r2.1 amount of chemical change Flashcards
what is stoichiometry?
the ratio of the amount of reactants and products in a reaction
why do we balance equations?
the law of the conservation of mass states that mass is conserved in a chemical reaction. therefore, there must be the same number of each atom in the reactants and products
what are the reacting masses equations?
solid: moles = mass / molar mass
gas: moles = volume / 22.7
aqueous: moles = concentration x volume
liquid: density = mass / volume
how do we do reacting masses calculations?
- balance the equation. identify the known and unknown reactant or product
- convert the ‘known’ mass, concentration or volume given into moles
- use the mole ratio of a balanced chemical equation to determine the moles of the unknown value
- convert the moles into the required quantity
what are the limiting and excess reactants?
the limiting reactant is the reactant that limits the amount of products that can be formed.
the excess reactant is the reactant that remains when the limiting reactant is consumed.
how do you determine the limiting and excess reactant?
- determine the amount in moles of each reactant
- divide the amount of each reactant by its coefficient in the balanced equation
- the lowest value is the limiting reactant and the highest is the excess reactant
what is theoretical yield and actual yield?
the theoretical yield is the maximum amount of product that can be formed in a chemical reaction (based on the stoichiometry of the reaction and amount of the limiting reactant).
the actual yield is the amount of product that is formed in a chemical reaction which may be lower than the theoretical yield.
what factors affect percentage yield?
loss of product: some of the product may evaporate or simply be spilled. it may also be lost when isolating or purifying the product.
incomplete reaction: there may be impurities present or the reaction may not go to completion due to insufficient time.
alternative reactions: an unexpected side reaction may occur which results in a different product
one or more of the reactants is impure
how do you calculate percentage yield?
percent yield = actual yield / theoretical yield x 100
what is titration?
volumetric analysis is a process that uses the volume and concentration of one chemical reactant (a standard solution) to determine the concentration of another unknown solution. the technique most commonly used is a titration. the volumes are measured using two precise pieces of equipment, a volumetric or graduated pipette and a burette.
what are the steps of a titration?
- measuring a known volume of one of the solutions with a graduated pipette and placing it into a conical flask
- the other solution is placed in the burette
- a few drops of indicator are added to the conical flask
- the tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator just changes colour
- multiple trials are carried out until concordant results are obtained
what is a back titration?
a back titration is a common technique used to find the concentration or amount of an unknown substance indirectly. the principle is to carry out a reaction with the unknown substance and an excess of a further reactant, such as an acid or an alkali. the excess reactant, after reaction, is the analysed by titration and the mole ratios are used to deduce the moles or concentration of the original substance being analysed.
what does a high percentage yield indicate?
a high percentage yield indicates that the reaction was very efficient at converting reactants into products.
what does a low percentage yield indicate?
a low percentage yield indicates that there were some inefficiencies during the practical process (incomplete reactions, side reactions, or practical issues such as product being left in the apparatus or lost during purification)
what is atom economy?
the atom economy of a chemical reaction is a measure of the amount of starting materials that become useful products. the ideals scenario would be if the atom economy for a reaction was 100%, indicating that all reactant atoms are used to make the desired product with no by-product waste.
how do you calculate atom economy?
atom economy = mr of desired product / total mr of all products x 100
what is gravimetric analysis?
it is a quantitative method in analytical chemistry in which the concentration of a substance present in a sample is evaluated based on the measurement of its mass.
this is done by precipitating the analyte (substance being analysed) in a sample as a solid compound which is then separated, washed, dried and weighed.
by finding out the stoichiometry of the chemical reaction which produced the precipitate, the amount of analyte present in the original sample can be calculated.
