3(a) Flashcards
What is observed when we burn magnesium in oxygen?(3)
The magnesium burns with a bright white flame.
Magnesium oxide, a white powder, is produced.
The mass of the magnesium will increase at the end of the experiment because it will have formed Magnesium oxide with the air.
Define enthalpy change.
Enthalpy change is the amount of heat energy taken in or given out in a chemical reaction, the differences between the energy of the products and reactants.
State the equation to calculate enthalpy change.
DH(measured in kJ/mol) =
-Heat energy transferred(Q) / moles(N)
State the equation to calculate heat energy transferred.
Q = mc X change in temperature
Where m is mass of water in grams and c is the specific heat capacity.
What is an exothermic reaction?(3)
A reaction that gives out heat to the surroundings is exothermic.
Exothermic reactions could include the reactions of metals with acids, neutralisations, displacement reactions and of course combustions.
In an exothermic reaction, the reactants have more energy than the products, ^H is negative because energy is given out.
What is an endothermic reaction?(2)
A reaction that absorbs heat from the surroundings is endothermic.
DH is positive because energy is absorbed and therefore gained.
Define calorimetry.
Calorimetry is the technique that uses the heat from a reaction to heat water, in order to calculate the amount of heat released from Q = mcT
When measuring enthalpy changes for displacement reactions, what is assumed?(3)
The density of the solution and therefore mass of solution.
The specific heat capacity of the mixture being that same as water’s heat capacity.
Why do reactions give out or absorb heat?(2)
During reactions, bonds are broken down in the reactants and new ones are formed to make the products.
Breaking bonds requires energy while making bonds release energy.
How can you calculate whether a reaction will be endothermic or exothermic?(2)
The energy released or absorbed from a reaction can be calculated using the bond energy’s of specific molecules.
If we take the energy released as negative and energy absorbed as positive and then add the two together, a positive total will mean that it is endothermic(gains energy) while a negative total will mean that it is exothermic(loses energy).
How is the accuracy of measuring enthalpy change in combustion reactions maximised(in a class)?(4)
The warm water gives out heat to the air, heat is also lost from the flame because it goes straight into the air rather than into the water.
The combustion of of the alcohol is also incomplete combustion which releases less heat than complete combustion.
However, a claimed copper calorimeter is used because it transfers heat to the water much better, an insulating card is also used.
Furthermore, draught-shields are used to make sure there is no wind.
Define collision theory and how it links with temperature.(3)
Reactions can only happen when particles collide, but in order for a reaction to happen upon collision the particles must have the minimum activation energy.
By increasing temperature, there will be a higher frequency of collisions as well as greater activation energy for all of the particles and therefore a higher frequency of successful collisions.
