Energetics Flashcards
Is energy released or gained when bonds are made?
Energy is released
Is energy released or gained when bonds are broken?
Energy is gained
What is an exothermic reaction?
- Transfers energy to the surroundings so temp of surroundings increases
- Energy released from forming new bonds is greater than the energy needed to break existing bonds
What is an endothermic reaction?
- Takes in energy from the surroundings so the temperature of surroundings decreases
- Energy needed to break existing bonds is greater than the energy released from forming new bonds
What is thermochemistry?
Study of heat changes during chemical reactions
Example of exothermic reaction
Neutralising an acid with an alkali
Burning fuels
Examples of endothermic reaction
Breakdown of limstone (calcium carbonate) to lime (calcium oxide) and carbon dioxide
Cold packs
Definition of enthalpy change
The heat energy change measured under conditions of constant pressure
What does (delta)H mean?
Enthalpy change
What are standard condition?
100kPa
298K (25 degrees Celsius)
How do you convert between K and degrees C?
To covert C to K, +273
Are exothermic reactions (delta)H + or -?
Why?
(delta)H-
Remember: Ex’s are negative
Products end up with less heat energy than the starting materials as they have lost head energy when they heated up their surroundings
Are endothermic reactions (delta)H + or -?
Why?
(delta)H+
Products end up with more energy than the staring materials
Why is it important to have a standard of pressure when measuring energy changes?
Pressure affects the amount of heat energy given out my reactions that involve gases
If gas is given out, some energy is required to push away the atmosphere
The greater the atmospheric pressure, the more energy used for this, and less energy remains to be given out as heat by the reaction
What effect does the states of the reactants and products have?
Affect the enthalpy change of a reaction
E.g. heat put in to change liquid into a gas
E.g. heat given out to change gas into liquid
What is the unit for ‘standard molar enthalpy change’?
KJmol-1
Definition of ‘standard molar enthalpy of formation’
The enthalpy change when ONE MOLE of a substance is formed from its CONSTITUENT ELEMENTS under STANDARD CONDITIONS, all reactants and products being in their STANDARD STATES
Definition of ‘standard molar enthalpy of combustion’
The enthalpy change when ONE MOLE of a substance is COMPLETELY burnt in oxygen under STANDARD CONDITIONS, all reactants and products being in their STANDARD STATES
Definition of ‘specific heat capactity’
Amount of heat needed to raise the temperature of 1g of substance by 1K
What is the unit of specific heat capacity?
Jg-1K-1
What is the equation to find enthalpy change?
Enthalpy change = mass of substance x specific heat capacity x temperature change
What apparatus can you use the find an approximate enthalpy change when a fuel burns?
Calorimeter
Definition of Hess’s Law
Enthalpy change of a chemical reaction is independent of the route taken
What way do the arrows point in the thermochemical cycle of ‘enthalpy changes of formation’?
Upwards
From elements in states to reactants and produces
What is the enthalpy of an element in their standard state?
Zero 0
What way do the arrows point in the thermochemical cycle of ‘enthalpy changes of combustion’?
Both go down
Definition of ‘bond dissociation enthalpy’
Enthalpy change required to break a covalent bond with all species in the gaseous state
Definition of ‘mean bond enthalpy’
The enthalpy change when one mole of a specified covalent bond is broken averaged out across the range of compounds
What is the equation to find overall enthalpy change?
Delta H = energy to break bonds (+ve) + energy to make bonds (-ve)
Definition of ‘bond enthalpy’
Enthalpy change to break one mole of covalent bonds in gaseous state
