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OCR A A-Level Chemistry > Chapter 9 - Enthalpy > Flashcards

Chapter 9 - Enthalpy Flashcards

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1
Q

Define the conversation of energy

A
  • energy cannot be made/destroyed

* only converted from one form to another

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2
Q

Define Enthalpy

A

•the heat content is stored in a chemical system (chemical energy)

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3
Q

Define enthalpy change

A
  • heat exchanges with surroundings during chemical change

* difference between the enthalpy of the product and reactants

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4
Q

Calculate enthalpy change out of the reactants and the products enthalpy change

A

🔺H = H (products) - H (reactants)

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5
Q

Calculate the percentage of uncertainty

A

(n x maximum uncertainty) / (size of measurement) x 100

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6
Q

Define exothermic

A

Enthalpy of products is smaller than enthalpy of reactants - heat lost to surrounding (negative)

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7
Q

Define endothermic

A

Enthalpy of products is greater than the enthalpy of the reactants - heat taken in by surrounding (positive)

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8
Q

What do enthalpy profiles involve?

A
  • enthalpy on y-axis
  • progress of reaction on x-axis
  • reactants
  • products
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9
Q

Does an EXOTHERMIC enthalpy profile got down to up or up to down?

A

Up to down

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10
Q

Does an ENDOTHERMIC enthalpy profile go down to up or up to down?

A

Down to up

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11
Q

Define activation energy (Ea)

A

It’s the minimum energy required to start a chemical reaction by breaking of bonds

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12
Q

What are the standard conditions?

A
  • 🔺H(s/c/f/r) ^Ø
  • T = 25°C / 298 K
  • P = 100kPa = 1 atm
  • C = 1 moldm^-3
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13
Q

Calculate the energy exchange

A

q = mc🔺T

  • q = energy exchange with surrounding (J)
  • m = mass of surrounding (g)
  • c = specific heat capacity of surrounding (Jg^-k^-)
  • 🔺T = temperature change
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14
Q

Calculate enthalpy change

A 
🔺H = -q/n
•🔺H = enthalpy change (Jmol^-)
•-q = enthalpy change with surrounding (J)
•n = mol
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15
Q

Define the observer effect

A

Changes in act of observation will make on phenomenon being observed. The result of the instruments that alter the state of what they measure in some manner

E.G a thermometer may heat or cool the solution

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16
Q

Define the standard enthalpy change of combustion

A

Enthalpy change takes place when one mole of a substance reacts completely with oxygen, understand the standard conditions, all of the reactants and products in standard state

17
Q

Define bond dissociation enthalpy (🔺Hd^ø)

A

Enthalpy change that takes place when breaking, by homily’s is fission, 1 mol of a given bond in the molecules of gaseous species

18
Q

Define homolytic fission

A
  • same number of electrons on either atom
  • covalent bond
  • endothermic
19
Q

Overall enthalpy change = ………………………………

A

Bond broken - bonds formed

20
Q

Define average bond enthalpy

A

Enthalpy change when one mole of bond of gaseous bond is broken

21
Q

Why may the data book value of enthalpy bond be different from enthalpy bond calculated?

A

Bond enthalpy are averages of bond energies in a variety of compounds

22
Q

Define Hess’ Law

A

If a reaction can take place by more than one route and the initial and final concentration are the same, the total energy change is the same for each route

23
Q

Define enthalpy cycle

A

Allows indirect determination of enthalpy change from other enthalpy changes using Hess’ law

24
Q

Calculation using enthalpy cycle

A

Route 1 = route 2

🔺H1 = 🔺H2 + 🔺H3

25
Q

Define enthalpy changes of combustion

A

Enthalpy change takes place when one mole of a substance reacts completely with oxygen under standard conditions

26
Q

State why the standard enthalpy of formation for hydrogen gas is zero

A

It is an element

27
Q

Define enthalpy change of formation

A

enthalpy change takes place when one mole of a compound is formed from its constituent elements in standard states

28
Q

Suggest two reasons why endothermic reaction needs heating to a higher temperature

A
  • to overcome activational energy

* higher temperature is needed to break the bonds

29
Q

Define enthalpy change of neutralisation

A

Energy change that accompanies the reaction of an acid by a base to form one mole of water, under standard conditions

30
Q

Give two examples of exothermic reactions

A

Combustion

Oxidation, e.g. of carbohydrates

31
Q

Give two examples of endothermic reactions

A

Thermal decomposition

Photosynthesis

OCR A A-Level Chemistry (28 decks)

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