AS Chemistry Unit 2

Exothermic Reactions

• At the end of the reaction energy has been given out
• Heat is given out due to reaction
• Bonds forming gives out energy
• eg. Neutralising an acid with an alkali

Endothermic Reactions

• At the end of reaction energy has been taken in
• Heat is taken in from surroundings during reaction
• Bonds breaking takes in energy
• eg. Breakdown of Calcium carbonate to Calcium oxide

Measuring energy values from reactions allows us to compare the efficiency of different fuels.

We measure enthalpy (heat energy) change at standard conditions-

• Pressure of 100kPa
• Temperature of 298 K (25 degrees)

Exothermic reactions have a negative enthalpy change as they give out heat

Endothermic reactions have a positive enthalpy change as they take in heat

The standard molar enthalpy of formation is- the enthalpy change when one mole of substance is formed from its constituent elements under standard conditions with products and reactants in their standard states.

The standard molar enthalpy of combustion is- the enthalpy change when one mole of compound is completely burned in oxygen under standard conditions with products and reactants in their standard states.

Enthalpy change is meaured using the temperature change during the reaction. This is then multiplied by the mass of substance being heated or cooled and the specific heat capacity of the substance, so:

Enthalpy change= mc(delta)T = q

Hess's Law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Here the route from A-->C-->B has the same enthalpy change as A-->B and as A-->D-->E-->B

Mean bond enthalpies can be used to work out the overall enthalpy change in a reaction- this is approximate as the enthalpies used are averaged.                                                               Mean bond enthalpy= The average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds.

You will be given individual Bond enthalpies in a table and a reaction.

eg.          C2H6 + Cl2 ---> C2H5Cl + HCl

• Draw out the molecules in the reaction equation given to easier see the bonds involved
• Imagine EVERY bond in the reactants breaks and add up the enthalpies given --if the bond enthalpy of C-H were 413 kJ/mole and the question involved ethane, you'd multiply 413 by 6 and add the bond enthalpy of the C-C bond. Then add the Cl-Cl bond enthalpy.
• Imagine EVERY bond in the products join and add up the enthalpies given --here there are 5 C-H bonds, 1 H-Cl bond, 1 C-C bond and 1 C-Cl bond.
• Find the difference between these two enthalpies you have worked out.
• Then work out if the enthalpy change is negative- If more energy was given out than put in or positive- If more energy was put in than given out.

These factors will increase the rate of a reaction:

• Increasing Temperature- Increases speed of molecules, which increases energy of molecules and number of collisions

• Increasing Concentration of solution- More particles present in given volume so collisions are more likely

• Increasing Pressure of gas- More particles present in a given volume so collisions are more likely

• Increasing Surface area of solids- eg. breaking solid into smaller pieces. More particles are availble to react with gas or liquid particles

• Using a Catalyst- Gives a route with a lower activation energy so more particles successfully react when they collide.

Le Chatelier's Principle- If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.

If pressure is increased the equilibrium will move to favour the reaction that results in fewer molecules

If temperature is increased the equilibrium will move to favour the endothermic reaction to oppose the increase in temperature

If concentration of a certain substance is increased the equilbrium will shift in the direction that favours the reaction resulting in fewer molecules of that substance