Enthalpy

Enthalpy

What we need to know for AS91164

Multiple Choice

Enthalpy is what?

• The measure of disorder in a system

• The measure of heat energy in the system

• An impossible concept

Poll

How confident are you at identifying whether a reaction is exothermic or endothermic?

• Not confident at all

• A little bit confident

• Very confident

• Super duper confident

Multiple Choice

Does this diagram show an endothermic or exothermic reaction?

• Endothermic as there is more energy in the reactants than products

• Exothermic as there is more energy in the reactants that products

Open Ended

As exothermic reactions are where the reactants (what you start with) have more energy that the products (what you end up with). What happens to the energy that has been released?

Fill in the Blanks

___________ reactions absorb energy from the surroundings. An example of this is ___________. If you ____ ice in your hand it feels _____ as heat energy is moving from your hand into the ice

Change in enthalpy =  $\Delta$  H

• This is calculated by subtracting the sum of the bond energy of the products from the sum of the bond energy from the reactants

•  $\Delta H=\sum_x^y\ \left(products\right)-\sum_x^y$  (reactants)

• Exothermic reactions have a -ve  $\Delta H$  as there is less energy in the products than in the reactants

• Endothermic reactions have a +ve  $\Delta H$  as there is more energy in the products than there is in the reactants

Open Ended

How could we calculate bond energy?

Poll

How confident are you in identifying if a reaction is endothermic or exothermic?

• I'm still confused

• It has become clearer, but I need to do some practice questions

• EZ as

Fill in the Blanks

Steps for finding bond energy:
1) Draw the _______ structure
2) Count the ______ of bonds
3) ______ what bonds are present
4)Use the _____to add the individual bond energies to get the ___of the bond energies in the molecule

Open Ended

If the bond energy of the products is 600kJ/mol and the bond energy of the reactants is 700kJ/mol is this reaction endothermic or exothermic? Show you reasoning for your answer