Determining enthalpy change of reaction Flashcards
(16 cards)
What is enthalpy change of reaction?
The enthalpy change when a reaction takes place in the molar quantities shown by the balanced equation under standard conditions with all chemicals in standard states
What type of reaction are we doing?
One that isn’t neutralisation, formation or combustion
Eg reacting magnesium with copper sulfate
Step 1
With a polystyrene cup in a beaker, pour 100cm3 of 1moldm-3 copper sulfate solution
Step 2
Take readings every 30s
Step 3
At 1 min 30s, add Mg powder and stir with thermometer
Step 4
Continue to take temp readings every 30s until 5 mins
Step 5
Plot the temp change on a graph, with time on x axis and temperature on y axis
How can we find an accurate temp change using this graph?
Extrapolate from the start of the reaction (when the mg was added at 1min 30s) upwards
Then extrapolate the point where temp begins to decrease
Point these lines meet = the maximum temp reached
Why do we use a graph to find the maximum temperature reached?
To account for the heat loss at all points of the reaction
To find the maximum temperature reached had no cooling occurred.
How to find enthalpy change of formation of this reaction?
Q = mc△T
Mass of copper sulfate solution x specific heat capacity x temperature change
What values have we found in this experiment to find enthalpy change?
Mass of copper sulfate solution = 100
Shc = 4.18
Temp change = found from graph
How to find the enthalpy change PER MOL?
Divide the enthalpy change by the mol of magnesium that reacted
If a polystyrene cup was NOT used, what would happen?
More heat energy transfers from the cup to surroundings
So maximum temperature would be lower
And more steep decrease in temp of the solution on the graph, as more energy is being given off
Finding enthalpy change to form a hydrated salt from a dehydrated salt
Measure enthalpy change when dehydrated salt is dissolved in water to form a solution
Repeat for hydrated salt
uSE HESS CYCLE to work out enthalpy change
Why can we not directly work out enthalpy change from dehydrated salt to hydrated salt
it is impossible to add the correct volume of water to dehydrated salt to hydrate it without dissolving it
Hard to measure enthalpy change of a solid
What start temp is used if both reagents are solutions?
Average temp of 2 solutions by measuring start temp of both solutions
