1.5 Rate Of Chemical Change (WJEC GCSE Chemistry): Flashcards

On a rate of reaction graph, a reaction at higher concentration has a steeper gradient at the start, becomes horizontal sooner, and forms the same amount of product as the lower concentration reaction.

Surface area is the total area of a solid reactant that is exposed and available to react. Increasing surface area by breaking a solid into smaller pieces exposes more particles and increases the rate of reaction.

Increasing pressure increases the rate of reaction because the same number of gas particles are contained in a smaller volume, so they collide more frequently, leading to more successful collisions per second.

False.

Increasing surface area speeds up the rate of reaction but does not change the total amount of product formed. The reaction is simply completed in a shorter time.

Increasing the surface area of a solid reactant is the same as decreasing its particle size. Both result in more reactant particles being exposed to collisions.

Increasing temperature makes the gradient steeper at the start of the graph. The reaction reaches completion sooner but produces the same total amount of product.

When sodium thiosulfate reacts with hydrochloric acid, sulfur is formed as a solid precipitate, which causes the solution to turn cloudy.

In the sodium thiosulfate experiment, the conical flask is placed over a white piece of paper with a cross on it. The stopwatch is stopped when the cross can no longer be seen from above.

The independent variable is the factor deliberately changed by the experimenter. In the sodium thiosulfate temperature experiment, this is the temperature of the sodium thiosulfate solution.

False.

As temperature increases, the particles have more energy and collide more frequently. This increases the rate of reaction, so the cross disappears in a shorter time.

In the sodium thiosulfate experiment, temperature is plotted on the x-axis (independent variable) and the time for the cross to disappear is plotted on the y-axis (dependent variable).

The word equation for the reaction is:

sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur

The specified practical does not include repeats, so repeatability cannot be checked. However, reproducibility can be assessed by comparing results with other groups.

The precipitation method is considered subjective because different people may disagree on the exact moment the cross disappears. This means the result depends on the observer's judgement rather than an objective measurement.

A successful collision is a collision between reactant particles that results in the atoms rearranging to form the products of the reaction.

According to particle theory, chemical reactions occur only when reactant particles collide with each other with sufficient energy to react.

True.

The number of collisions is proportional to the number of particles present. Doubling the concentration places twice as many particles in the same volume, doubling the collision frequency.

Increasing temperature increases the kinetic energy of particles. This causes more collisions per second AND a greater proportion of collisions to have energy above the activation energy.

Activation energy is the minimum amount of energy that colliding particles must have for a reaction to occur. Particles colliding with less than this energy bounce off each other without reacting.

Increasing surface area increases the rate of reaction because a greater area of reactant particles is exposed. This means more collisions can occur per second, leading to more frequent, successful collisions.

False.

Only temperature directly affects the energy of collisions by increasing the kinetic energy of particles. Concentration, pressure, and surface area affect only the number (frequency) of collisions.

An unsuccessful collision results in reactant particles bouncing off each other and remaining unchanged. No products are formed.

A catalyst is a substance that speeds up the rate of a chemical reaction without being chemically changed or used up in the reaction.

False.

A catalyst is not used up in the reaction. The mass of the catalyst at the start and end of the reaction is the same.

A catalyst does not affect the total amount of product formed. It increases the rate of reaction so the same amount of product is produced in a shorter time.

Catalysts work by providing an alternative reaction pathway that has a lower activation energy, so more particles have sufficient energy to react.

False.

Different reactions require different catalysts. A catalyst that works for one reaction will not necessarily work for another.

Industrial catalysts need to be replaced regularly because they are affected by impurities over time, which reduces their activity and makes them less effective at speeding up the reaction.

On a rate of reaction graph, adding a catalyst produces a steeper initial gradient and the curve becomes horizontal sooner. The final amount of product is the same.

A lower activation energy means a higher proportion of reactant particles have energy greater than or equal to the activation energy. This results in more successful collisions per second and a faster rate of reaction.

An enzyme is a biological catalyst, made in living cells, that speeds up the rate of a chemical reaction without being changed or used up in the reaction.

Denaturation is the process by which an enzyme loses its specific shape when exposed to high temperatures (over ~60 degrees C), causing the chemical bonds holding the shape to break and the enzyme to stop functioning.

The optimum temperature of an enzyme is the temperature at which it is most effective as a catalyst, producing the maximum rate of reaction.

False.

Enzymes catalyse specific reactions. Each enzyme has a specific shape that means it can only act on particular reactions.

Enzymes are used in industry because they allow reactions to occur at lower temperatures and pressures than normally required, saving money and energy.

Three uses of enzymes are:\n\n1. Food processing (e.g. breaking starch into sugar in bread making)\n2. Fermentation (yeast enzymes convert glucose to ethanol)\n3. Biological detergents (enzymes break down stains such as blood or grass)

False.

Enzymes are denatured by high temperatures, not low ones. At low temperatures, enzymes still function but there are fewer successful collisions, so the rate of reaction is slower.

In fermentation, the enzyme in yeast converts glucose into ethanol and carbon dioxide.

Hydrogen gas is produced when magnesium reacts with dilute hydrochloric acid. The volume of hydrogen collected over time is measured to determine the rate of reaction.

In the gas collection experiment, the independent variable is time and the dependent variable is the volume of hydrogen gas produced.

False.

Using a higher concentration produces the same total volume of hydrogen gas but it is produced faster. The steeper gradient shows a faster rate, but the final volume is the same.

A gas syringe is a piece of laboratory apparatus used to collect and measure the volume of gas produced during a reaction. It provides an alternative to collecting gas over water using an inverted measuring cylinder.

A concentration of 0.5 mol/dm³ acid is prepared by mixing 10 cm³ of 1 mol/dm³ hydrochloric acid with 10 cm³ of water, giving a total volume of 20 cm³ at half the original concentration.

To collect gas over water, the measuring cylinder is filled with water and clamped upside down in a trough. Gas from the reaction displaces the water.

False.

The two specified practicals use different reactants. One investigates sodium thiosulfate and hydrochloric acid (measuring rate by precipitation). The other investigates magnesium and hydrochloric acid (measuring rate by gas collection).

The flask must be connected straight after adding the magnesium to avoid gas escaping before collection begins. Any gas lost at the start would make the measured volumes lower than the actual amount produced.