Global challenges Study Guide

Chemical reactions involve reactants transforming into products. The rate of a chemical reaction quantifies how quickly this transformation occurs. It's typically measured by observing the change in concentration of a reactant or product over time. For example, you might measure the rate at which a reactant is used up or a product is formed. Various factors influence reaction rate, including temperature, concentration of reactants, pressure (for gases), surface area (for solids), and the presence of a catalyst.

When we plot reaction conditions against reaction rate, we can gain valuable insights into the kinetics of a reaction. Such graphs usually show an independent variable (the condition being changed, e.g., temperature, concentration) on the x-axis and the dependent variable (reaction rate) on the y-axis. The shape of the curve reveals the relationship between the condition and the rate. A steep upward slope indicates that a small change in the condition leads to a significant increase in rate, while a flatter slope suggests a less pronounced effect. A plateau suggests that the reaction rate has reached its maximum or is no longer limited by that particular factor. By analysing these graphs, we can determine optimum conditions for a reaction, predict how changes will affect its speed, and even deduce information about the reaction mechanism. For instance, if increasing the concentration of a reactant has no effect on the rate, it suggests that reactant is not involved in the rate-determining step. Conversely, a linear increase in rate with concentration points to a direct relationship. These interpretations are crucial in industrial processes to maximise product yield efficiently and safely.

Alloys are mixtures containing at least one metal and at least one other element, which can be either a metal or a non-metal. The alloying process involves melting the constituent elements together and then allowing them to cool and solidify. The atoms of the added element disrupt the regular arrangement of atoms in the pure metal. This disruption makes it harder for layers of atoms to slide past each other, which is why alloys are generally harder and stronger than pure metals. The specific properties of an alloy, such as its strength, hardness, corrosion resistance, or electrical conductivity, depend on the type and proportion of the elements added to the main metal. For example, steel is an alloy of iron and carbon; the carbon atoms, being smaller than iron atoms, fit into the gaps in the iron lattice, making the iron much stronger. Stainless steel, another iron alloy, contains chromium and nickel, which provide excellent corrosion resistance, making it suitable for cutlery and surgical instruments. Brass, an alloy of copper and zinc, is harder and more durable than pure copper, while bronze, an alloy of copper and tin, is even harder and resistant to corrosion. Duralumin, an alloy of aluminium with copper, magnesium, and manganese, is lightweight yet strong, ideal for aircraft construction. Understanding the relationship between an alloy's composition and its properties allows chemists and engineers to design materials for specific applications, optimising their performance.