Rate of Reaction (AQA GCSE Combined Science: Trilogy: Chemistry): Flashcards

Step 1: moles = 6.0 ÷ 44 = 0.136 mol.

Step 2: rate = 0.136 ÷ 150 = 9.1 × 10^-4 mol/s.

Always convert mass to moles first, then divide by time in seconds.

The answer is 60 times too large: 6 minutes = 6 × 60 = 360 seconds.

Rate is in mol/s, so time must always be converted to seconds first.

Forgetting this conversion is a common error in multi-step rate calculations.

The gradient represents the rate of reaction.

A steeper gradient means a faster rate. The gradient decreases over time as reactants are used up and the reaction slows down.

The curve becomes horizontal because a limiting reactant has been used up.

Common wrong answers: "both reactants are used up" (wrong if one is in excess) and "the syringe is full" (wrong reason). The time must be read from the graph, not from the data table.

Mean rate of reaction = change in y-value ÷ change in time.

Using volume: mean rate = (V₂ − V₁) ÷ (t₂ − t₁).

False.

Rate graphs must show a curved line: the rate slows as reactants are used up, so the line curves and eventually becomes horizontal.

Drawing a straight line of best fit is one of the most commonly penalised plotting errors.

You must use a superlative: Catalyst A gave the "steepest curve", or "highest initial gradient", or "greatest volume of gas produced in a given time."

Using only "steeper" (comparative) is insufficient and will not score the mark.

The student must explicitly state that the rate of reaction decreases over time. Required wording: "the rate slows down" or "the rate decreases as the reaction proceeds."

Describing only the volume trend, without mentioning rate, does not score the mark.

A tangent is a straight line drawn to touch a curve at exactly one point without crossing it; the gradient of this tangent gives the instantaneous rate of reaction at that moment in time.

A line from the origin to a point on the curve gives the average rate over that time period, not the instantaneous rate.

To find the instantaneous rate at a specific time, you must draw a tangent to the curve at that point; without a tangent, no marks for the instantaneous rate can be awarded.

The gradient of a tangent is calculated as the change in y divided by the change in x (using a large triangle), not a single y-value divided by a single x-value.

False.

A large triangle reduces the error when reading coordinate values from the axes. Using a small triangle magnifies any reading errors, making the gradient calculation less accurate.

Marks cannot be awarded for numbers whose origin cannot be determined.

Annotating the triangle (labelling the Δy and Δx values read from the graph) shows the examiner where your figures came from; error carried forward credit is also available if the method is clearly shown but a value was misread.

Rate = Δy ÷ Δx = 72 cm³ ÷ 20 s = 3.6 cm³/s.

This is the initial rate because the tangent is drawn at time = 0, where the gradient (and therefore the rate) is greatest.

The five factors are:

• concentration of dissolved reactants

• pressure for reacting gases

• temperature

• surface area of solid reactants

• the presence of a catalyst.

Mark schemes require reference to frequency of collisions, not just number.

You must state there are 'more frequent collisions' or 'more collisions per second.' Simply writing 'more collisions' (with or without 'successful') is not creditworthy.

False.

Changing temperature (or concentration, surface area, or catalyst) only affects the rate: how quickly product forms. The total amount of product remains the same; it is simply reached sooner when the rate is higher.

When explaining the effect of concentration on rate, stating 'there are more particles' is insufficient. You must state there are more particles per unit volume. Also, references to particle speed or energy of collisions are not accepted.

Smaller chips have a larger surface area for the same volume, so there are more frequent collisions between acid particles and the surface of the marble.

A common misconception is thinking the marble chips collide with each other — it is always the acid particles that collide with the solid surface. References to particle speed or energy are not accepted.

A catalyst is a substance that increases the rate of reaction and is not used up during the reaction.

A common error is defining a catalyst as 'anything that speeds up a reaction' — for example, stating 'temperature is a catalyst' contradicts the definition and loses the mark.

The dependent variable is the time taken for the cross to no longer be seen through the flask. Vague answers such as 'how fast it goes cloudy' or simply 'the time' do not gain credit.

Control variables include the volume of sodium thiosulfate solution, the volume of hydrochloric acid, and the concentration of hydrochloric acid.

Using the word 'amount' instead of volume or concentration is not creditworthy.

False.

Separate measuring cylinders (or pipettes or burettes) must be used for each liquid. Using the same piece of equipment contaminates the solutions and can start the reaction before the timer begins, making the results invalid.

The timer must be started when the hydrochloric acid is added to the thiosulfate, and stopped when the cross is no longer visible.

As the reaction proceeds, there are fewer sodium thiosulfate particles per unit volume (concentration decreases). This means the frequency of particle collisions decreases, so the rate of reaction decreases.

43 s is an anomalous result: it differs significantly from the other two repeats. The mean must be calculated excluding anomalous values, giving 53 s.

Including the anomaly is one of the most common errors in mean calculation questions for this practical.

Activation energy (E_a) is the minimum amount of energy that colliding particles must have for a chemical reaction to occur. It is different for each reaction; reactions with a high E_a have fewer successful collisions and a lower rate.

A successful collision occurs when colliding particles have energy greater than or equal to the activation energy (E_a): the atoms rearrange to form products.

An unsuccessful collision occurs when the energy is below E_a: the particles simply bounce off each other unchanged.

False.

Increasing concentration only increases the frequency of collisions (more particles per unit volume). It does not increase the energy of individual collisions.

Mark schemes explicitly reject references to particle energy or speed when explaining concentration's effect on rate.

Increasing temperature increases the rate of reaction because particles have more kinetic energy, so the frequency of collisions increases, and a greater proportion of collisions have energy greater than or equal to the activation energy.

Temperature is the only factor that increases both the frequency of collisions (particles move faster) and the energy of collisions (a greater proportion exceed E_a).

Concentration and surface area only increase collision frequency, not energy. Of the two effects of temperature, the increase in energy is the more important.

For every 10°C increase in temperature, the rate of reaction approximately doubles.

This applies to aqueous and gaseous systems and explains why a small rise in temperature has a disproportionately large effect on rate.

A catalyst is a substance that increases the rate of a chemical reaction but is not used up during the reaction. Its mass is the same before and after the reaction.

A catalyst provides an alternative reaction pathway that has a lower activation energy. More particles then have energy greater than or equal to the activation energy, so there are more successful collisions per second.

Fewer than 20% state the 'alternative pathway' link; many only say the activation energy is lowered, which is insufficient for full marks.

False.

A catalyst increases the rate of the forward and reverse reactions equally, so the equilibrium position does not change. It only allows equilibrium to be reached more quickly.

Claiming the catalyst shifts the equilibrium right is one of the most common misconceptions in this topic.

A catalyst works by providing an alternative reaction pathway with a lower activation energy, so a greater proportion of particles have enough energy to react.

Since three lines are being compared, a superlative is required, not a simple comparative.

The correct wording is that the best catalyst has the steepest curve, produces the greatest volume of gas in a given time, or has the highest initial gradient.

'Steeper' is insufficient when comparing three options.

A catalyst reduces the energy required (a lower temperature can be used), which cuts fuel costs. It also increases the rate of production, so more product is made in the same time, improving efficiency.