Monitoring and controlling chemical reactions Revision Notes

What is Percentage Yield?

Percentage yield is a measure of the efficiency of a chemical reaction. It compares the actual amount of product obtained in an experiment (actual yield) to the maximum possible amount of product that theoretically could be formed based on the chemical equation and the amount of reactants used (theoretical yield). It is expressed as a percentage. A high percentage yield indicates that a reaction is efficient, minimising waste and maximising product formation. A low percentage yield suggests inefficiencies, which chemists often investigate to improve the process.

Calculating Percentage Yield

To calculate percentage yield, you need two key pieces of information: the actual yield and the theoretical yield. The actual yield is the mass of the product you obtain when you perform the reaction in the lab. The theoretical yield is calculated using stoichiometry from the balanced chemical equation and the mass of the limiting reactant; this represents the maximum possible product. The formula to use is: Percentage Yield = (Actual Yield / Theoretical Yield) × 100%. Always ensure that both actual and theoretical yields are in the same units (e.g., grams or kilograms) before performing the calculation to avoid errors.

Reasons for Less Than 100% Yield

It is very rare to achieve a 100% percentage yield in a chemical reaction. Several factors contribute to yields being less than perfect. Common reasons include the reaction not going to completion, meaning some reactants may not react to form products. Side reactions can occur, forming unwanted by-products, which reduces the amount of the desired product. Product can be lost during transfers, filtration, or purification steps. Some products might also remain dissolved in the solvent, or evaporate if volatile. These losses reduce the actual yield, leading to a lower percentage yield.

Importance of Percentage Yield

Knowing the percentage yield is vital for both laboratory and industrial chemistry. In research, it helps chemists evaluate the effectiveness of new synthetic routes. Industrially, a high percentage yield is crucial for economic viability, as it means less raw material is wasted and more product is made, reducing costs. It also has environmental implications, as higher yields often mean less waste generation. Improving percentage yield is a constant goal in chemical manufacturing to make processes more sustainable and cost-effective.

Defining Concentration (Higher)

Concentration quantifies the amount of solute dissolved in a specific volume of solution. It indicates how 'strong' or 'weak' a solution is. A high concentration means there is a large amount of solute packed into a small volume, whereas a low concentration indicates less solute or a larger volume of solvent, leading to more diluted conditions. We typically express concentration in grams per decimetre cubed (g/dm³) in this context. Understanding this fundamental definition is crucial for quantitative chemical analysis and practical applications, such as preparing solutions of known strength for experiments or industrial processes.

The Concentration Formula (Higher)

The core relationship between mass of solute, volume of solution, and concentration is given by the formula: Concentration (g/dm³) = Mass of solute (g) / Volume of solution (dm³). This formula is exceptionally versatile. You can rearrange it to calculate any of the three variables if the other two are known. For example, to find the mass of solute needed, rearrange to: Mass of solute (g) = Concentration (g/dm³) × Volume of solution (dm³). To find the volume of solution: Volume of solution (dm³) = Mass of solute (g) / Concentration (g/dm³). Proficiency in these rearrangements is key for solving problems.

Impact of Mass and Volume on Concentration (Higher)

The concentration of a solution is directly proportional to the mass of the solute and inversely proportional to the volume of the solution. This means that if you increase the mass of solute while keeping the volume constant, the concentration will increase. For example, dissolving 20 g of salt in 1 dm³ of water will result in a higher concentration than dissolving 10 g of salt in the same volume. Conversely, if you increase the volume of the solution while keeping the mass of solute constant, the concentration will decrease. For instance, diluting a solution by adding more solvent reduces its concentration. This inverse relationship explains why adding water makes a drink taste less strong.

Units and Conversions (Higher)

It is critical to use consistent units when working with concentration calculations. The standard unit for concentration in this context is grams per decimetre cubed (g/dm³). Often, volumes are given in cubic centimetres (cm³). Therefore, you must convert cm³ to dm³ before using the concentration formula. Remember that 1 dm³ = 1000 cm³. To convert cm³ to dm³, divide the volume in cm³ by 1000. Neglecting this conversion is a common source of error in calculations. For instance, 250 cm³ is equivalent to 0.25 dm³ (250 / 1000).

Avogadro's Law and Molar Volume (Higher)

Avogadro's Law states that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules. This means that one mole of any gas will occupy the same volume under identical conditions of temperature and pressure. This volume is known as the molar volume. Understanding this law is fundamental for calculations involving gaseous reactants and products in chemical reactions. The specific value of molar volume depends on the temperature and pressure conditions.

Standard Conditions for Molar Volume (Higher)

Two common sets of conditions are used when referring to molar volume:<br>1. Standard Temperature and Pressure (STP): 0°C (273 K) and 1 atmosphere (101.325 kPa). At STP, one mole of any gas occupies a volume of approximately 22.4 dm³.<br>2. Room Temperature and Pressure (RTP): 20°C (293 K) and 1 atmosphere. At RTP, one mole of any gas occupies a volume of approximately 24 dm³. It is important to know which conditions are being referred to in a problem, as this will determine the molar volume value to be used in calculations.

Calculations involving Molar Volume (Higher)

The relationship between the volume of a gas and its molar amount can be expressed by the formula: Volume of gas (dm³) = Moles of gas (mol) × Molar volume (dm³/mol). This formula can be rearranged to find the number of moles if the volume is known: Moles of gas (mol) = Volume of gas (dm³) / Molar volume (dm³/mol). This principle is indispensable for stoichiometric calculations involving gases, allowing chemists to predict quantities of gaseous products or reactants.

Collision Theory Fundamentals

For a chemical reaction to occur, reactant particles must collide. However, not all collisions are successful. A successful collision requires two conditions: sufficient energy (at least the activation energy) and correct orientation. The rate of reaction is determined by the frequency of these successful collisions. Any factor that increases the frequency of collisions or the proportion of collisions that meet the energy requirement will increase the reaction rate.

Effect of Temperature

Increasing the temperature of a reaction mixture significantly increases the reaction rate. This is because higher temperatures provide particles with more kinetic energy. Faster-moving particles lead to two key effects: 1. Increased frequency of collisions: Particles move around more rapidly, colliding more often. 2. Increased energy of collisions: A greater proportion of collisions will meet or exceed the activation energy, making them successful. Both factors contribute to a faster reaction rate.

Effect of Concentration (for solutions) and Pressure (for gases)

Increasing the concentration of reactants in a solution means there are more reactant particles packed into the same volume. This increases the chance of particles colliding, leading to a higher frequency of collisions. For gaseous reactants, increasing pressure also means more particles are forced into a smaller volume, achieving the same effect of increased particle density. In both cases, the increased collision frequency directly leads to a higher frequency of successful collisions, thus increasing the reaction rate.