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(A-level) Chemistry > Module 3.2 > Flashcards

Module 3.2 Flashcards

1
Q

What is enthalpy

A

A measure of heat energy in a chemical system

It can be thought of the energy stored in bonds

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2
Q

What is the law of conservation of energy

A

The law that:

energy cannot be created or destroyed only transferred

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3
Q

How is enthalpy change measured

A

By measuring the energy transferred from the system to the surroundings

And the energy transferred from the surroundings to the system.

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4
Q

What is an exothermic reaction

A

A reaction with a negative change in energy between products and reactants

  • The temperature of the surroundings increase
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5
Q

What is an endothermic reaction

A

A reaction where enthalpy change is positive between products and reactants

  • the temperature of the surroundings decreases
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6
Q

What is activation energy

A

The minimum energy required for a reaction to begin

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7
Q

How do you draw an enthalpy diagram

A

write the reactants and products on the diagram

Draw the activation energy from the reactants to the top of the curve

Draw the enthalpy change from the reactants to the products

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8
Q

What are the standard conditions for change in enthalpy

A

100kpa pressure
298k temperature
1mol/dm3 concentration

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9
Q

What is the standard state of a substance

A

the physical state of the substance under standard conditions

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10
Q

What is the standard enthalpy change of reaction (ΔrH)

A

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions and in standard states

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11
Q

What is the standard enthalpy change of formation (ΔfH)

A

The enthalpy change that takes place when 1 mol of a substance IS FORMED FROM ITS PRODUCTS under standard conditions, with all products and reactants in their standard states,

e.g

C(s) + 2H2(g) → CH4(g)

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12
Q

What is the standard enthalpy change of combustion (ΔcH)

A

The change in enthalpy that takes place when 1 mole of a substance reacts with oxygen under standard conditions with all reactants and products in their standard states

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13
Q

how is energy calculated in a reaction under standard conditions (calorimetry)

A

q = mcΔT

q = energy (J)
m = mass (g)
c = specific heat capacity (J/g/k)
ΔT = change in temperature (k)

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13
Q

What is the standard enthalpy change of neutralisation (ΔneutH)

A

The enthalpy change that accompanies the reaction of an acid and base to form 1 mole of H2O under standard conditions whith all reactants and products in their standard states

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14
Q

How is the enthalpy change of combustion in a reaction calculated (equation)

A

Find the q=mcΔT of the solution

Then divide the energy (which must be converted to KJ) by the moles of a specific substance
q(KJ) / mol = ΔcH (KJ/mol)

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15
Q

What is Hess’s Law

A

The law that enthalpy changes in a chemical reaction are independent of the route they take

∆H1 + ∆H2 → ∆H

16
Q

How is a regular Hess cycle drawn

A

The direct route is between the products and reactants

The indirect route is from the products to the alternative route (alternative products / reactants) to the products

In the indirect route an arrow points down to the alternative products from the reactants
And an arrow points up from the alternative products to the products

17
Q

What is the ∆Hf of elements

A

0

If the element is on both sides of the reaction its enthalpy change is 0

18
Q

How do you draw the Hess cycle for enthalpy change of formation

A

Write the constituent elements below the reaction.

Draw arrows pointing up from the constituent elements to the reactants and the products

19
Q

How do you draw the Hess cycle for enthalpy change of combustion

A

Write the equation
Underneath, write the products of combustion (usually H2O and CO2)

Draw arrows pointing to the reactants of combustion, from the products and the reactants

20
Q

What are average bond enthalpies

A

The mean energy needed for 1 mole of a given gaseous bond to undergo homolytic fission (breaking covalent bonds)

21
Q

What type of reaction is bond breaking

A

Endothermic
- energy is absorbed to break bonds

22
Q

What type of reaction is bond forming

A

Exothermic
- energy is released when bonds are formed

23
Q

Why are reactions exothermic

A

If more energy is released when forming bonds than energy is absorbed to break bonds, there is a negative energy change

This means the enthalpy change is negative

24
Q

Why are reactions endothermic

A

If more energy is absorbed when breaking bonds, than is released forming new bonds there is a positive enthalpy change.

The reaction is endothermic

25
Q

How do you calculate ∆H using average bond enthalpies

A

∆H of reactants - ∆H products

(Break - make)

26
Q

When calculating using average bond enthalpies do moles matter

A

Yes - multiply the average bond enthalpy by the number of moles

27
Q

In Hess cycle calculations, do you need to do anything with moles

A

Yes
Multiply the correct enthalpy changes by the correct moles

28
Q

What is Calorimetry

A

A quantitative study of energy in a chemical reaction

A mathematical equation is then used to calculate enthalpy change ( q = mc∆T)

29
Q

Describe the experiment allows the direct measurement of enthalpy of combustion
(Specifically copper calorimetry)

A

Combusting a fuel can be used to increase the temperature of a known mass of water

-Measure the fuels starting mass
-Add a known mass of water to the copper calorimeter
-Mount the copper calorimeter above the fuel and measure the starting temperature of fuel
-Combust the fuel using a spirit burnerfor a few minutes (e.g 5) and record the waters final temperature
-Take the mass of the unused fuel and calculate the fuel used

Calculate the energy change of water
Find the moles of fuel burnt
Energy ÷ moles = enthalpy

30
Q

How can enthalpy of reaction be measured (experiment - the simpler one)

A

-Place an insulated polystyrene cup into a beaker, with a hole for a thermometer
-Add a known measurement of a known concentration liquid and record its temperature (every minute) until its stable
-Add the second reactant and do not record the temperature
- Record the temperature every minute for 5 minutes

Plot a graph to calculate the temperature change in the reaction
Then calculate the change in enthalpy

31
Q

How do you calculate density

A

Density = mass / volume

32
Q

Why might the calculated enthalpy change of an experiment be different to the actual value

A
  • Heat energy could be lost to the surroundings
  • The reaction may not go to completion ( e.g incomplete combustion)
  • Non - standard conditions
33
Q

Why might the calculated enthalpy change of an experiment be different to the actual value WHEN USING AVERAGE BOND ENTHALPY

A

It is an average bond enthalpy - the values are not exact

Bond energy / enthalpy assumes all molecules are gasseous

(A-level) Chemistry (7 decks)

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