Reactions and Mass Flashcards
(33 cards)
What is relative formula mass?
The relative mass of a molecule calculated by adding the atomic masses of the atoms present
Reacting mass equation
Mass= Mr x moles
Exothermic reaction
A reaction that releases energy in the form of heat, occurs when making bonds
Endothermic reaction
A reaction that absorbs energy in the form of heat, occurs when breaking bonds
Give two examples of an endothermic reaction
Cooking food and photosynthesis
What is Ea?
Activation Energy, the minimum amount of energy needed for particles to react
What is ∆H?
Total energy change in a reaction
How to calculate Mr?
Add the relative atomic masses of each element and times them by the quantity if needed, e.g (O2 would be 2x16).
How to calculate moles, mass and Mr?
moles= mass/mr, mass= mr x moles, mr= mass/moles
How to find percentage composition?
Find the Mr for all elements, then add them to find a total. use the Mr of the element asked and put it over the total mr. Divide and times by 100 to find the percentage.
Example for percentage composition
What is the percentage of oxygen in magnesium oxide (MgO)?
Mr of mg is 24
Mr of O is 16 - element asked for
16+24=40 - total Mr
16/40=40%
How to do limiting reactant equations
Draw a table with the reactants and products down the side and the headings; mass, Mr, moles, ratio and limiting. Then add the masses given in the question into the table. Using a periodic table, find the Mr and divide the mass by the Mr to get the moles. Find the ratio. Keeping the ratio in mind, use the moles to find which reactant is limiting. Whichever reactant is limiting, the moles in that reactant will be the same amount of moles in the product. Use the mr of the product and times it by the moles to find the mass of the product.
What does the law of conservation of mass state?
No atoms are lost or made during a chemical reaction so the mass of the products is equal to the mass of the reactants
What is Avogadro’s number?
6.02 x 10^23
What is another name for this number?
One mole, it is the same number of atoms present in 12 grams of carbon
What is a limiting reactant?
A limiting reactant means no matter how much excess reactants are added, if no more of a limiting reactant is added no more product is made.
Why are limiting reactants useful?
They are useful so exact measurements do not need to be taken
How do people in the chemical industry ensure there is as little waste as possible in reactions?
In order to limit economic loss and increase profit, chemists consider the percentage yield of a reaction and the atom economy of a reaction
What is percentage yield?
Percentage yield is a measure of how effective an industrial process is in producing a desired product.
How is percentage yield calculated?
Actual yield/ theoretical yield= %
Why is percentage yield rarely 100%
You may lose some product during separation/ purification, the desired product may be very reactive or the reaction may be reversible
What is atom economy?
The efficiency of a reaction based on how many atoms are used- the aim is to minimise waste while maximising the use of atoms in a reaction
What is the equation for atom economy?
(Mr of desired products/ total Mr of reactants) x 100
Lithium reaction with air
Soft but firm silver metal turns black quite quickly
