Chemistry Unit 2- Energetics

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  • Created by: FireDwarf
  • Created on: 11-01-14 16:17
What happens during a chemical reaction?
Chemical bonds break and new ones form.
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Which one has energy going in, and which one going out?
breaking bonds take in energy while when bonds are formed, energy is given out.
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What does the overall energy change indicate?
Whether it is exothermic or endothermic.
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What is an exothermic reaction?
A chemical reaction that gives out more energy then it takes in. (Products have less energy)
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Endothermic reaction?
A chemical reaction that takes in more energy then it gives out. (Products have more energy)
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So which one gives out heat?
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Example of an exothermic reaction?
Neutrilising an acid with an alkaline.
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Breakdown of limestone to lime and Co2.
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What are all reactions?
If they are exo in one direction, must be endo in the other (and vice versa).
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Heat blue copper sulphate, forms anhydrous copper sulphate + water (endo), add water to anhydrous sulphate then will form blue (exo).
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What is enthalpy change?
Measurement of heat energy which has changed in a chemical reaction, at constant pressure.
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Standard conditions?
100kpa 298k
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Why do we need standard pressure?
Gas needs energy to be pushed out into the atmosphere. If the pressure isdiffrent then the energy will sldo change (greater pressure, more energy needed).
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Why are physical states important?
Because diffrent amounts of enthalpy for a reaction occurs dependant upon the state.
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Endo, products have more energy the reactants, exo the products have less.
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What is standard molar enthalpy of formation?
Enthalpy change whenone mole of a substance is formed from its elements under standard conditions & reactants / products in standard state.
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What s standard molar enthalpy of combustion?
Enthalpy change when one mole of substance is burnt in O2 under S conditions and reactants and products S state.
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How to measure enthalphy change?
Heat certiain amount of water.
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Equation 1?
q = m x c x change in T
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(q- E change, m mass of water, c constant and temp, change in temp)
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What have we worked out? What needs doing now?
We now have the J, but not the kjmol-1. we need to find out moles of the substance and then convert into kj. Then add sign to signify if its exo or endo (exo is -).
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So, what how do we work out the enthalpy change of solutions eg; neutrilization.
1) Balance the equation 2) use the q= m c change in t equation. 3) Their both liquids, therefore your M is the combination of both.
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4) Once worked out q, your moles can be worked out by n = c x v. Note, you only work out the smallest moles amount. 5) Divide your q by the moles to get H.
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What is displacement?
When a more reactive metal removes the less reactive metal from its compound and puts itsself in its place.
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How to work it out for displacement reactions?
1) balance 2) q= m x c x t. Your M is the liquid. 3) work out q 4) Work out both moles and use the smallest number of moles. 5) Divide q by the moles to work out H.
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pros and cons about a simple calorimeter? (SEE PAGE 118)
Cheap & simple to use BUT lots of heat loss & therefore reduced accuracy. Eg: No draught shield, could lose heat that way. No lid, lose heat there.
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Improved caliometer.
Draught shield, funnels the heat reduces loss. Lid on the cup, reduces heat loss there.
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Flame caliometer
Water surrounds the heat and therefore increases surface area, more accurate. Copper, good insulator. Spiral, increae SA. Supply of O2, keeps it buring.
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Bomb caliometer?
Water surrounds, electric (accurate measurements).
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What is Hess's Law?
The enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products.
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How do we use this to calcluate enthalpy change of formation?
If its the same in any direction, we can use the elements which form the reactants & products to work out their enthalphy change>
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Put on the top line the reaction (reactants and products). On a line below, put the elements which form both the products and the reactants. Draw arrows which demostrate they form the reactants and products.
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Then follow the cycle. The reactants want to form products so they will go agianst the first line, but follow the second. If it goes agianst the line, then the enthalphy change is the opposite sign.
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How to find out the enthalphy change of combustion?
Put on the first line reactants and products (minus co2 and h2o). On the line below put co2 and h20.
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Using the combustion values, all of the lines will point downwards (reactants combusts into co2 and water). But, you will want to follow 2 of these lines (the reactants one) and up one to get to products.
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If it says 2 moles, you need to times it by 2 because the values are for one mole only.
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How do I find Hf from Hc?
Put the Hf reaction on the top line, and co2 and h2o on the bot. Plot the Hc values (all pointing down).
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What is bond dissociation enthalpy?
The enthalpy change required to break a convalent bond with all species in gaseous state
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what is mean bond enthalpy?
The average value of the bond dissociation enthalpy for a given type of bond. It is average because the value is taken from a variety of compounds.
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How to do it?
Write the equation. Put down all bonds formed on both sides of the equation. Times the bond by its mean bond enthalphy (watch out for moles)
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On the left hand side, the sign is a + while on the right a -. Add them together and you have youe enthalphy change
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How to identify an actual mean bond enthalphy amount?
Work out the enthalphy change, and then ratio out whatever you have left over you dont make.
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why can we get diffrent bond enthalphy amounts?
Because the mean is only an average and they can have diffrent amounts therefore.
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Other cards in this set

Card 2


Which one has energy going in, and which one going out?


breaking bonds take in energy while when bonds are formed, energy is given out.

Card 3


What does the overall energy change indicate?


Preview of the front of card 3

Card 4


What is an exothermic reaction?


Preview of the front of card 4

Card 5


Endothermic reaction?


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