Chemical reactions Study Guide

Concentration is a fundamental concept in chemistry that quantifies the amount of solute dissolved in a given volume of solvent or solution. For GCSE Gateway Science (Higher Tier), it's crucial to understand how the mass of a solute and the volume of the solution relate to its concentration. The concentration of a solution is defined as the amount of solute divided by the volume of the solution. The standard unit for concentration in this context is grams per cubic decimeter (g/dm³), although other units like grams per liter (g/L) are equivalent. Sometimes, it can be useful to express concentration in terms of mass per unit volume such as g/100 cm³.

The relationship is directly proportional to the mass of the solute and inversely proportional to the volume of the solution. This means if you increase the mass of the solute while keeping the volume of the solution constant, the concentration will increase. For example, dissolving 20 g of salt in 1 dm³ of water results in a concentration of 20 g/dm³. If you then dissolve an additional 10 g of salt in the same 1 dm³ of water (now a total of 30 g), the concentration becomes 30 g/dm³.

Conversely, if you keep the mass of the solute constant but increase the volume of the solution, the concentration will decrease. Imagine having 20 g of salt in 1 dm³ of water (20 g/dm³). If you add another 1 dm³ of water, making the total volume 2 dm³, the 20 g of salt is now spread over a larger volume, resulting in a concentration of 10 g/dm³. This process is known as dilution. Decreasing the volume of the solution while keeping the mass of the solute constant would increase the concentration, assuming the solute remains dissolved.

The key formula to calculate concentration is: Concentration (g/dm³) = Mass of solute (g) / Volume of solution (dm³). It is important to ensure consistent units. Often, volume is given in cm³, so you must convert it to dm³ by dividing by 1000 (since 1 dm³ = 1000 cm³). For instance, if 5 g of solute is dissolved in 250 cm³ of solution, the volume in dm³ would be 250 cm³ / 1000 = 0.25 dm³. The concentration would then be 5 g / 0.25 dm³ = 20 g/dm³.

Mastering this relationship and the associated calculations is essential for understanding many chemical reactions and processes, including titrations and rate of reaction studies. It helps in preparing solutions of specific strengths and in interpreting experimental results where reactant concentrations are varied.

When carbonates react with acids, three main products are always formed: a salt, water, and carbon dioxide gas. This is a classic characteristic reaction for carbonates. The specific salt produced depends on the metal in the carbonate and the acid used. For example, if calcium carbonate reacts with hydrochloric acid, the products are calcium chloride (the salt), water, and carbon dioxide. The carbon dioxide gas is observable as effervescence (fizzing). To test for carbon dioxide, the gas can be bubbled through limewater (calcium hydroxide solution); if carbon dioxide is present, the limewater will turn cloudy due to the formation of insoluble calcium carbonate.

Some metals, particularly those above hydrogen in the reactivity series (like magnesium, zinc, iron, and aluminium), also react with acids. The general products of this reaction are a salt and hydrogen gas. The reactivity of the metal affects the vigour of the reaction; more reactive metals react more vigorously. For instance, magnesium reacting with hydrochloric acid produces magnesium chloride and hydrogen gas. Hydrogen gas can be identified by the 'squeaky pop' test, where a lit splint held near the gas produces a distinctive sound. Less reactive metals, such as copper, silver, and gold, typically do not react with dilute acids.

Writing balanced chemical equations for these reactions is crucial for demonstrating an understanding of the conservation of mass. This involves ensuring that the number of atoms of each element is the same on both the reactant and product sides of the equation. For example, for the reaction of magnesium with hydrochloric acid, the unbalanced equation is Mg + HCl → MgCl₂ + H₂. To balance this, we need two molecules of HCl: Mg + 2HCl → MgCl₂ + H₂. Similarly, for calcium carbonate with hydrochloric acid, the unbalanced equation is CaCO₃ + HCl → CaCl₂ + H₂O + CO₂. Balancing requires two molecules of HCl: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.