Describe the practical where burning a fuel can determine the enthalpy change of that fuel (PAG 3)
- in a copper calorimeter put in a known mass of water, using a clamp secure this calorimeter above a spirit burner with the fuel you want to find the enthalpy change of.
- before lighting the spirit burner weigh the mass and record, then place it directly below the calorimeter.
- place a thermometer in the water and measure the initial temperature and record.
- remove the lid of the spirit burner just before you light it
- wait for the temperature to rise by 15°, record the final temperature and replace the lid
- weigh the final mass of the spirit burner
- then input your data into q=mc ▵t
- then Input that answer into J\1000/mols of fuel
Why does PAG 3 not always give a perfect answer?
- heat lost to surroundings
- incomplete combustion
- heat loss to calorimeter
- some fuel may evaporate
- ▵h under standard conditions is for standard states. Which for water is liquid, but water produced in this is a gas
Describe how to calculate the enthalpy change of a reaction (acid+alkali) using a polystyrene cup
- add one reactant into a polystyrene cup and place on a lid with a thermometer
- record the temperature every 30 seconds
- at 3 minutes add in your other reactant and stir continuously
- measure the temperature every 30 seconds till the temperature begins to fall
- input the data into q=mc ▵t to find your ▵H change
Define standard enthalpy change for a reaction (▵rH°)
The enthalpy change when molar quantities of the reactants, as stated in the equation, react together under standard conditions, with all substances in their standard states.
Define standard enthalpy change of combustion (▵cH°)
The enthalpy change when one mole of the substance burns completely in oxygen under standard conditions, with all substances in their standard states
CH4(g) + 2O2(g) => CO2 (g) + 2H2O (l)
Define the standard enthalpy change of formation (▵fH°)
The enthalpy change when 1 mole of the substance is made from its elements in their standard states under standard conditions
Define the standard enthalpy change of neutralisation (▵neutH°)
The enthalpy change when one mole of H+ ions react with one mole of OH- ions to form one mole of water under standard conditions
Why are ▵H bond enthalpy values from calculations not the same as the actual bond enthalpy values?
- the bond enthalpy values used in a calculation are the averages not the actual values
- for bond enthalpy calculations everything is in their gaseous state, when for actual bond enthalpy values everything is in their standard states.
Define bond breaking
Energy must be put in to break a bond. So bond breaking is exothermic.
Define bond making
Energy is given out when a bond is made. So bond making is exothermic.
Define bond enthalpy values
The energy needed to break a particular type of covalent bond, for one mole of bonds in the gas state, is called the bond enthalpy, and is always positive.
Compare bond strengths
The stronger a covalent bond, the bigger the bond enthalpy value, and also the shorter the bond length.
How do you use bond enthalpy values to calculate the enthalpy change for a reaction?
- Write a balanced equation and draw the structural formula of each substance
- Imagine breaking all the bonds in the reactants. List the number of each type of bond to be broken and total these up (+)
- Imagine making all the bonds in the products. List the number of each type of bond being made and total these up (-)
- Add these numbers together with these respective signs to get the overall enthalpy change
What is Hess’ law?
The overall enthalpy change for a reaction is the same whatever route is taken.
What are the steps to completing a cell cycle
- Write a balanced equation for the enthalpy change you want to find
- Fill in the alternative route using the bond enthalpy values you know
- Use alternative route to calculate the bond enthalpy values
Which way round does the arrow go for combustion and formation?
- combustion both arrows point down
-formation both arrows point up
