Rates of Reaction Flashcards

All 26 cards in this deck

What apparatus can be used to collect and measure the volume of gas produced when hydrochloric acid reacts with marble chips?

A gas syringe, or an upturned measuring cylinder (or burette) full of water over a water trough, joined to the sealed flask by a delivery tube.

Name three practical methods that can be used to follow the rate of a reaction.

Measuring the volume of gas given off; measuring the loss in mass on a balance; timing a colour change or the formation of a precipitate (cloudiness).

In an investigation of how acid concentration affects the rate of reaction between marble chips and hydrochloric acid, what are the independent, dependent and control variables?

Independent: concentration of the acid. Dependent: volume of gas collected in a fixed time. Control: mass and size of marble chips, volume of acid, temperature.

State one risk in the sodium thiosulfate and hydrochloric acid rate experiment and a suitable precaution.

Toxic sulfur dioxide gas is produced — work in a well-ventilated room or fume cupboard and wear eye protection.

What should you do with an anomalous result when drawing a line of best fit for rate results?

Ignore it — do not let it affect the line of best fit, and repeat that measurement if possible.

What are the steps to calculate the mean rate of reaction?

e.g. 30 cm$^3$ of gas collected in 60 s

1. Find the change in volume (or mass): 30 cm$^3$
2. Divide by the time taken: $30 \div 60$
3. Mean rate = 0.5 cm$^3$ per second

What are the steps to find the rate of reaction at a given instant from a volume–time graph?

e.g. the rate at 100 s

1. Draw a tangent touching the curve at 100 s.
2. Read the vertical and horizontal differences along the tangent: e.g. 40 cm$^3$ over 100 s.
3. Rate = gradient = $40 \div 100$ = 0.4 cm$^3$ s$^{-1}$

On a graph of volume of gas produced against time, where is the rate of reaction greatest?

At the start, where the curve is steepest.

True or false? When the line on a volume of gas against time graph becomes horizontal, the reaction has finished.

True. The volume has stopped rising because a reactant has been used up.

Why does the curve on a volume–time graph become less steep as the reaction goes on?

The reactant particles are being used up, so collisions become less frequent and the rate falls.

How does the curve change if the same mass of a solid reactant is used as a powder instead of large pieces, with all other conditions the same?

e.g. magnesium ribbon replaced by magnesium powder

The curve is steeper and levels off sooner, but reaches the same final volume of gas.

According to collision theory, what two conditions must be met for a reaction to happen?

Particles must collide, and the collision must have enough energy (the activation energy) — a successful collision.

In terms of collisions, why does increasing the temperature increase the rate of reaction?

Particles move faster so collide more frequently, and more collisions have enough energy — so more frequent successful collisions.

In terms of collisions, why does increasing the concentration of a solution increase the rate of reaction?

There are more particles in the same volume, so collisions are more frequent.

In terms of collisions, why does increasing the pressure of reacting gases increase the rate of reaction?

The same number of particles are squeezed into a smaller volume, so collisions are more frequent.

In terms of collisions, why does using smaller pieces of a solid reactant increase the rate of reaction?

Smaller pieces have a larger surface area to volume ratio, so more particles are exposed and collisions are more frequent.

True or false? Increasing the concentration of an acid makes its particles move faster.

False. Higher concentration means more particles in the same volume, so collisions are more frequent; only raising the temperature makes particles move faster.

What is a catalyst?

A substance that speeds up the rate of a reaction without altering the products, and is itself unchanged chemically and in mass at the end.

True or false? A catalyst is used up during a reaction, so less of it is present at the end.

False. A catalyst is chemically unchanged and has the same mass at the end of the reaction.

In terms of activation energy, how does a catalyst increase the rate of a reaction?

It provides an alternative route with a lower activation energy, so a greater proportion of collisions are successful.

True or false? Adding a catalyst changes the products made in a reaction.

False. The products are the same — only the rate of the reaction increases.

What is an enzyme?

A biological catalyst — a protein made by living cells that speeds up a reaction.

Give one use of enzymes in industry.

The production of alcoholic drinks such as beer and wine.

True or false? Like other catalysts, an enzyme is chemically unchanged at the end of the reaction.

True. Enzymes are biological catalysts, so they are not used up.

What are the steps in the 'disappearing cross' method for finding the rate of the reaction between sodium thiosulfate and hydrochloric acid?

e.g. the cross disappears after 40 s

1. Draw a black cross on paper and stand the conical flask on top of it.
2. Add the hydrochloric acid to the sodium thiosulfate solution and start the timer at once.
3. Stop the timer when the cross can no longer be seen through the cloudy sulfur: time = 40 s.

In the 'disappearing cross' experiment, how does the time taken for the cross to disappear relate to the rate of reaction?

The shorter the time, the faster the rate (rate $\propto \frac{1}{\text{time}}$).