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Flashcards in Energy Deck (15)
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1

Enthalpy

Conservation of energy
Enthalpy change is 🔺H

2

Exothermic

Heat loss from system to the surroundings
🔺H is negative
Products have a lower enthalpy than reactants

Examples
self-heating cans
Oxidation of fuels for cooking
Respiration

3

Endothermic

Enthalpy of products is greater than the reactants
Heat is gained from the surroundings
🔺H is positive

Examples
self-cooling beer cans
Thermal decomposition
Photosynthesis

4

Activation energy

The minimum amount of energy required to start a reaction by breaking bonds

5

Standard conditions

Pressure of 100kPa or 1 atmosphere
Stated temperature usually 289K or 25oc
And a concentration of 1.0 mol dm -3 for reactions with aqueous solutions

6

Standard state

Physical state of a substance under standard conditions of 100kPa and 298K

7

The standard enthalpy change of reaction 🔺Hr

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

8

The standard enthalpy change of combustion 🔺Hc

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions and a standard state

9

The standard enthalpy change of a formation 🔺Hf

Enthalpy change that takes place when one mole of a compound is formed when its constituent elements in their standard states and standard conditions

10

Specific heat capacity, c

Energy required to raise the temperature of 1g of a substance by 1oc

q=mc🔺t

11

Hess's law

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

12

Rates of reaction

Change in concentration of a reactant or a product in a given time

Factors that affect it:
Temperature
Pressure
Concentration
Surface area
Adding a catalyst

13

Catalysts

Increases the rate of reaction without being used up in the process
Provides an alternative route
Require lower temperature, reducing energy demand and emissions
Better atom economy
Reduce waste
Harber process:
ammonia is made by reacting nitrogen and hydrogen
Nitrogen triple bond has to be broken
Enzymes act as a biological catalysts
- can work at lower temperatures
- biodegradable
-eg: washing detergents

14

The Boltzmann distribution

The distribution of energies of molecules at a particular temperature often shown as a graph

Only molecules with an energy greater than the activation energy, Ea, are able to react

15

Equilibrium

Reversible reactions

Dynamic equilibrium:
Closed system
Rates are equal

Le Cheatelier's principle: when a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change

Concentration:
Increase - left
Decrease - right

Pressure:
Increase - fewer molecules
Decrease - more molecules

Temperature:
Increase - endothermic
Decrease - Exothermic