The Rate & Extent of Chemical Change (AQA GCSE Combined Science: Synergy: Physical Sciences): Flashcards

False.

Only successful collisions result in a reaction. These are collisions where particles collide with energy greater than or equal to the activation energy. Particles with insufficient energy simply bounce off each other unchanged.

A collision is successful when the particles collide with energy greater than or equal to the activation energy. This provides enough energy for the chemical bonds in the reactants to break and new bonds in the products to form.

According to collision theory, increasing the concentration of particles in a solution increases the number of collisions per second, leading to more successful collisions and a faster rate of reaction.

Increasing kinetic energy (e.g. by raising temperature) means more particles have energy greater than or equal to the activation energy. This increases both the frequency of collisions and the proportion that are successful, so the rate of reaction increases.

False.

A reaction with a high activation energy will have fewer successful collisions, because fewer particles will have enough energy to exceed the threshold. Reactions with a low activation energy are generally faster at the same temperature.

The rate of reaction depends on:

1. The number of collisions per second — more frequent collisions increase the rate

2. The proportion of collisions with energy greater than or equal to the activation energy — only these collisions are successful

The four factors are:

1. Temperature

2. Concentration of reactants in solution (or pressure of reacting gases)

3. Surface area of solid reactants

4. The presence of a catalyst

The surface area to volume ratio describes how much surface is exposed relative to the total volume of a solid. Increasing it (e.g. by grinding a solid into powder) exposes more particles to reactants, increasing the rate of reaction.

True.

Higher temperature gives particles more kinetic energy, causing more frequent collisions. Of these two effects, the increase in collision energy (meaning more collisions exceed the activation energy) is the more important factor.

Increasing the concentration of a solution increases the rate of reaction because there are more reactant particles in a given volume, leading to more frequent collisions.

Increasing pressure forces the same number of gas particles into a smaller volume. This increases the number of particles per unit volume, causing more frequent collisions and increasing the rate of reaction.

True.

A 10°C rise in temperature causes a large increase in rate. This is because many more particles now have energy exceeding the activation energy, so the proportion of successful collisions increases significantly.

Breaking a solid into smaller pieces increases the surface area exposed to the other reactant. This means more particles can collide per second, increasing the frequency of successful collisions and therefore the rate of reaction.

At higher concentration, the line:

1. Has a steeper gradient at the start

2. Becomes horizontal sooner

3. Reaches the same final amount of product

This shows the reaction is faster but produces the same total quantity of product.

The concentration is varied by changing the ratio of sodium thiosulfate solution to water added to the flask. As more water is added and less thiosulfate is used, the concentration decreases but the total volume stays constant.

The disappearing cross experiment is a method for measuring reaction rate. Sodium thiosulfate and hydrochloric acid react to form a sulfur precipitate that clouds the solution. The time for the cross beneath the flask to disappear is recorded.

The reaction begins as soon as the reactants mix, producing hydrogen gas. If the flask is not sealed immediately, gas will escape before collection begins, making the volume measurements inaccurate and the results unreliable.

False.

As concentration increases, the reaction is faster, so the cross disappears more quickly. A higher concentration means more particles per unit volume, causing more frequent successful collisions.

Repeating each concentration three times allows a mean time to be calculated and any anomalous results to be identified and excluded. This improves the reliability of the results.

In Required Practical 19, the concentration of sodium thiosulfate is the independent variable, the time taken for the cross to disappear is the dependent variable, and temperature is an example of a control variable.

The disappearing cross method is subjective — different observers may disagree on the exact moment the cross disappears. It also produces only one data point per experiment, so a continuous rate graph cannot be drawn.

The activation energy (symbol E_a) is the minimum amount of energy that reacting particles must have to react successfully. All reactions have an activation energy because bonds in the reactants must first be broken.

A reaction profile (energy level diagram) is a graph showing the relative energies of reactants and products during a reaction. It displays the activation energy, the overall energy change, and whether the reaction is exothermic or endothermic.

False.

In an exothermic reaction, the products have a lower energy than the reactants. Energy is released to the surroundings, so the overall energy change is negative. The arrow on the reaction profile points downwards.

The peak of the curve represents the transition state — the highest energy point during the reaction. The height from the reactants to the peak is the activation energy (E_a), the minimum energy needed for the reaction to proceed.

In an endothermic reaction, the products have more energy than the reactants, so the overall energy change is positive, and the arrow on the reaction profile points upwards.

Reactions with a higher activation energy require more energy for particles to react. At a given temperature, fewer particles will have enough energy to exceed this threshold, so fewer successful collisions occur per second and the reaction is slower.

A fully labelled reaction profile for an exothermic reaction must show:

1. Reactants and products labelled on the y-axis (energy)

2. The activation energy (E_a) arrow

3. The overall energy change arrow (negative, pointing downwards)

4. A curved line showing energy as the reaction proceeds

A bond energy is the amount of energy required to break a specific chemical bond, or the energy released when that bond is formed. Bond energies are measured in kJ per mole.

False.

Bond breaking is an endothermic process — energy must be taken in from the surroundings to break chemical bonds. Bond making is exothermic, as energy is released when new bonds form.

Overall energy change = energy taken in (to break bonds in reactants) − energy given out (from forming bonds in products)

• A negative value means the reaction is exothermic

• A positive value means the reaction is endothermic

In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break the existing bonds.

Energy in (bonds broken): 436 + 242 = 678 kJ

Energy out (bonds formed): 2 × 431 = 862 kJ

Overall energy change: 678 − 862 = −184 kJ

The reaction is exothermic because more energy is released than taken in.

True.

In an endothermic reaction, energy taken in to break bonds exceeds energy released from forming bonds, giving a positive overall energy change. The products end up at a higher energy level than the reactants.

The energy change of a reaction is calculated as: energy taken in − energy given out. It is negative for exothermic reactions (energy released) and positive for endothermic reactions (energy absorbed).

Each bond type has a bond energy value per bond. If the balanced equation shows that multiple bonds of the same type are broken or formed (shown by the balancing numbers), the bond energy must be multiplied by the number of those bonds to get the total energy.

A catalyst is a substance that increases the rate of a chemical reaction without being altered or consumed in the reaction. It provides an alternative reaction pathway with a lower activation energy.

A catalyst provides an alternative pathway for the reaction that has a lower activation energy. This means a greater proportion of reactant particles have energy above the activation energy, so more successful collisions occur per second.

False.

A catalyst is not used up during a reaction. Its mass is the same at the start and end of the reaction. This is why only small amounts are needed and why catalysts are cost-effective in industrial processes.

On a reaction profile, a catalysed reaction shows a lower activation energy compared to the uncatalysed reaction, but the same overall energy change.

Iron is used as a catalyst in the Haber process to produce ammonia from nitrogen and hydrogen. Iron beads are used to maximise surface area for catalysis.

True.

Different reactions have different activation energies and require different alternative pathways. For example, iron catalyses the Haber process, while enzymes act as biological catalysts for specific reactions in living organisms.

With a catalyst, the curve:

1. Has a steeper gradient at the start

2. Becomes horizontal sooner

3. Reaches the same final amount of product

This shows the reaction is faster but produces the same total quantity of product.

An enzyme is a biological catalyst made from protein. Enzymes increase the rate of specific chemical reactions in living organisms by providing a reaction pathway with a lower activation energy.

An enzyme is a biological catalyst made from protein. Enzymes speed up chemical reactions in cells and remain unchanged at the end of the reaction.

The active site is the specific region on an enzyme where the substrate binds. It has a complementary shape to the substrate, which explains the specificity of enzymes.

In the lock and key model:

1. The enzyme acts as the lock and the substrate as the key

2. The substrate has a complementary shape to the enzyme's active site

3. They bind to form an enzyme-substrate complex

4. Products form and are released; the enzyme is unchanged and can catalyse further reactions

False.

Beyond the optimum temperature, bonds that hold the enzyme's shape begin to break. The active site distorts, the substrate can no longer fit, and the enzyme is said to be denatured. Denaturation is largely irreversible.

Enzymes work fastest at their optimum temperature. In the human body, this is approximately 37 °C.

Each enzyme has an optimum pH at which it works fastest. Moving too far above or below the optimum pH breaks bonds that maintain the enzyme's shape, changing the active site so the substrate no longer fits. Extreme pH causes denaturation.

False.

Most enzymes have an optimum pH of around 7, but some differ based on where they work. For example, enzymes in the stomach have a lower optimum pH (around pH 2) due to the acidic environment.

Enzymes are specific because the shape of each enzyme's active site is complementary to only one particular substrate (or group of substrates). Usually one enzyme catalyses one particular reaction.

An amylase is an enzyme that catalyses the breakdown of starch (a polysaccharide) into maltose (a disaccharide). It is used in Required Practical 20 to investigate the effect of pH on enzyme activity.

A drop of the reaction mixture is placed on iodine solution every 30 seconds. Iodine turns blue-black in the presence of starch. When the iodine remains orange-brown, all the starch has been digested, indicating the reaction is complete.

False.

A shorter time for the iodine to remain orange-brown means all the starch was digested more quickly, indicating a faster rate of enzyme activity. A longer time means the enzyme is working more slowly at that pH.

In Required Practical 20, buffer solution is added to set the pH, amylase enzyme is added, and the mixture is tested with iodine to detect remaining starch.

A pH buffer solution maintains the pH at a fixed, known value throughout the experiment. This ensures the pH is the only independent variable being changed, making the investigation a fair test.

All solutions (amylase, starch, and buffer) are placed in a water bath and allowed to reach the target temperature before being mixed. A thermometer is used to confirm the temperature has been reached.

True.

A colorimeter measures the absorbance or transmission of light through the solution. As starch is broken down, the iodine solution becomes lighter (less starch = less blue-black colour), providing a more precise and objective measure of reaction rate.

A reversible reaction is one where the products can react to reform the original reactants. It can proceed in both the forward and reverse directions, shown by the symbol ⇌.

False.

In a reversible reaction, if the forward reaction is exothermic, the reverse reaction must be endothermic. The same amount of energy is transferred in each direction.

A reversible reaction is shown using the symbol ⇌ (two half-arrows, one pointing right and one pointing left). This indicates the reaction can proceed in both the forward and reverse directions.

When hydrated copper(II) sulfate is heated, the blue crystals turn white and water is released. Adding water reverses this reaction, turning the crystals blue.

An anhydrous substance has had its water of crystallisation removed, usually by heating. For example, anhydrous copper(II) sulfate is a white powder formed when the blue hydrated crystals are heated.

ammonium chloride ⇌ ammonia + hydrogen chloride

Heating causes decomposition (endothermic forward reaction); cooling causes the gases to recombine to reform ammonium chloride (exothermic reverse reaction).

False.

A reversible reaction does not go to completion. The forward and reverse reactions occur simultaneously. In a closed system, the reaction reaches dynamic equilibrium where reactants and products are both present.

A dynamic equilibrium occurs in a closed system when the rates of the forward and reverse reactions are equal. The concentrations of reactants and products remain constant but both reactions are still occurring.

False.

At dynamic equilibrium, the concentrations of reactants and products are constant (they do not change over time), but they are not necessarily equal. The relative amounts depend on the conditions of the reaction.

1. The reaction must be in a closed system — reactants and products cannot escape

2. The rates of the forward and reverse reactions must be equal

At dynamic equilibrium, the rates of the forward and reverse reactions are equal, and the concentrations of all species remain constant.

False.

At dynamic equilibrium, both the forward and reverse reactions are still occurring continuously — it is this ongoing activity that makes it "dynamic." The reactions have not stopped; they are simply proceeding at the same rate.

If the conditions change, the system is no longer at equilibrium. It will automatically shift the position of equilibrium to oppose the change and re-establish equilibrium with new constant concentrations.

In a closed system, no reactants or products can escape. If the system were open, products could be lost, preventing the reverse reaction from occurring at the same rate as the forward reaction, so equilibrium could never be established.

The equilibrium shifts to the right (towards the products) to reduce the effect of the increased reactant concentration. More product is formed until a new equilibrium is established.

False.

Increasing temperature shifts the equilibrium in the direction of the endothermic reaction. If the forward reaction is exothermic, increasing temperature shifts the equilibrium to the left (towards reactants).

The position of equilibrium describes the relative amounts of reactants and products present when a system is at equilibrium. If it shifts right, more products are formed; if it shifts left, more reactants are present.

Increasing pressure shifts the equilibrium towards the side with fewer molecules of gas. This reduces the pressure. If both sides have equal numbers of gas molecules, changing pressure has no effect on the position of equilibrium.

For the reaction 2NO₂ (g) ⇌ N₂O₄ (g), increasing pressure shifts the equilibrium to the right because the forward reaction produces fewer molecules of gas.

Decreasing temperature shifts the equilibrium in the direction of the exothermic reaction. If the forward reaction is endothermic, the reverse (exothermic) reaction is favoured, so the equilibrium shifts to the left, producing more reactants.

False.

Changes in pressure only affect reactions involving gases. For reactions where all species are solids or liquids, changing pressure has no effect on the position of equilibrium.

The equilibrium shifts to the right (towards the products) to replace the product that was removed, until a new equilibrium is established. This is an example of the system opposing the change.