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A-LEVEL CHEMISTRY > ENTHALPY CHANGE > Flashcards

ENTHALPY CHANGE Flashcards

1
Q

What is enthalpy change? (1)

A

Enthalpy change (ΔH) is the heat energy transferred in a reaction at constant pressure.

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2
Q

What can enthalpy change be? (2)

A

It can be exothermic (ΔH < 0), releasing heat energy to the surroundings, or endothermic (ΔH > 0), absorbing heat energy from the surroundings.

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3
Q

What is enthalpy change measured in? (1)

A

Enthalpy change is measured in joules (J) or kilojoules (kJ) per mole of reactant or product.

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4
Q

What is the standard enthalpy change? (1)

A

Standard enthalpy change (ΔH°) is the enthalpy change when reactants and products are in their standard states (usually 1 atm pressure and 298 K).

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5
Q

What is the standard enthalpy change of formation? (1)

A

Standard enthalpy change of formation (ΔH°f) is the enthalpy change when one mole of a compound is formed from its elements in their standard states.

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6
Q

What is the standard enthalpy change of combustion? (1)

A

Standard enthalpy change of combustion (ΔH°c) is the enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions.

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7
Q

What does Hess’s law state? (1)

A

Hess’s Law states that the total enthalpy change for a reaction is the same, regardless of the route taken.

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8
Q

What does Hess’s law allow us to do? (1)

A

It allows us to calculate ΔH for a reaction by summing the enthalpy changes of a series of reactions that add up to the desired reaction.

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9
Q

What is bond enthalpy? (1)

A

Bond enthalpy (also called bond energy) is the energy required to break one mole of a particular bond in a gaseous molecule.

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10
Q

How can average bond enthalpies be used? (1)

A

Average bond enthalpies can be used to estimate the enthalpy change of a reaction by comparing the energy required to break bonds in the reactants with the energy released by forming bonds in the products.

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11
Q

How do you calculate enthalpy change from bond enthalpies? (1)

A

ΔH = Σ(bond enthalpies of bonds broken) - Σ(bond enthalpies of bonds formed).

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12
Q

What is calorimetry? (1)

A

Calorimetry is the experimental technique used to measure heat changes in chemical reactions.

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13
Q

What is a calorimeter used for? (1)

A

A calorimeter is a device used to measure the heat absorbed or released in a chemical or physical process.

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14
Q

How do you calculate Heat absorbed or released? (1)

A

using the equation: q = mcΔT, where q is heat, m is mass, c is specific heat capacity, and ΔT is temperature change.

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15
Q

What is the enthalpy change in an exothermic reaction? (1)

A

the enthalpy change (ΔH) is negative, indicating that the system loses heat energy.

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16
Q

What are some characteristics of exothermic reactions? (2)

A

-Exothermic reactions often involve the formation of more stable products with lower energy than the reactants.
-They are typically spontaneous, meaning they occur naturally without the input of external energy.

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17
Q

What are some examples of exothermic reactions? (3)

A

combustion, neutralization, and many oxidation reactions.

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18
Q

How is an exothermic reaction represented on an energy profile diagram? (2)

A

-In an exothermic reaction, the energy released by the reaction is shown as a downward slope on the energy profile diagram.
-The reactants have higher energy than the products, and the difference in energy is released as heat to the surroundings.

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19
Q

What is the activation energy? (1)

A

The activation energy (Ea) represents the minimum energy required for the reaction to occur.

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20
Q

How are exothermic reactions used? (3)

A

-heating systems
-energy production
-chemical synthesis

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21
Q

How do exothermic reactions affect the environment? (2)

A

Exothermic reactions that involve the combustion of fossil fuels release carbon dioxide (CO2) and other pollutants into the atmosphere, contributing to air pollution and climate change.

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22
Q

What is the enthalpy change of an endothermic reaction? (1)

A

In an endothermic reaction, the enthalpy change (ΔH) is positive, indicating that the system gains heat energy.

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23
Q

What are the characteristics of an endothermic reaction? (2)

A

-Endothermic reactions often involve the formation of less stable products with higher energy than the reactants.
-They are typically non-spontaneous under standard conditions and require the input of external energy to occur.

24
Q

What are some examples of endothermic reactions? (4)

A

thermal decomposition, some dissolution reactions, and many reactions involving phase changes (e.g., melting, vaporization).

25
Q

What is the energy profile diagram of an endothermic reaction? (2)

A

-In an endothermic reaction, the energy absorbed by the reaction is shown as an upward slope on the energy profile diagram.
-The reactants have lower energy than the products, and the difference in energy is absorbed from the surroundings.

26
Q

How are endothermic reactions used? (3)

A

-cooling systems
-thermal insulation
-chemical synthesis

27
Q

What are the safety considerations of using endothermic reactions? (1)

A

Endothermic reactions can result in temperature decreases in the surroundings, potentially leading to frostbite or hypothermia if not properly controlled.

28
Q

How do endothermic reactions have an effect on the environment? (1)

A

Endothermic reactions that involve the absorption of heat energy can contribute to energy consumption and greenhouse gas emissions if fossil fuels are used to provide the required energy.

29
Q

What is the principle of calorimetry? (1)

A

The principle of calorimetry is based on the conservation of energy, stating that energy cannot be created or destroyed but can only change forms.

30
Q

What are the types of calorimeters? (2)

A

-coffee cup calorimeter
-bomb calorimeter

31
Q

What is a coffee cup calorimeter? (1)

A

Used for reactions in solution. It consists of a Styrofoam cup with a lid, equipped with a thermometer.

32
Q

What is a bomb calorimeter? (1)

A

Used for combustion reactions. It consists of a sealed vessel (bomb) surrounded by water in an insulated container.

33
Q

How is calorimetry used? (3)

A

-determination of heat capacity
-enthalpy changes
-physical processes

34
Q

What factors affect the measurements from a calorimeter? (3)

A

-Insulation: Proper insulation minimizes heat exchange with the surroundings.
-Stirring: Ensures uniform temperature distribution.
-Calibration: Calorimeters should be calibrated using known standards for accuracy

35
Q

What are the limitations of calorimetry? (3)

A

-Heat loss: Heat loss to the surroundings can lead to underestimation of heat changes.
-Incomplete reactions: Incomplete or side reactions can affect accuracy.
-Assumption of constant pressure: The assumption may not hold for all reactions, leading to errors.

36
Q

What is meant by standard states in standard enthalpy of formation? (1)

A

Standard states refer to the most stable form of an element at a specified temperature and pressure.

37
Q

What is the value of an element in standard enthalpy of formation? (1)

A

Defined as zero

38
Q

How is standard enthalpy of formation determined? (1)

A

determined experimentally using calorimetry or other thermodynamic methods.

39
Q

What is the equation to calculate standard enthalpy of formation? (1)

A

ΔH°f = ΣΔH°f(products) - ΣΔH°f(reactants), where Σ denotes the sum of the enthalpies of formation for the products and reactants.

40
Q

How is the standard enthalpy of formation used? (3)

A

-thermochemistry
-energy conservation
-rection stoichiometry

41
Q

What are the standard conditions for standard enthalpy of combustion? (1)

A

the substance is completely burned in excess oxygen to form carbon dioxide (CO2) and water (H2O).

42
Q

What are standard enthalpy of combustion usually? (1)

A

Combustion reactions are typically highly exothermic, releasing heat energy to the surroundings.

43
Q

How is standard enthalpy of combustion calculated? (1)

A

ΔH°c = ΣΔH°f(products) - ΣΔH°f(reactants), where Σ denotes the sum of the enthalpies of formation for the products and reactants.

44
Q

How is standard enthalpy of combustion used (3)

A

-energy content
-fuel efficiency
-environmental impact-release carbon dioxide

45
Q

What is Maxwell-Boltzmann distribution? (1)

A

The Maxwell-Boltzmann distribution is a probability distribution that describes the speeds and energies of particles in a gas at a given temperature.

46
Q

What is meant by speed distribution, in terms of Maxwell-Boltzmann distribution? (1)

A

The distribution shows the range of speeds at which gas molecules move in a gas sample.

47
Q

What is meant by energy distribution, in terms of Maxwell-Boltzmann distribution? (1)

A

It also illustrates the distribution of kinetic energies among gas molecules.

48
Q

What is meant by shape, in terms of Maxwell-Boltzmann distribution? (1)

A

The Maxwell-Boltzmann distribution curve is a smooth, symmetrical curve that peaks at the most probable speed/energy.

49
Q

What is meant by temperature dependence, in terms of Maxwell-Boltzmann distribution? (1)

A

The distribution curve shifts to the right (higher speeds/energies) as temperature increases, reflecting the increased kinetic energy of gas molecules.

50
Q

What does the are under the curve represent? (1)

A

The total area under the curve represents the total number of gas molecules in the sample.

51
Q

What does the maxwell-Boltzmann distribution allow? (4)

A

-allows calculation of the mean speed of gas molecules
-provides information on the fraction of molecules that have speeds or energies above or below a specified value.
-explains collision theory and reaction rates, as molecules must have sufficient energy and proper orientation for a successful collision to occur.
-helps in understanding effusion and diffusion processes

52
Q

What are the limitations of the maxwell-Boltzmann distribution? (2)

A

-ideal gas assumption- assumes that gas molecules behave as ideal gases, which may not always hold true under certain conditions
-Monoatomic vs. Polyatomic Molecules- distribution may differ for monoatomic and polyatomic gases due to differences in degrees of freedom and molecular shapes.

53
Q

What does Hess’s law state? (1)

A

The total enthalpy change of a reaction is independent of the route taken.

54
Q

What is the basic equation for enthalpy change of formation? (2)

A

The sum of enthalpy change of formation (reactants) + enthalpy change of reaction = the sum of enthalpy change of enthalpy change of formation (products)

Enthalpy change of reaction = the sum of enthalpy change of formation (products) - the sum of enthalpy change of formation (reactants)

55
Q

What is the basic equation for enthalpy change of combustion? (2)

A

Enthalpy change of formation (products) + enthalpy change of combustion (products) = sum of enthalpy change (reactants)

56
Q

What should you do if you follow an arrow backwards in a Hess’s law diagram? (1)

A

You subtract the enthalpy change that goes with that arrow.

57
Q

What is the basic equation for enthalpy change of a reaction? (2)

A

Enthalpy change of a reaction = (+) enthalpy change of formation (reactants) + enthalpy change of formation (products)

A-LEVEL CHEMISTRY (42 decks)

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