CSEC Chemistry Paper 3 May/June 2011 - Free CXC Past Paper with Answers

Chemical reactions are accompanied by heat changes. Heat may be absorbed from (or given out to) the surroundings. Such heat changes can be measured experimentally. In this experiment, you are required to determine the heat change which occurs when solid potassium nitrate is dissolved in water.

The procedure for the experiment is outlined below.

PROCEDURE

50 cm³ of distilled water were measured into a polystyrene cup.
The temperature of the water in the cup was measured at one-minute intervals for four minutes, and the data were recorded in Table 1.
Potassium nitrate (5.0 g) was poured into the cup. The contents were stirred with the thermometer and the temperature was recorded at one-minute intervals over a further period of 6 minutes. The temperature for minute 5 is also recorded in Table 1.

TABLE 1. TEMPERATURE CHANGES UPON DISSOLVING POTASSIUM NITRATE IN WATER

$\Delta T$ (temperature reading at a particular time) – (temperature reading at 4 minutes)

Read and record in Table 1 the temperature readings for 6 minutes to 10 minutes from the diagrams in Figure 1.

Figure 1. Thermometer readings at different times upon dissolving potassium nitrate in water

Calculate the change in temperature ( $\Delta T$ ) EACH minute from 6 minutes to 10 minutes and record your answers in Table 1. [See note below Table 1.]

(i) Using the axes provided in Figure 2 on page 5, plot a graph of temperature against time using the data in Table 1.

From your graph, calculate the MAXIMUM change in temperature which occurs when 5.0 g of potassium nitrate is dissolved in 50 cm³ of water. You should indicate on your graph how you arrived at your answer.

Is the reaction exothermic or endothermic? State a reason for your answer.

Calculate EACH of the following:

(Relative atomic mass: K = 39, N = 14, O = 16)

(Heat change = mass of solution x specific heat capacity of solution x temperature change that is, Q = m x c x $\Delta T$ ; Assume that the specific heat capacity of the solution is 4.2 J g $^{-1}$ °C $^{-1}$ and the mass of 1 cm³ of solution is 1 gram.)

a) The number of moles of potassium nitrate used in the experiment.

The heat change which occurs when 5.0 g of potassium nitrate is dissolved in 50 cm³ of water.

The heat change when 1 mole of potassium nitrate is dissolved in water.

Describe how you would prepare a saturated solution of potassium nitrate (KNO₃) at 25 °C.

< ![CDATA[A student carries out the following tests on Solid R and makes the inferences recorded in Table 2. Complete Table 2 to show ALL possible observations.

TABLE 2: RESULTS OF TESTS ON SOLID R

Metals A and B are two unknown metals. Design an experiment to determine which of the metals, A or B, is more reactive. The hypothesis is given below.

Hypothesis: Metal A is more reactive towards acids than Metal B.

Your answer should include the following:

Apparatus and materials

Variables to control

Data to be collected

Discussion of results as they relate to the hypothesis

ONE possible source of error

CSEC Chemistry Paper 3 May/June 2011 Questions

Question 1 (14 marks)

PROCEDURE

Question 2 (10 marks)

Question 3 (12 marks)