1.4 Energetics Flashcards

1
Q

What energy change is breaking bonds associated with?

A

Energy is taken in to break bonds —> endothermic reaction

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2
Q

What energy change is making bonds associated with?

A

Energy is released to make bonds —> exothermic reaction

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3
Q

What are some uses of thermochemistry?

A

Measuring and comparing the energy values of fuels

Calculating the energy requirements for industrial processes

Working out the theoretical amount of energy released/ taken in in a reaction

Predicting if a reaction will take place or not

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4
Q

What is an endothermic reaction?

A

One with an overall positive enthalpy change —> energy in breaking bonds> energy out making bonds

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5
Q

What is an exothermic reaction?

A

One with an overall negative enthalpy change —> energy in breaking bonds < energy out making bonds

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6
Q

If a reversible reaction is endothermic one way, what type of the reaction is the other way?

A

Exothermic

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7
Q

Give 2 examples of exothermic reactions

A

Combustion of fuels

Neutralisation

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8
Q

Give an example of an endothermic reaction

A

Thermal decomposition

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9
Q

Define enthalpy change; what symbol is used to represent it?

A

Heat energy change of a system at a constant pressure

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10
Q

What are the standard conditions?

A

100kPa/ 1atm pressure

289K / 25 degree Celsius temperture

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11
Q

What does in standard state mean?

A

The state an element/compound exists at in standard conditions (100kPa, 298K)

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12
Q

Draw an enthalpy change diagram for an endothermic reaction and one for an exothermic reaction

A

Slide 26

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13
Q

Define standard enthalpy of formation

A

The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions (100kPa, 298K) with reactants and products in their standard states

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14
Q

Give an example of an equation which represents standard enthalpy of formation

A

There are many H2 (g) +1/2O2(g) —> H2O(l)

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15
Q

Define standard enthalpy of combustion

A

The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPa,298K) with reactants and products in their standard states

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16
Q

Give an example of an equation which represents standard enthalpy of combustion

A

C(s) + O2(g) —> CO2 (g)

17
Q

What is the difference between heat and temperature?

A

Heat is the sum of all particles’ energy therefore it is affected by the ammount of substance

Temperature is related to the mean kinetic energy of the particles in a system, so is independent of the number of particles present

18
Q

How can you calculate enthalpy change from experimental data?

A

Use the equation Q=MCΔT

M is mass of substance being heated
C is specific heat capacity of that substance being heated
ΔT change in temperature

19
Q

Draw a simple calorimeter

A

Slide 40

20
Q

How could this calorimeter be made more accurate?

A

Add draught screens at the sides, add a lid on the top of the beaker, add mineral wool around the beaker —> all to insulate and reduce heat lost to the surroundings

21
Q

What is a flame calorimeter

How does it differ to a simple calorimeter?

A

Reduces heat lost to the surrounding to give more accurate results:has spiral chimney made of copper, enclosed flame and burnt in pure oxygen not air

22
Q

How would you measure the enthalpy change for a reaction occurring in (aq)?

A

Use an expanded polystyrene cup as a calorimeter (good insulator therefore reduces heat loss)
Heat is generated in the solution; measure this temperature change. Take heat capacity I’d solution to be 4.18 and density of solution = 1gcm^-3

23
Q

What can you use to make experimental determination of enthalpy change of reaction more accurate?

A

Cooling curves

24
Q

What is hess’s law?

A

Enthalpy change in a reaction is independent of the route taken.

25
Q

What is the enthalpy of an element?

A

The enthalpy of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 0

26
Q

Define bond dissociation enthalpy

A

The enthalpy change required to break a covalent bond, with all species in the gaseous state; differs for the same bond type in different molecules

27
Q

Define mean bond enthalpy

A

Average value( across different chemical environments) for the bond dissociation enthalpy of a given bond.

28
Q

Why may experimental methods for enthalpy determination not be very accurate?

A

Heat is lost to the surroundings

Not in standard conditions

Reaction may not go to completion

29
Q

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

A

Bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply to that molecule therefore it is more accurate