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AS Chemistry unit 2 > Energetics > Flashcards

Energetics Flashcards

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1
Q

Definition of exothermic

A

Reaction in which the enthalpy of the products is less than the enthalpy of the reactants

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2
Q

Endothermic definition

A

A reaction in which the enthalpy of products is greater than the enthalpy of reactants

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3
Q

Write the equation for the enthalpy change of a reaction

A

(Enthalpy of products) - (enthalpy of reactants)

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4
Q

Draw the graph of the enthalpy change for an exothermic process

A

See notes

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5
Q

Draw the graph of the enthalpy change of an endothermic reaction

A

See notes

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6
Q

Describe the characteristics of an exothermic reaction

A

Enthalpy change is negative
Products have a smaller enthalpy than the reactants
Temp of surroundings rises due to energy given out to surroundings

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7
Q

Describe the characteristics of an endothermic reaction

A

Enthalpy change is positive
Energy is absorbed from surroundings
Temperature of surroundings falls
Products have a higher enthalpy than reactants

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8
Q

What does the value of H depend on

A
  1. The amounts of reactant used
  2. The physical states of reactants/products
  3. The temperature and pressure at which the reaction is carried out
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9
Q

Name the “standard conditions”

A

298K (20C) and 100kPa

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10
Q

Definition of standard enthalpy change

A

The change in heat energy at a constant pressure; measured at standard conditions

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11
Q

Definition of the standard enthalpy of a reaction

A

The enthalpy change when the number of moles of substances are as written in the equation

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12
Q

Definition of the standard enthalpy of formation

A

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

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13
Q

Definition for the standard enthalpy of combustion

A

The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions

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14
Q

What is the enthalpy of formation of an element

A

It is always 0

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15
Q

Definition of the standard enthalpy of neutralisation

A

The enthalpy change when one mole of water is produced in a neutralisation reaction under standard conditions

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16
Q

Draw the diagram for the enthalpy of combustion

A

See diagram on page 5

17
Q

Write the equations used to find the standard enthalpy of combustion

A

See page 5

18
Q

What do the letters stand for in the equation q=mc 🔼T

A 
q = enthalpy change (J)
m = mass of water/solution (g)
c = specific heat capacity (JK^-1g^-1)
🔼T = change in temperature (K)
19
Q

What is meant by “specific heat capacity”?

A

It is the energy required to raise the temperature of 1g of substance by 1K

20
Q

What are the units for standard enthalpy changes?

A

KJmol^-1

21
Q

The method for finding the enthalpy of neutralisation

A
  1. Measure 50cm^3 of HCl (1M) into a polystyrene cup
  2. Record the temp
  3. Add 50cm^3 of NaOH (1M)
  4. Attach the lid and stir
  5. Record max temp
  6. Use the two equations
22
Q

The law of conservation of energy

A

Energy cannot be created or destroyed but it can change from one form to another

23
Q

Hess’s Law

A

The enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same

24
Q

Definition for average bond enthalpy

A

The energy required to break one mole of a given bond averaged over many compounds

25
Q

Write the method for calculating the enthalpy of a reaction using bond energies

A
  1. Write a balanced equation using structural formulae
  2. List the bonds present in reactants and products
  3. List the bond energies (bond breaking in reactants is endo) (bond making in product is exo)
26
Q

Why are values using bond energies different from Hess’s law?

A

Average bond enthalpies give an estimate of the value as they are not specific to the compounds involved in the reaction

27
Q

What is an endothermic reaction in terms of bonds, is it positive or negative KJmol^-1

A

Bond breaking

Positive

28
Q

What is an exothermic reaction in terms of bonds and is it positive or negative KJmol^-1

A

Bond making

Negative

AS Chemistry unit 2 (10 decks)

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