surroundings to the system.

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3
Q

What is the definition of standard enthalpy of change of reaction?

A

The enthalpy change measured at standard conditions, when the number of moles of substances in the equation as written react.

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4
Q

Chemical energy is made up of two conditions…

A
  • Kinetic energy (measure of motion of particles in a substance)
  • Potential energy (measure of how strongly these particles interact with each other, e.g. attract/repel.)
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5
Q

What is heat energy?

A

The portion of potential energy and the kinetic energy of a substance that is responsible for the temperature of the substance. Heat energy is directly proportional to absolute temperature.

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6
Q

What is enthalpy?

A

The measure of total energy of a system.

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7
Q

The enthalpy change of a process is…

A

the heat energy that is transferred between the system and the surroundings at constant pressure.

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8
Q

You can’t measure enthalpy directly, but…

A

you can measure the enthalpy change.

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9
Q

What are standard conditions?

A

100kPa pressure and 298K temperature.

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10
Q

Under standard conditions, the enthalpy change measured is called…

A

the standard enthalpy change.

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11
Q

Exothermic reactions have what value?

A

-

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12
Q

Endothermic reactions have what value?

A

+

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13
Q

Enthalpy has the symbol…

A

H

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14
Q

What is the equation to show the relationship between enthalpy and internal energy?

A 
H = U + PV
(P = pressure, V = volume)
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15
Q

Give some examples of exothermic reactions?

A

Freezing water, condensing water vapour, combustion of petrol, dissolving sodium hydroxide in water.

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16
Q

Give some examples of endothermic reactions?

A

Melting ice, evaporating water, dissolving ammonium nitrate in water, photosynthesis.

17
Q

The change in enthalpy is given by …

A

products - reactants

18
Q

What does an enthalpy level diagram look like for an exothermic reaction?

A

Reactants ↓ Products

19
Q

What does an enthalpy level diagram look like for an endothermic reaction?

A

Reactants ↑ Products

20
Q

What points do you need to remember when constructing enthalpy level diagrams?

A
  • You only need to label the vertical axis (Enthalpy, H).
  • The formulae for both reactants and products should be giving, including state symbols.
  • The value for ΔH, correct sign included.
  • Not essential to show activation energy.
21
Q

What is the definition of standard enthalpy change of combustion?

A

The enthalpy change measured at standard conditions when one mole of a substance is completely burned in oxygen.

22
Q

What are the two common ways of writing the combustion of hydrogen?

A

H₂(g) + ½O₂ → H₂O(l) and 2H₂(g) + O₂ → 2H₂O(l)

23
Q

What are the steps of calculating enthalpy change of combustion?

A
  1. Calculate heat energy, Q, transferred to the water using the equation: Q = mcΔT
  2. Calculate the amount of n (moles) of ethanol burned.
  3. Calculate the Δ𝒸H, using the equation: ΔH=-Q/n
24
Q

What is the experimental determination of enthalpy change of combustion of a liquid?

A
  • A spirit burner containing the liquid under test is weighed.
  • A known volume of water is added to a copper can.
  • The temperature of the water is measured.
  • The burner is lit.
  • The mixture is constantly stirred with the thermometer.
  • When the temperature of the water has reached approximately 20°C about its initial temperature, the flame is extinguished and the burner is immediately reweighed.
  • The final temperature is measured.
25
Q

What are some errors and assumptions made in the experiment determining the enthalpy change of combustion?

A
  • Some of the heat energy produced in burning is transferred to the air and not the water.
  • Some ethanol may not have burned completely to form carbon dioxide and water.
  • Some heat produced is transferred to the copper can and not the water.
  • The conditions are not standard, e.g. water vapour produced not liquid water.
  • Not all of the heat energy transferred from the water to the surroundings is compensated for.
26
Q

What is the standard enthalpy change of neutralisation?

A

The enthalpy change measure at standard conditions when one mole of water is produced by the neutralisation of an acid with an alkali.

27
Q

What is the experimental determination of enthalpy change of neutralisation?

A
  • Use a pipette fitted with a safety filter, place 25.0cm³ of 1.00 mol dm⁻³ acid into a polystyrene cup.
  • Measure the temp. of the acid.
  • Using a pipette, place 25.0cm³ of the alkali into the beaker.
  • Measure the temp. of alkali.
  • Add the alkali to the acid, stir with the thermometer and measure the maximum temp. reached.
28
Q

What are the sources of error and assumptions in the neutralisation reaction?

A
  • Uncertainties of measurements involved with the use of the pipette and the thermometer.
  • Some heat energy will be transferred to the polystyrene cup and the thermometer.
29
Q

How do you calculate standard enthalpy change of neutralisation?

A
  • Find mean starting temp.
  • Calculate temperature change.
  • Calculate the volume of solution heated.
  • Use Q = mcΔT
  • Amount of acid neutralised = amount of water formed
  • Then, ΔneutH=-Q/n
  • Value should be negative.
30
Q

What is the standard enthalpy change of formation?

A

The enthalpy change when one mole of a substance is formed from its elements in their standard states.

31
Q

What is Hess’s Law?

A

States that the enthalpy change of a reaction is independent of the path taken in converting reactants into products, provided the initial and final conditions are the same in each case.

32
Q

How do you calculate the standard enthalpy change of formation using Hess’s Law?

A

ΔfH = sum of ΔcH of elements - ΔcH of compound

33
Q

How do you calculate enthalpy change of formation with two equations?

A

-Add theΔH of the two equations.

CHEMISTRY TOPIC 8 GENERAL (2 decks)

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