Enthalpy Changes COPY Flashcards
What is meant by the term enthalpy
A measure of the heat change in a chemical system
Define system
The reactants and products
Define surroundings
The rest of the univers- lab and equipment
Define exothermic
Energy is released form the system to the surroundings- △H = -
Define endothermic
Energy is transferred from the surroundings to the system-△H = +
Draw an energy level diagram for an exothermic reaction
- y axis labelled enthalpy
- X axis labelled progress of reaction
- First line is higher up than final line and is labelled reactants
- An arrow pointing down from the reactants line to the products line labelled △H
Draw an energy level diagram for an endothermic reaction
- Y axis labelled enthalpy
- X axis labelled progress of reaction
- First line is lower down than final line and is labelled reactants
- An arrow pointing up from the reactants line to the products line labelled △H
Explain why, when the reaction mixture is measured the temp has increases despite the reaction losing energy,
Because it is an exothermic reaction, so the system is transferring energy to the surroundings and the thermometer is part of the surroundings.
Define the term activiation energy
The energy required to start a reaction by the breaking of bonds
Why do most chemical reactions have an activation energy
It is necessary to break the bonds in the reactant molecule.
Draw an energy diagram for an exothermic reaction with activation energy
- The same basis as needed for a normal exothermic energy diagram
- Have a dotted curved line which starts at the reactant line goes up and then down to the products line.
- The height from the reactant line to the peak of the curve is the Ea (activation energy)
Write equation of the energy change in a reaction
q=mc△T m= mass of substance heated (g) c= specific heat capacity (J g^-1 K^-1) △T = Change in temperature (K but will be the same in Celsius) q= heat energy change (J)
State the assumptions made when using the q=mc△T equation
- Density of solution = density of H2O
- No heat loss or gain to surroundings or calorimeter
- The Specific heat capacity of solution= Specific heat capacity of water ( means it works best if solution is quite dilute)
What do you use as your measurement for mass and why
The mass of all the solution and not the solid
Because the spc of most solids will be closer to 0 than 4.18 so it is more accurate to not count it.
Define specific heat capacity
Energy required to heat 1g of a substance by 1 degree celsius
Write formula for enthalpy change
Enthalpy change = q/mol
Enthalpy change should include the correct sign(-/+) Depending on whether the temp rose or fell in the experiment.
Outline a method for an experiment you might use to determine the enthalpy change of solution △solH of ammonium bromide
- Measure out a know volume of water in measuring cylinder
- Pour water into a polystyrene cup and measure the initial temp suing a thermometer
- Weigh a weighing bole containing some solid NH4Br
- Put solid in water and stir using thermometer
- Measure the highest/lowest temp reached by the solution
- Reweigh empty weighing bottle
- Calculate temp change
- Find energy change using q=mc△T
- Find number of moles of NH4Br and use this to find enthalpy change= q/mol
State the standard conditions
Temp= 25 degrees or 298K Pressure= 100kPa Concentration= 1 mol dm^-3
Define Standard enthalpy change of combustion △cH (planet symbol)
The enthalpy change that takes place when 1 mole of substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of formation △fH (planet symbol)
The enthalpy change that takes place when 1 mole of compound is formed from its elements under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of neutralisation △neutH (planet symbol)
The enthalpy change that accompanies the reaction of an acid and a base to form one mole of H2O (l) under standard conditions, with all the reactants and products in their standard states.
State Hess’s law
The △rH is always the same irrespective of the reaction route.
If a reaction can take place by more than one route and he initial and final conditions are the same, the total enthalpy change is the same for each route.
Practice drawing Hess cycles using the Hess’s law sheets
Do it
Define average bond enthalpy
The average enthalpy change that takes place when breaking by homolytic fission 1 mole of a given bond in the molecules of a gaseous species
What is the sign of bond enthalpy
+ always requires energy to be put in to break bonds. It is endothermic
What is the sign of the enthalpy change when forming a bond
- Energy will be released when forming bonds.
Describe how to find the overall enthalpy change from the mean bond enthalpies
- List the bonds broken and the bonds formed
- Attribute values to each bond to find the total energy used for forming the bonds and the total energy to break the bonds.
- The bonds formed should have a - sign and bonds broken should have a + sign
- Add the values together to get the overall enthalpy change △rH for the reaction.
Explain why the value for overall enthalpy change is not always equal to the standard enthalpy change
Standard enthalpy change requires standard states and standard conditions. Using bond enthalpies assumes gaseous states.
State the formula for calculating percentage error in measurements
Total inaccuracy in apparactus/ actual measurement *100
State procedural errors that may have occurred when measuring the enthalpy change of combustion which may lead to differences in your result compared the standard enthalpy changes
- Heat loss to the surroundings
- Incomplete combustion of alcohol- seen by soot
- Evaporation of alcohol or water
- Non-standard conditions
- Specific heat capacity of apparatus ignored
