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Module 3 (7-10) > Chapter 9 > Flashcards

Chapter 9 Flashcards

1
Q

What is Enthalpy (H)?

A

A measure of the heat energy in a chemical system

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2
Q

What is a chemical system?

A

The atoms/molecules/ions making up the chemicals

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3
Q

How do you work out enthalpy change ΔH?

A

ΔH= enthalpy of products - enthalpy of reactants

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4
Q

Is enthalpy change negative or positive?

A

It can be either depending on if the products contain more or less energy then the reactants

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5
Q

What is the law of conservation of energy?

A

Energy cannot be created or destroyed.

So heat in a chemical system is transferred between the system and the surroundings.

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6
Q

What is an exothermic reaction and when does it occur?

A

A reaction is exothermic when the products have less energy than the reactants.

Heat is given off by the reaction to the surroundings. The temperature of the environment increases and can be measured with a thermometer.

The energy of the system decreases.

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7
Q

How does the enthalpy profile diagram of an exothermic reaction look?

A

Goes from reactants up to the peak and down steeply to products (products have less energy).

enthalpy change is negative and the activation energy goes up.

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8
Q

What is an endothermic reaction and when does it occur?

A

A reaction is endothermic when the products have more energy than the reactants.

Heat energy is absorbed by the reaction form the surroundings.

The temperature of the environment decreases and can be measured by a thermometer. The energy of the system increases.

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9
Q

How does the enthalpy profile diagram of an endothermic reaction look?

A

Starts low energy with reactants and goes up to peak and down slightly to products. (reactants have less energy)

The enthalpy change is positive.

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10
Q

What Is an enthalpy profile diagram?

A

Diagram that shows the:
-energy level of reactants and products
-the peak (transition state)
-the activation energy (Ea)
-the enthalpy change of the reaction (ΔH)

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11
Q

What is activation energy (Ea)?

A

The minimum amount of energy needed for reactant molecules to have a successful collisions and start the reaction

minimum amount of energy needed for a reaction to occur

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12
Q

What is the definition of the enthalpy change of reaction (ΔH)?

A

This can be describes as the overall energy taken in from / given out to the surroundings OR the energy difference from reactants to products

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13
Q

What symbol is used to show something is ‘standard’?

A

Ꝋ

this means that the reaction was carried out in standard conditions.

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14
Q

What are the standard conditions?

A

-pressure of 100 kPa
-temperature of 298 K (25C)
-Each substance should be in its standard physical state.
-standard concentration- 1moldm-3 (only relevant for solutions)

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15
Q

What is the symbol for the enthalpy change of reaction?

A

ΔHꝊr

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16
Q

What is the symbol for the enthalpy change of formation?

A

ΔHꝊf

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17
Q

What is the symbol for the enthalpy change of combustion?

A

ΔHꝊc

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18
Q

What is the symbol for the enthalpy change of neutralisation?

A

ΔHꝊneut

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19
Q

What is the definition for the enthalpy change of reaction?

A

The enthalpy change that accompanies a reaction in the stoichiometry shown in a chemical equation under standard conditions with all reactants and products in their standard states.

20
Q

What is the definition for the enthalpy change of formation?

A

The enthalpy change that takes place when one mole of a compound is formed from its own elements standard conditions with all reactants and products in their standard states.

21
Q

What is the definition for the enthalpy change of combustion?

A

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states.

22
Q

What is the definition for the enthalpy change of neutralisation?

A

The energy change that accompanies the reaction of an acid by a base to form one mole of water under standard conditions with all reactants and products in their standard states.

23
Q

What is 0 degrees Celsius in Kelvin?

A

273K

24
Q

What 3 quantities do you use to calculate the energy change of the surroundings?

A

1) The mass of the surroundings (m)
2)The specific heat capacity of the surroundings (c)
3) the temperature change of the surroundings (ΔT)

25
Q

How do you get the mass of the surroundings for the Q= mcΔT equation?

A

You weigh the materials that are changing temperature.

26
Q

How do you get the specific heat capacity of the surroundings for the Q= mcΔT equation?

A

Every substance has a specific heat capacity. We mainly use the SHC of water in experiments (4.18jg-1K-1)

27
Q

What is specific heat capacity?

A

The energy required to raise the temperature of 1g of a substance by 1K.

28
Q

How do you get the temperature change of the surroundings for the Q= mcΔT equation?

A

temperature reading at end - temperature reading at start

29
Q

What equation do we use to calculate an energy change?

A

Q= mcΔT

30
Q

What unit is Q in in the Q= mcΔT equation and what does it mean?

A

J - joules
It is the heat transferred

31
Q

What unit is Q in in the Q= mcΔT equation and what does it mean?

A

J - joules
It is the heat transferred

32
Q

Why could the value of an enthalpy change of combustion be inaccurate? (calculated value differs from data book value)

A

-Heat loss to the surroundings other than water (beaker/air)
-Incomplete combustion (CO produced and C so black layer of soot)
-Evaporation of the alcohol from the wick of the spirit burner (burner must be weighed as soon as possible after you extinguish the flame)
-Non-standard conditions

33
Q

What can we use to calculate enthalpy changes without having to do any experiments?

A

Average bond enthalpies

34
Q

What is an Average bond enthalpy?

A

The energy required to break one mole of a specified type of bond in a gaseous molecule.

35
Q

Are bond enthalpies endothermic or exothermic?

A

Endothermic (so always have a positive value)

36
Q

How is a bond enthalpy for a specific bond calculated?

A

an average of a number of the same type of bond but in different environments is calculated.

So C-H in methane but also C-H in ethene.

37
Q

What Is a limitation of using average bond enthalpies?

A

The average bond enthalpy won’t be representative of the exact bond that you are trying to use to calculate the enthalpy change (eg OH in methanol needs a lot less energy then an OH bond in water) so the values aren’t that accurate.

38
Q

Is bond breaking endothermic or exothermic?

A

Endothermic (positive enthalpy change)

Energy is needed to break the bonds

39
Q

Is bond making endothermic or exothermic?

A

exothermic (enthalpy change is negative)

Energy is released when bonds are made

40
Q

How do you calculate enthalpy change with average bond enthalpies?

A

Enthalpy change of reaction = sum of bond enthalpies of reactants - sum of bond enthalpies of products.

41
Q

What do we use when we can’t calculate enthalpy changes directly?

A

Hess’ Law and Hess’ cycle

42
Q

What does Hess’ law state?

A

If a reaction can take place by 2 routes and the starting and finishing conditions are the same then the total enthalpy change is the same for each route.

43
Q

How do the arrows look in a standard enthalpy of formation Hess’ cycle?

A

They both go up

44
Q

How do the arrows look in a standard enthalpy of combustion Hess’ cycle?

A

they both go down

45
Q

What do you put at the bottom equation in a standard enthalpy of formation Hess’ cycle?

A

All of the separate elements balanced (Co2 would be broken up to C + O2)

46
Q

What do you put at the bottom equation in a standard enthalpy of combustion Hess’ cycle?

A

The combustion products (CO2 and H2O)

Module 3 (7-10) (4 decks)

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