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Chemistry > Energeitcs > Flashcards

Energeitcs Flashcards

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1
Q

What is enthalpy?

A

A measure of the heat energy of a substance

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2
Q

What is enthalpy change ΔH?

A

The change in heat energy of a substance at constant pressure

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3
Q

What is the unit of ΔH?

A

KJmol-1

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4
Q

What is ΔHθ?

A

The change in heat energy under standard conditions - 100kPa + 298K

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5
Q

Are breaking bonds exo/endothermic? Explain your answer

A

Endo - energy is taken in to break the bonds

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6
Q

Are making bonds exo/endothermic? Explain your answer

A

Exo- energy is given out when bonds are made

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7
Q

How do you calculate the overall energy change of a reaction?

A

Energy released when bonds are made in the products- energy needed to break bonds in the reactants

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8
Q

In an exothermic reaction is the enthalpy change positive or negative?

A

negative

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9
Q

Why is the enthalpy change in an exothermic reaction negative?

A

The products have less energy than the reactants because energy is lost to the surroundings

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10
Q

In an endothermic reaction in the enthalpy change positive or negative?

A

positive

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11
Q

Why is the enthalpy change in an endothermic reaction positive?

A

The products have more energy than the reactants because energy is taken in from the surroundings

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12
Q

What is the standard enthalpy of formation ∆Hf?

A

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

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13
Q

Write an equation to show the standard enthalpy of formation of ethanol

A

2C(s) + 3H2(g) + 1/2 O2(g) → C2H5OH(l)

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14
Q

What is the standard enthalpy of combustion ∆Hc?

A

The enthalpy change when one mole of a compound is completely burned in oxygen in standard conditions

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15
Q

Write an equation to show the standard enthalpy of combustion of ethene.

A

C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l)

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16
Q

What is the mean bond enthalpy (bond dissociation enthalpy)?

A

The average energy needed to break a certain type of bonds in a range of compounds

17
Q

Why are bond enthalpies always positive?

A

energy is required to break bonds - endothermic

18
Q

Why might the bond enthalpy calculated using Hess’s law be different to the mean bond enthalpy from a data book?

A

mean bond enthalpies aren’t exact, they are averaged over a range of compounds

19
Q

How can you use bond enthalpies tocalculate the enthalpy change for a reaction?

A

(∑ bond enthalpies of products)-(∑ bond enthalpies of reactants)

20
Q

What is the specific heat energy of a substance ©

A

The amount of energy needed to raise the temperature 1g of substance by 1K

21
Q

What is the unit of specific heat capacity

A

Jg-1K-1

22
Q

Describe the experiment you would do to calculate the enthapy change of a combustion reaction?

A

Calorimetry-burn a known amount of reactant and record the ∆T of known mass of water

23
Q

What is the equation you can use to calculate enthalpy change?

A

q=mc∆T

24
Q

Describe each of the components in the q=mc∆T with units

A

q = heat lost/gained in J, m-mass of solution in g/water in calorimetry, ∆T change in temperature of the water Kelvin(K), c=specific heat capacity of solution/water JK-1K-1

25
Q

Describe the experiment you would do to calculate the enthalpy change of an exo/endo reaction

A

carry out the reaction in a polystyrene cup, measure the temperature every minute starting before mixing, plot time vs temp on a graph to obtain ∆T on mixing by extrapolating

26
Q

Why is extrapolation used to find an accurate ∆T for an exo/endo reaction?

A

to allow for a heat loss from the polystyrene cup

27
Q

Suggest why the experimental ∆Hc of ethanol is lower than the data book value

A

Some heat is lost to the surroundings and is not all transferred to the water

28
Q

Suggest what could be done to reduce the heat loss in a calorimetry experiment

A

use a heat shield to prevent the heat lost from the burning substance

29
Q

What is Hess’s law?

A

The overall enthalpy change for a reaction is the same independent of the route taken

30
Q

What is the ∆Hf of O2 and why?

A

Zero, because the ∆Hf of elements in their standard states are zero

Chemistry (21 decks)

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