# Energetics Flashcards Preview

## A-level AQA Chemistry - Physical > Energetics > Flashcards

Flashcards in Energetics Deck (15)
1
Q

Enthalpy Change

Symbol

Units

Standard Conditions

A

The heat energy transferred in a reaction at constant pressure.

Delta H

kJ mol -1

100 kPa pressure and a temperature of 298 K (25oC)

2
Q

Exothermic Reactions

A

Give out energy

Delta H is negative

3
Q

Endothermic Reactions

A

Absorb energy

Delta H is positive

4
Q

Endothermic reactions and bonds

A

Energy is needed to break bonds, so bond breaking is endothermic (delta H is positive).

Stronger bonds take more energy to break

5
Q

Exothermic reactions and bonds

A

Energy is released when bonds are formed, so bond making is exothermic (delta H is negative).

Stronger bonds release more energy when they form

6
Q

Enthalpy Change of a Reaction

A

The overall effect of bonds broken and bonds being formed.

If you need more energy to break bonds than is released when bonds are made, delta H is positive.

If it’s less, delta H is negative

7
Q

Bond Enthalpy

A

The energy required to break bonds

8
Q

Mean Bond Enthalpy

A

The average energy needed to break a certain type of bond, over a range of compounds

9
Q

Formula for calculating enthalpy change of reaction

A

Total energy absorbed - total energy released

10
Q

Standard Enthalpy of Formation

A

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

11
Q

Standard Enthalpy of Combustion

A

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

12
Q

Formula for calculating enthalpy changes from a calorimetry experiment

Describe each part and give the units

A

q=mc delta T

where:
q= heat lost or gained (in joules). Same as the enthalpy change if pressure constant
m= mass of water (or other solution) in the calorimeter (in grams)
c= specific heat capacity of water (4.18 J g-1 K-1)
Delta T= the change in temperature (K) of the water or solution

13
Q

Hess’s Law

A

The total enthalpy change of a reaction is independent of the route taken

14
Q

Two assumptions when calculating enthalpy changes

A
1. You can assume that all solutions have the same density as water.
2. Although the official unit for delta T is K, the value is actually the same in oC
15
Q

Specific Heat Capacity of Water

A

The amount of energy it takes to raise the temperature of 1 g of water by 1 K