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Chem > Energetics > Flashcards

Energetics Flashcards

1
Q

2 types of reaction

A

Endothermic
Exothermic

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2
Q

Enthalpy change

A

Heat energy change measured under conditions of constant pressure

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3
Q

standard enthalpy change symbol

A

ΔH°

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4
Q

What are standard conditions for standard enthalpy changes

A

100kPa
And a stated temp eg 298 Kelvin

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5
Q

Enthalpy of formation

A

ΔfH is the enthalpy change when one mole of compound is formed from its constituent elements in their standard states under standard conditions (298K and 100 kPa).

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6
Q

enthalpy of combustion

A

ΔcH is the enthalpy change when one mole of substance in its standard state is burnt in excess oxygen under standard conditions.

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7
Q

Exothermic definition

A

Heat energy given out
Reactants have more energy than products
🔼H = neg

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8
Q

Endothermic def

A

Heat given in
Reactants have less energy
🔼H = positive

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9
Q

Enthalpy change units

A

Kj/mol
Kjmol^-1

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10
Q

Axis of Enthalpy change diagrams

A

X axis = progress of reaction
Y axis = energy

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11
Q

Activation energy

A

From highest point to reactants

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12
Q

Making bonds is …

A

Exothermic
Releases energy
-ve 🔼H

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13
Q

Breaking bonds is …

A

Endothermic
Req energy
🔼H = positive

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14
Q

Whne is a reaction exothermic

A

When energy rel from bond making is greater than energy taken in from bond breaking

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15
Q

calorimetry equation

A

q = m x c x ΔT

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16
Q

what is calorimetry

A

Calorimetry is the measurement enthalpy changes in chemical reactions

17
Q

q ?

A

heat change (j)

18
Q

m?

A

mass (g)
of water whereby 1cm3 = 1g

19
Q

c?

A

specifc heat capacity
4.18

20
Q

ΔT

A

change in temp
K

21
Q

After calc q you …

A

convert to kj
divide by number of mol

22
Q

hess law states

A

enthalpy change is independent of the route taken

23
Q

how to calc enthalpy change of combustion

A

arrows point down toward
co2 and h2o at bottom
enthalpy chage = reac - proud

24
Q

State the meaning of the term average bond Enthalpy for the _____ bond

A

bond Enthalpy change to break 1 mol of _____ bonds

25
Q

How to calc Enthalpy of formation

A

arrows point upwards
Elements at the bottom
Prod - reactants

26
Q

define mean bond enthalpy

A

The average heat energy required to break one mol of covalent bonds over a range of compounds

27
Q

How to calculate mean bond enthalpy

A

Change in H = sum bonds broken - sum bonds formed
This gives 1 mark

28
Q

Why may Enthalpy change of an element thag you would expect to be 0 not be 0

A

Element not in its standard state

29
Q

State why standard Enthalpy of formation of oxygen is 0

A

By definition 🔼Hf is formation from an element

Chem (18 decks)

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