Energetics Flashcards
What is thermochemistry?
the study of heat changes during chemical reactions.
What are the principles of thermochemistry?
> When a chemical reaction takes place, chemical bonds break and new ones are formed.
> Energy must be put in to break bonds and energy is given out when bonds are formed. As a result energy change happens
> The overall change may result in energy being given out or taken in.
> At the end of the reaction, if energy has been given out, the reaction is exothermic.
> At the end of the reaction, if energy has been taken in, the reaction is endothermic.
> Always the case that a reaction that is endothermic in one direction is exothermic in the reverse direction.
Give examples of exothermic reaction?
Reactions involving neutralising acid with alkali
Give an example of an endothermic reaction?
The breakdown of limestone (calcium carbonate) to lime (calcium oxide) and carbon dioxide
endothermic reaction is heating of copper sulfate.
What does endothermic mean?
Reactions that take in heat from their surroundings to keep the reaction going.
What does exothermic reaction mean?
reactions give out heat as they proceed
What factors affect the amount of energy given out in a reaction?
Depends on the quantity of reactants.
What is the amount energy given off measured in?
Kilojoules per mole, kJ mo1- 1
What is enthalpy change?
When you measure a heat change at constant pressure
What is the standard conditions for measuring enthalpy change?
> Pressure of 100 kPa
> Temperature of 298K
What is standard molar enthalpy of formation?
Is the enthalpy change when one mole of substance is formed from its constituent
elements under standard conditions. all reactants and products being in their standard states.
What is Standard Molar Enthalpy Of Combustion?
Is the enthalpy change when one mole of substance is completely burnt in
oxygen under standard conditions, all reactants and products being in their standard states.
What is specific heat capacity?
Is the amount or heat needed to raise the temperature of Jg of substance by I K
What can a calorimeter be used for?
it can be used to approximate
Why is the heating of Blue copper sulfate crystals a endothermic reaction in product to reactant side?
The water molecules are bonded to the copper sulfate - so in order to break these bonds and make white, anhydrous copper sulfate, heat energy must be supplied
Why can thermochemistry be seen as important in real life?
it enables us to compare the efficiency of different fuels
What does standard molar enthalpy change mean?
The general name for the enthalpy change for any reaction
What is heat?
measure of the total energy of all the particles present in a given amount of substance
What is temperature? `
related to the average kinetic energy of the particles in a system
How can you measure enthalpy change?
By arranging the heat to be transferred into a particular mass of a substance, often water
You then find out:
1) mass of the substance that is being heated up or cooled down
2) work out the temperature change
3) work out the specific heat capacity of the substance.
Formula for enthalpy change?
Mass of substance (m) x specific heat capacity (c) x temperature change
Why is a flame calorimeter better than a simple calorimeter?
- This is because the spiral chimney is made of copper
- The flame is enclosed
- The fuel burns in pure oxygen, rather than air.
{which reduces heat loss}
What is hess’s law?
Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
Why are polyester cups best suited to be used for calorimetery?
They are good insulators (this reduces heat loss through their sides)
They have a low heat capacity so they absorb very little heat
What are enthalpy diagrams used for?
To represent the enthalpy changes in chemical reactions - they show the energy (enthalpy) levels of the reactants and products of a chemical reaction on a vertical scale, so you can compare their energies
What is the enthalpy of elements in their standard state?
0
What is bond dissociation enthalpy?
As the enthalpy change required lO break a covalent bond with all species in the gaseous state.
